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AS3.2

EDI
Clouds, Aerosols, Radiation and Precipitation (General Session)

Clouds and aerosols play a key role in climate and weather-related processes over a wide range of spatial and temporal scales. An initial forcing due to changes in the aerosol concentration and composition may also be enhanced or dampened by feedback processes such as modified cloud dynamics, surface exchange or atmospheric circulation patterns. This session aims to link research activities in observations and modelling of radiative, dynamical and microphysical processes of clouds, aerosols, and their interactions. Studies addressing several aspects of the aerosol-cloud-radiation-precipitation system are encouraged. Contributions related to the EU project "Constrained aerosol forcing for improved climate projections" (FORCeS) are also invited.

Topics covered in this session include, but are not limited to:
- Cloud and aerosol macro- and microphysical properties, precipitation formation mechanisms and their role in the energy budget
- Observational constraints on aerosol-cloud interactions
- Use of observational simulators to constrain aerosols, clouds and their radiative effects in models
- Experimental cloud and aerosol studies
- High-resolution modelling, including large-eddy simulation and cloud-resolving models
- Parameterization of cloud and aerosol microphysics/dynamics/radiation processes
- Interactions between aerosols and regional circulation systems and precipitation patterns
- Aerosol, cloud and radiation interactions and feedbacks on the hydrological cycle, regional and global climate

Convener: Edward Gryspeerdt | Co-conveners: Annica Ekman, Geeta PersadECSECS, Anna Possner
Presentations
| Thu, 26 May, 08:30–11:50 (CEST), 13:20–16:40 (CEST)
 
Room F1

Thu, 26 May, 08:30–10:00

Chairpersons: Edward Gryspeerdt, Anna Possner

08:30–08:40
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EGU22-5540
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solicited
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Highlight
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On-site presentation
Laura Wilcox et al.

The uncertainty in aerosol radiative forcing is currently the largest source of uncertainty in estimates of the magnitude of the total anthropogenic forcing on climate, and changes in aerosol emissions are likely important for regional climate over the next few decades. This is especially the case for Africa and Asia where large aerosol emission changes are anticipated, and where aerosol has played an important role in historical changes. Uncertainty in near-term projections due to the substantial spread in aerosol (or their precursor) emissions pathways is compounded by uncertainty in the simulated response to these emissions, so a multi-model framework is needed to identify robust changes.  

Several earlier studies have explored the climate response to regional aerosol perturbations, with interesting, but not always consistent, results. Using these studies to inform our understanding of the potential role of aerosol in near-future changes is not straightforward. Many are based around equilibrium experiments that are challenging to use to interpret transient simulations, and the effects of different experimental designs are difficult to separate from the effects of structural differences between the models. In Regional Aerosol MIP, we will perform a set of transient experiments based on emissions from the Shared Socioeconomic Pathways. Regional Aerosol MIP will better enable us to assess the potential contribution of aerosol to near-future climate change, to describe the robust features of the response to regional aerosol changes, and to identify where the key uncertainties lie. In this presentation we will introduce the experiment design, alongside some early analysis.

How to cite: Wilcox, L., Allen, R., Bauer, S., Bollasina, M., Ekman, A., Keeble, J., Lewinschal, A., Lund, M., Merikanto, J., O'Donnell, D., Paynter, D., Persad, G., Rumbold, S., Samset, B., Takemura, T., Tsigaridis, K., Undorf, S., and Westervelt, D.: Regional Aerosol Model Intercomparison Project , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5540, https://doi.org/10.5194/egusphere-egu22-5540, 2022.

08:40–08:46
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EGU22-4108
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On-site presentation
Gunnar Myhre et al.

Enhanced emissions of both greenhouse gases and aerosols generate climate responses on a wide range of time scales. An initial radiative response triggers a set of rapid adjustments, which are eventually followed by surface-temperature-driven feedbacks. While a lot happens during the first days and months after a perturbation, the monthly mean data typically used in climate studies are too coarse to show the temporal evolution of responses. In these analyses, we take a closer look at how the climate system responds during the very first hours and days after a sudden increase in carbon dioxide (CO2), in black carbon (BC) or in sulfate (SO4). Five models have performed PDRMIP simulations with hourly output, and we also compare results to monthly PDRMIP and CMIP6 results. We find that the effect of increasing ocean temperatures kicks in after a couple of months. Rapid precipitation reductions are for all three climate perturbations established after just a couple of days, and does for BC not differ much from the full-time response.  For CO2 and SO4, the magnitude of the precipitation response gradually increases with surface warming, and for CO2 the sign of the response changes for negative to positive after two years. Rapid cloud adjustments are typically established within the first 24 hours and while the magnitude of cloud feedbacks for CO2 and SO4 increases over time, the latitude-height pattern of the total cloud changes is clearly present after one year. While previously known that climate responses to BC are dominated by rapid adjustments, this work underlines the swiftness of the processes involved.

How to cite: Myhre, G., Stjern, C., Samset, B., Forster, P., Quaas, J., Takemura, T., Voulgarakis, A., Jia, H., Jouan, C., Sand, M., and Olivie, D.: The timescales of climate responses to carbon dioxide and aerosols, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4108, https://doi.org/10.5194/egusphere-egu22-4108, 2022.

08:46–08:52
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EGU22-13274
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On-site presentation
Marianne Pietschnig et al.
Recent studies have revealed biases in sea surface temperature trends in CMIP6 between about 1970 and 2015, and other studies have suggested a possible lack of aerosol emissions over Asia in the same period. Motivated by these findings, we investigate the effect of doubling anthropogenic aerosol emissions over Asia from 1950 onward on sea surface temperatures in NorESM2-LM. While the perturbation of historical aerosol emissions does not yield a robust improvement in modeled sea surface temperature trends, we discover other changes which are worthy of further investigation: 
 
  • We find that the ocean heat transport decreases significantly in the Northern Hemisphere extratropics, which is counter-intuitive given the increasing temperature gradient between the tropics and the polar regions due to the enhanced aerosol emissions.
  • When doubling SO2 emissions, we find increases in Southern Hemisphere sea surface temperatures in contrast to cooling in the Northern Hemisphere. 

Furthermore, we compare the fully-coupled simulations to atmosphere-only simulations where historical sea surface temperatures are prescribed and the same perturbation of aerosol emissions over Asia is imposed. The atmosphere-only simulations show much weaker changes in cloud cover compared to the fully-coupled simulations. Hence, sea surface temperature changes – possibly caused by changes in the oceanic circulation – must play an important role in setting the atmospheric response.

How to cite: Pietschnig, M., Olivié, D., Moseid, K. O., Hofer, S., Madan, G., Lacasce, J. H., and Storelvmo, T.: Effects of increased aerosol emissions over Asia on global sea-surface temperatures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13274, https://doi.org/10.5194/egusphere-egu22-13274, 2022.

08:52–08:58
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EGU22-6250
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On-site presentation
Dirceu Herdies et al.

The Sun’s radiation is the primary energy source for chemical, biological, and physical processes that happen in the climate system. Thus, the earth-atmosphere radiative balance system is one of the main aspects of climate change. Natural fluctuations in incident solar radiation caused by the sunspot cycle can influence the energy balance. In addition, human activities can also affect this balance. Changes in gases and aerosols emissions to the atmosphere can modify its composition because they are involved in complex chemical reactions, such as ozone concentrations. Gases and aerosols can absorb, scatter, and reflect incident solar radiation, thereby affecting the balance. This study assesses the direct impact of aerosol (through changing the optical depth of aerosol-AOD in the radiation subroutine) on the surface atmospheric temperature and radiation balance. Three simple experiments for 1998-2017 were carried out through numerical modeling using BAM-v1.2 (Brazilian Global Atmospheric Model), the operational weather and climate forecasting model at CPTEC/INPE (Center for Weather Forecasting and Climate Studies/National Institute for Space Research). These experiments were zero aerosols, fixed AOD over the land and ocean, and AOD climatology with a spatial and temporal variation (AOD-C). The surface atmospheric temperature was validated against the ERA5 reanalysis from December to February (DJF) and June to August (JJA). Also, the downward shortwave solar radiation on the clear-sky variable was validated against CERES-EBAF satellite data. We performed the bias, the difference between the model and the reanalysis data (ERA5) and EBAF-CERES, correlation and RMSE of the model results against ERA5 and EBAF-CERES for surface temperature and downward shortwave solar radiation on clear-sky respectively. Our results have shown a positive bias atmospheric surface temperature for the northern hemisphere continent and a negative bias for the southern hemisphere continent during JJA. We observed a decrease in this positive bias in the northern hemisphere in the experiments with fixed aerosols, but an important improvement (vies, correlation, and RMSE) was observed in the experiment with AOD-C. On the other hand, during the DJF period, the model has a positive bias only in some continental areas, such as southwestern South America and South Africa, North Africa, and the Australian continent. Similarly, to JJA, we observed improvements in these regions in the experiments that use fixed and AOD-C. The downward shortwave solar radiation on clear-sky results for both DJF and JJA showed an inversion from the positive bias to a negative bias in the model version without aerosols to the model with fixed aerosols and AOD-C, due to the presence of the aerosol, which reduces short wave flow. An important improvement (vies, correlation, and RMSE) in the downward shortwave solar clear sky was observed in the version that uses AOD-C during JJA.

How to cite: Herdies, D., Alvim, D., Basso, L., Pendharkar, J., Castilho, D., Oyerinde, G., Costa, S., Kubota, P., and Figueroa, S.: Validadion of three Brazilian Global Atmospheric Model experiments (without, fixed and monthly climatological aerosol) against ERA5 and CERES-EBAF, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6250, https://doi.org/10.5194/egusphere-egu22-6250, 2022.

08:58–09:04
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EGU22-4247
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ECS
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Highlight
Peter Kuma et al.

Uncertainty in cloud feedback in climate models is a major limitation in projections of future climate. We analyse cloud biases and trends in climate models relative to satellite observations, and relate them to equilibrium climate sensitivity, transient climate response and cloud feedback. For this purpose, we develop a deep convolutional artificial neural network for determination of cloud types from low-resolution daily mean top of atmosphere shortwave and longwave radiation images, corresponding to the World Meteorological Organization (WMO) cloud genera recorded by human observers in the Global Telecommunication System. We train this network on a satellite top of atmosphere radiation dataset, and apply it on the Climate Model Intercomparison Project phase 5 and 6 (CMIP5 and CMIP6) historical and abrupt-4xCO2 experiment model output and the ERA5 and Modern-Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2) reanalyses. We compare these with satellite observations, link cloud type occurrence biases and trends to climate sensitivity, and compare our cloud types with an existing cloud regime classification based on the Moderate Resolution Imaging Spectroradiometer (MODIS) and International Satellite Cloud Climatology Project (ISCCP) satellite data. We show that there is a strong linear relationship between the root mean square error of cloud type occurrence and model equilibrium climate sensitivity, transient climate response and cloud feedback (Bayes factor 7×102, 4×102 and 13, respectively). This indicates that models with a better representation of the cloud types have a more positive cloud feedback and higher climate sensitivity. Along with other studies, our results point to a choice between two explanations: either high sensitivity models are plausible, contrary to combined assessments of climate sensitivity and cloud feedback in previous review studies, or the accuracy of representation of present-day clouds in models is negatively correlated with the accuracy of representation of future projected clouds.

How to cite: Kuma, P., Bender, F., Schuddeboom, A., and McDonald, A.: Cloud type machine learning shows better present-day cloud representation in climate models is associated with higher climate sensitivity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4247, https://doi.org/10.5194/egusphere-egu22-4247, 2022.

09:04–09:10
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EGU22-11653
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ECS
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Highlight
Hendrik Andersen and Jan Cermak

In this contribution, statistical and machine learning techniques are applied to quantify the response of clouds to changes in environmental factors.

Clouds play a key role for the Earth’s energy balance; however, their response to climatic and anthropogenic aerosol emission changes is not clear, yet. Here, 20 years of satellite cloud observations are analyzed together with reanalysis data sets in multivariate-regression and machine-learning approaches to quantitatively link the variability of observed cloud radiative effects to changes in environmental factors, or cloud-controlling factors (CCFs). In this data-driven approach, a large number of CCFs, including aerosol proxies, are used as predictors at a large spatial and temporal scale typical of CCF analyses. The analysis reveals distinct regional patterns of CCF importance for shortwave and longwave cloud radiative effects. In stratocumulus cloud regions, the main controls of shortwave CRE are the sea surface temperature and the estimated inversion strength, but also zonal winds in the lower free troposphere are relevant controls of CRE. Aerosol proxies are shown to be most important for shortwave CRE in the regions of stratocumulus to cumulus transition. Future analyses of interactions between different CCFs and comparisons to global climate models are outlined.

How to cite: Andersen, H. and Cermak, J.: A data-driven approach to understanding global controls of cloud radiative effects, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11653, https://doi.org/10.5194/egusphere-egu22-11653, 2022.

09:10–09:16
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EGU22-7521
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ECS
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On-site presentation
George Datseris et al.
Understanding planetary cloudiness is of major importance for Earth's energy balance and potential for warming, but so far we lack pathways to approach planetary cloudiness theoretically. On the one hand, it is difficult to connect the microphysics of cloud formation to planetary wide cloudiness. On the other hand, a representation of cloudiness in energy balance models simply does no exist yet. In this work we want to provide simple means to treat planetary cloudiness in an energy balance model. We utilize a top-down approach and directly decompose the energetic signature of planetary cloudiness into a simple model composed of simple components. Vertical wind speed and estimated inversion strength are enough to capture all major characteristics of cloudiness in both shortwave and longwave spectral signatures. Other variables provide only minor improvements to the fits, while surface horizontal wind speed seems to be important for capturing hemispheric asymmetries in cloudiness. We use our results to argue that cloudiness can be incorporated into conceptual models based on mean temperature and equator-to-pole temperature difference.

How to cite: Datseris, G., Blanco, J., Bony, S., Caballero, R., Hadas, O., Kaspi, Y., and Stevens, B.: Minimalistic approach to planetary cloudiness, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7521, https://doi.org/10.5194/egusphere-egu22-7521, 2022.

09:16–09:22
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EGU22-2856

A wide range of aerosol effects on precipitation have been proposed, from the scale of individual clouds to that of the globe.

This presentation, based on the findings of an expert workshop under the umbrella of the GEWEX Aerosol Precipitation initiative, reviews the evidence and scientific consensus behind these effects and the underlying set of physical mechanisms, categorised into i) radiative effects via modification of radiative fluxes and the energy balance and ii) microphysical effects via modification of cloud droplets and ice crystals.

There exists broad consensus and strong theoretical evidence that, because global mean precipitation is constrained by energetics and surface evaporation, aerosol radiative effects (aerosol-radiation interactions and aerosol-cloud interactions) act as drivers of precipitation changes. Likewise, aerosol radiative effects cause well-documented shifts of large-scale precipitation patterns, such as the Inter-Tropical Convergence Zone (ITCZ). The extent to which aerosol effects on precipitation are applicable at smaller scales and driven or buffered by compensating microphysical and dynamical mechanisms and budgetary constraints is less clear. Although there exists broad consensus and strong evidence that suitable aerosol perturbations increase cloud droplet numbers, reducing the efficiency of warm rain formation across cloud regimes, the overall aerosol effect on cloud microphysics and dynamics as well as the subsequent impact on local, regional and global precipitation is less constrained.

This presentation provides a review of the physical mechanisms of aerosol effects on precipitation backed up by evidence from recent cloud-resolving and global modelling simulations as well as from satellite observations.

How to cite: Stier, P. and the GEWEX Aerosol Precipitation (GAP) initiative expert workshop team: Multifaceted Aerosol Effects on Precipitation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2856, https://doi.org/10.5194/egusphere-egu22-2856, 2022.

09:22–09:28
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EGU22-5393
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ECS
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On-site presentation
Ying Liu et al.

In recent decades, WRF has been widely used in regional rainfall simulations, and many studies have shown it has good performance in reproducing rainfall distribution. However, the WRF simulated rainfall amounts are often significantly underestimated which might be due to insufficient consideration of aerosol-cloud-precipitation-meteorology interaction in its mdoelling. WRF-chem as a meteorology-chemistry coupling model is expected to improve such a shortcoming. In this study, we carry out a series of WRF and WRF-chem simulations of a large-scale extreme rainfall event (occurred from October 11th to 15th, 2018) over the UK to explore whether different aerosol effects could help improve the rainfall simulation performance.

 

To compare and evaluate the influences of different aerosol effects, four types of simulations using WRF and WRF-chem were conducted. The baseline simulation (called WF_B) was simulated by WRF without any emission data and chemical boundary conditions. The sensitivity simulation (WC_NE) was simulated by WRF-chem with emission data and chemical boundary conditions as well as used a chemical mechanism. But it turned off aerosol direct and indirect effects. The other two sensitivity simulations WC_DE and WC_DAIE were conducted by turning on the direct aerosol effect and turning on all (direct and indirect) aerosol effects, respectively. All simulations used the same domain configurations, physical schemes, and meteorological boundary conditions. Through comparing the difference between the four simulated rainfall distributions and amounts, the impact of aerosol direct effect, indirect effect, and net (direct + indirect) effect on extreme rainfall simulation were estimated.

 

The simulation results were compared with UK radar observations. The sensitivity study shows that the rainfall intensity performance greatly improved with the inclusion of the aerosol-cloud interaction in the modelling (indirect effect). However, aerosol-radiation feedback (direct effect) does not have a significant impact on rainfall intensity estimations. One of the reasons was because the aerosol indirect effect has a great influence on droplet/particle concentration, precipitation efficiency and cloud life in nature. Statistics show that there are 115 grids in radar observation with rainfall greater than 100 mm, while WF_B, WC_NE, WC_DE and WC_DAIE simulations have respectively 44, 44, 44 and 117 grids with rainfall greater than 100 mm. In addition, the Root Mean Square Error of WF_B, WC_NE, WC_DE and WC_DAIE accumulated rainfall is 2.501, 2.501, 2.484 and 0.779 respectively. On the other hand, the rainfall spatial performances of the four simulations are relatively close, which were not improved obviously with the inclusion of aerosol effects. Their probability of detection (POD), frequency bias index (FBI), critical success index (CSI), and false alarm ratio (FAR) performances were averaged at 0.941, 0.946, 0.936, and 0.006, respectively. Finally, using the chemical mechanism and chemical data but turning off aerosol effects resulted in similar rainfall estimations of WRF-chem and original WRF. In summary, it is highly recommended to turn on WRF aerosol effects, especially the indirect aerosol effects in extreme rainfall simulations.

How to cite: Liu, Y., Zhuo, L., and Han, D.: Investigating the influence of aerosol effects on extreme rainfall simulations over the UK using WRF and WRF-chem model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5393, https://doi.org/10.5194/egusphere-egu22-5393, 2022.

09:28–09:34
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EGU22-13520
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On-site presentation
Olivier Champagne et al.

The region of Western Central Africa (WCA) is covered by a large deck of stratocumulus or stratus during the long dry season (June-September). These low clouds are an important component to sustain the Gabonese and Congolese forests, but they are not properly simulated by global climate models. The Pan-African convection-permitting decadal regional climate simulation (4 km resolution), conducted with the Met-office unified model (CP4-Africa), has so far facilitated great advances in scientific understanding on characteristics of organised deep-convection in the climate change context. However, it remains unclear whether there may also be added value in the simulation of extensive low-level clouds. Here, we concentrate on the CP4 historical period (1997-2006) and evaluate its representation of low clouds in WCA. We will present preliminary results on the ability of CP4 to simulate the diurnal and seasonal evolution of low clouds in WCA compared to in-situ observations, ERA5 reanalyses and the non-convection-permitting regional simulation (R25). R25 was run with a similar setup and global driving data as CP4, but using a convective parametrization, thus allowing direct attribution of simulation differences to resolution and the representation of convection. This work is relevant for our understanding of the processes responsible for the development and persistence of low clouds in WCA. Our results may also be used to assess whether future projections at km-scale such as from CP4 can provide more plausible depictions of low cloud changes.

How to cite: Champagne, O., Moron, V., Philippon, N., and Klein, C.: Representation of Low-Level Clouds in West Central Africa in a convection-permitting regional climate simulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13520, https://doi.org/10.5194/egusphere-egu22-13520, 2022.

09:34–09:40
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EGU22-9020
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ECS
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On-site presentation
Rose Miller et al.

Biomass burning (BB) and anthropogenic aerosols and their influence on clouds represent one of the poorly quantified uncertainties in radiative forcing of the climate system. Cumulus clouds, common over maritime regions impacted by BB or anthropogenic aerosols, are important regulators of the global radiative energy budget and global hydrologic cycle. In 2019, a NASA-funded field campaign, the Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex) based in South East Asia sampled three distinct regions around the Philippines, the West Pacific, the South China Sea, and the Sulu Sea. The aerosols and clouds located in these three areas were sampled by instruments mounted on the NASA P3 aircraft, quantifying the microphysical properties of clouds and the chemical composition of aerosols. Our analysis focuses on analyzing the statistical properties of cloud-pass statistics as a means to compare the relationships between cloud properties and their aerosol environments.

During CAMP2Ex the NASA P3 penetrated 1698 clouds just above cloud base. The cloud pass diameters ranged from 0.11 km to 4.5 km with most clouds in the range of 0.2 - 0.3 km. Updraft strengths ranged from 0.1 to 3.0 m/s.  The cloud droplet concentrations ranged from 171.4 to 1971.6 cm-3 with the smallest number concentration found in marine cumulus outside the region of BB plumes and aerosol plumes originating from anthropogenic and natural sources of the Asian continent. The highest particle concentrations were within BB plumes. In biomass burning regions, organic aerosol ranged from 32.1 to 53.4 µg/m3, sulfate aerosols ranged from 4.9 to 7.6 µg/m3, and black carbon ranged from 90.1 to 181.3 µg/m3. In comparison, anthropogenic aerosol regions, where sulfate aerosol was dominant, had sulfate ranging from 1.5 - 15.0 µg/m3 and organics from 0.1 - 3.3 µg/m3. The Manila plume recorded a range of sulfate aerosols of 1.2 - 10.2 µ/m3, nitrate 0.8 - 3.4 µg/m3, and ammonium 0.7 - 4.7 µg/m3 and black carbon 20 - 615 ng/m3. These aerosol source regions were compared to open-ocean marine aerosol with chemical masses less than 0.1 µg/m3 for all species measured by the on board aerosol mass spectrometer. 

The relationship between cloud number concentrations (Nd), effective radii (re), liquid water content (LWC), and cloud drop size distributions just above cloud base within updrafts exceeding 0.4 m/s are related to aerosol chemical composition in four aerosol regimes, biomass burning, industrial anthropogenic aerosol over the South China Sea, the Manila plume around Metro Manila, and open ocean marine aerosol. 

How to cite: Miller, R., Rauber, R., Di Girolamo, L., McFarquhar, G., Nesbitt, S., Ziemba, L., and Wang, J.: Biomass burning and anthropogenic aerosol influence on cumulus cloud microphysical properties during CAMP2Ex, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9020, https://doi.org/10.5194/egusphere-egu22-9020, 2022.

09:40–09:46
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EGU22-9554
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ECS
Mengjiao Jiang et al.

The Sichuan Basin, which is located in southwest China, has become one of the most polluted regions in China. The frequency of atmospheric boundary layer in the Sichuan Basin are analyzed based on radiosonde profile from 2014 to 2016. The occurrence probability of multi-layer temperature inversions is about 46.86% in winter over the Sichuan Basin. The radiosonde data are divided into four equal parts according to the previous 12-hour average visibility and PM2.5 mass concentration. As the PM2.5 mass concentration increases (visibility decreases), the surface-based inversion frequency increases more consistently in the morning, while the elevated inversion increases more consistently in the evening. The mechanisms of the aerosol radiative effect on the boundary layer temperature inversion are further investigated by using the radiometer, the Mie scattering lidar, and the microwave radiometer in Chengdu. 1D-SBDART simulation is performed to better clarify the mechanisms. The simulation results show that: from clean to heavy pollution conditions, in clear-sky the surface shortwave radiation reduces by 135.04 w·m-2 and the heating rate increases by 0.75 k·d-1; in cloudy sky the surface shortwave radiation reduces by 46.15 w·m-2 and the heating rate increases by 0.35 k·d-1. Aerosols can enhance the boundary layer temperature inversion at both daytime and nighttime due to the radiative effect, while clouds mitigate the enhancement by aerosol effects. 

How to cite: Jiang, M., Yang, Y., Ni, C., and Chen, Q.: The impact of aerosols on the temperature inversion in the boundary layer over southwest China , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9554, https://doi.org/10.5194/egusphere-egu22-9554, 2022.

09:46–09:52
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EGU22-2863
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On-site presentation
Ross Herbert and Philip Stier

The long-lived and widespread nature of smoke, coupled with its ability to perturb the atmosphere simultaneously via aerosol-cloud and aerosol-radiation interactions, has proven a challenge to observe and simulate; as such, the impact of smoke on regional and global scales remains uncertain.

In this study we use an 18-year climatology from multiple instruments onboard AQUA and TERRA satellites to identify and characterise the relationships between aerosol-optical-depth (AOD) and the large-scale properties of the clouds, precipitation, and top-of-atmosphere radiation over the Amazon rainforest during the biomass burning season.

Our analysis provides robust evidence that localised smoke production drives widespread modification to the cloud regime over the region: in the morning (TERRA) cloud liquid water path increases with AOD, whereas in the afternoon (AQUA) convective activity is initially enhanced then supressed when AOD exceeds 0.4. During both time periods there is an increasingly pronounced presence of high-altitude, optically thin, clouds.

The result is a sharp contrast in the cloud-field properties and vertical distribution between low-AOD days and high-AOD days and a pronounced top-of-atmosphere radiative effect of -50 Wm­-2 (for AOD = 1.4), which persists throughout the day.

How to cite: Herbert, R. and Stier, P.: Amazon fires drive widespread changes to diurnal cloud regimes and radiation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2863, https://doi.org/10.5194/egusphere-egu22-2863, 2022.

09:52–10:00
Discussion

Thu, 26 May, 10:20–11:50

Chairpersons: Annica Ekman, Anna Possner

10:20–10:30
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EGU22-1845
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ECS
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solicited
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Highlight
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On-site presentation
Adele Igel and Abigail Williams

Observations and simulations show that an increase in aerosol concentration typically leads to an increase in liquid water path in precipitating stratocumulus clouds due to precipitation suppression, but once precipitation is fully suppressed, further increases in aerosol concentration typically lead to a reduction in liquid water path due to enhanced evaporation at cloud top. The increased evaporation is typically attributed directly to the presence of smaller, more numerous cloud droplets. However, observations suggest that the evaporation rate is primarily controlled by the entrainment mixing rate rather than the droplet properties at the tops of stratocumulus clouds. As such, aerosol-induced changes to droplet properties should not directly lead to faster evaporation. Our simulations suggest instead that the smaller, more numerous droplets enhance the cloud top maximum radiative cooling rate, which in turn increases the entrainment rate and speeds evaporation. Our results highlight that unlike integrated radiative cooling, maximum radiative cooling continues to increase with increasing liquid water path and remains sensitive to droplet properties at high liquid water path. As such, the role of radiation in driving aerosol-cloud interactions may need additional consideration in the future.

How to cite: Igel, A. and Williams, A.: What drives increased evaporation at cloud top in polluted stratocumulus clouds?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1845, https://doi.org/10.5194/egusphere-egu22-1845, 2022.

10:30–10:36
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EGU22-12323
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Highlight
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On-site presentation
Tom Goren et al.

Aerosol–cloud interactions in marine stratocumulus clouds (Sc) are among the most challenging frontiers in cloud–climate research. In particular, the cloud cover susceptibility to droplet concentration remained under-represented in the literature. We developed methodologies to estimate what would have been the cloud cover and the associated radiative effect of currently observed Sc, but in a hypothetical cleaner world. The first methodology uses a realistic Lagrangian large eddy simulation coupled with satellite observations and provides a process-oriented analysis. The other uses a simple model and provides a global estimate of the radiative impact. We found that overcast Sc decks would have broken up sooner had they not been influenced by anthropogenic aerosol, thereby causing a significant effective radiative forcing.

How to cite: Goren, T., Feingold, G., Gryspeerdt, E., Kazil, J., and Quaas, J.: Exploring the Effect of Aerosol on Marine Cloud Cover Using a Counterfactual Approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12323, https://doi.org/10.5194/egusphere-egu22-12323, 2022.

10:36–10:42
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EGU22-4075
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ECS
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On-site presentation
Emilie Fons et al.

Aerosol-cloud interactions (aci) are a key climate process causing significant cooling by perturbing Earth’s radiative budget, especially in stratocumulus clouds that cover extended areas of the planet. However, the exact value of the global radiative forcing associated with aci has proven difficult to quantify. In fact, estimates still differ between models and satellite data. Satellite studies are limited by the fact that the variable of interest, i.e. the aerosol concentration that the cloud forms on, remains unobservable. Instead, the satellite can only retrieve a proxy metric, such as Aerosol Optical Depth (AOD), which is retrieved in cloud-free pixels, requiring an aggregation technique -either spatial or temporal- to recreate aerosol-cloud data pairs. Furthermore, until now, aci has been most often quantified using polar-orbiting satellites, which only provide one daily snapshot of cloud and aerosol optical properties. In this study, newly available geostationary satellite aerosol products are used to explore the temporal and spatial dependence of aci in stratocumulus clouds. Preliminary results indicate that the sensitivity of cloud properties to AOD changes depending on the spatial and temporal scales chosen for the analysis, indicating either a shift of prevailing physical processes in time and space, or statistical biases and spuriousness. This dependence of sensitivity with the scale of analysis is also confirmed in reanalysis data. However, it seems that the sensitivities captured in reanalysis data are of opposite signs than in the satellite data. Ongoing work focuses on investigating the role of spatial and temporal scales as well as resolving the discrepancy between the observational and reanalysis estimates at these different scale

How to cite: Fons, E., Neubauer, D., and Lohmann, U.: Temporal and spatial dependence of aerosol-cloud interactions in marine stratocumulus clouds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4075, https://doi.org/10.5194/egusphere-egu22-4075, 2022.

10:42–10:48
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EGU22-8789
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On-site presentation
Sudhakar Dipu et al.

Important aspects of the adjustments to aerosol-cloud interactions can be examined using the relationship between cloud droplet number concentration (Nd) and liquid water path (LWP). Specifically, this relation can constrain the role of aerosols in leading to thicker or thinner clouds in response to adjustment mechanisms. This study investigates the satellite retrieved relationship between Nd and LWP for a selected case of mid-latitude continental clouds using high-resolution Large-eddy simulations (LES) over a large domain in weather prediction mode. Since the satellite retrieval uses the adiabatic assumption to derive the Nd (NAd), we have also considered NAd from the LES model for comparison. The joint histogram analysis shows that the NAd-LWP relationship in the LES model and the satellite is in approximate agreement. In both cases, the peak conditional probability (CP) is confined to lower NAd and LWP, and the corresponding mean LWP shows a weak relation with NAd. In contrast, at higher NAd (> 50 cm−3 ), the CP shows a larger spread; consequently, the mean LWP increases non-monotonically with increasing NAd in both cases. However, the NAd-LWP relation lacks, in particular, the negative sensitivity at higher NAd. This case over continent thus behaves differently compared to previously-published analysis of Oceanic clouds using satellite retrievals. Additionally, our analysis illustrates a regime dependency (marine and continental) in the NAd-LWP relation from the satellite retrievals. When considering the relationship of the simulated cloud-top Nd, rather than NAd, with LWP, the result shows a much more nonlinear (positive and negative) relationship and is inconsistent with the satellite retrievals. However, the difference is much less pronounced when the sensitivity (Nd-LWP) is considered for shallow stratiform (adiabatic) than convective (sub-adiabatic) clouds. Comparing local vs large-scale statistics from satellite data shows that continental clouds exhibit only a weak nonlinear Nd-LWP relationship. Hence a regime-based Nd-LWP analysis is even more relevant when it comes to continental clouds and its comparison to satellite retrievals. 

How to cite: Dipu, S., Schwarz, M., Ekman, A. M. L., Gryspeerdt, E., Goren, T., Sourdeval, O., Mülmenstädt, J., and Quaas, J.: Exploring satellite-derived relationships between cloud droplet number concentration and liquid water path using large-domain large-eddy simulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8789, https://doi.org/10.5194/egusphere-egu22-8789, 2022.

10:48–10:54
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EGU22-8212
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Virtual presentation
Silvia M. Calderón et al.

We carried out a closure study of aerosol-cloud interactions during stratocumulus formation using a large eddy simulation model UCLALES-SALSA and in situ observations from the 2020 sampling campaign at the Puijo SMEAR IV station in Kuopio, Finland. UCLALES-SALSA uses spectral bin microphysics for aerosols and hydrometeors and incorporates a full description of their interactions into the turbulent-convective radiation-dynamical model of stratocumulus. Typical closure studies use observations to assess agreement between aerosol properties and cloud droplet number concentration (CDNC) in updrafts. Here, the unique observational setup allowed a closer look into the aerosol size-composition dependence of droplet activation and droplet growth in turbulent boundary layer driven by surface forcing and radiative cooling. The model successfully described probability distribution of updraft velocities and aerosol activation efficiency curves, and nicely recreated the size distributions shapes for aerosol and cloud droplets. This is the first time such a detailed closure is achieved not only accounting for activation of cloud droplets in different updrafts, but also accounting for processes evaporating droplets and drizzle production through coagulation-coalescence. 
We studied two cases of cloud formation, one diurnal (24/09/2020) and one nocturnal (31/10/2020), with high and low aerosol loadings, respectively. Aerosol number concentrations differ more than an order of magnitude between cases and therefore, lead to CDNC  of less than 100 cm-3 up to 1000 cm-3. Different aerosol loadings affected the supersaturation at the cloud base, and thus the minimum size of aerosol particles producing cloud droplets. Also, as the mean size of cloud droplets in the diurnal-high aerosol case was lower, the droplet evaporation process was found to be decreasing the observed CDNC more than in the low aerosol case.   In addition, in the low aerosol case, the presence of large aerosol particles played a significant role on the droplet spectrum evolution as it promoted the drizzle formation through coalescence and collision processes enhanced by cyclic turbulence fluctuations. Also, during the event, the ice particle formation was observed due to subzero temperature at the cloud top. 
The studied cases are presented in detail and can be further used by the cloud modellers to test and validate their models in a well characterized modelling setup. We also provide recommendations on how increasing amount of information on aerosol properties could improve the understanding of processes affecting cloud droplet number and liquid water content in stratified clouds.

How to cite: Calderón, S. M., Tonttila, J., Buchholz, A., Joutsensaari, J., Komppula, M., Leskinen, A., Liqing, H., Moisseev, D., Tiitta, P., Virtanen, A., Kokkola, H., and Romakkaniemi, S.: Closure study of aerosol-stratocumulus interactions with UCLALES-SALSA, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8212, https://doi.org/10.5194/egusphere-egu22-8212, 2022.

10:54–11:00
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EGU22-11779
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ECS
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Virtual presentation
Lambert Delbeke and Chien Wang
Low-Level Stratiform Clouds (LLSC) appear frequently over southern West Africa (SWA). During the West African Monsoon (WAM) period, both local (air pollution) and remote (dust and biomass burning aerosols from North and Central Africa, respectively) aerosol sources can play a significant role in LLSC diurnal life cycle. The Dynamics-Aerosols-Chemistry-Cloud Interactions In West Africa (DACCIWA) campaign has produced a considerable number of clouds and aerosols measurements during the WAM in 2016. Numerical simulations using a Large Eddy Simulations (LES) model with detailed aerosol and cloud microphysical processes and constrained by DACCIWA observations have been conducted, driven by different atmospheric aerosol compositions in order to study the impacts of different aerosols on the key processes (formation, break-up, transition to cumulus) of LLSC. Detailed results alongside conclusions will be presented.

How to cite: Delbeke, L. and Wang, C.: Influence of Different Atmospheric Aerosol Compositions on the Life Cycle of Stratiform Clouds over Southern West Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11779, https://doi.org/10.5194/egusphere-egu22-11779, 2022.

11:00–11:06
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EGU22-4896
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ECS
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On-site presentation
Hailing Jia et al.

Aerosol–cloud interaction is the most uncertain component of the overall anthropogenic forcing of the climate, in which the Twomey effect plays a fundamental role. Satellite-based estimates of the Twomey effect are especially challenging, mainly due to the difficulty in disentangling aerosol effects on cloud droplet number concentration (Nd) from possible confounders. By combining multiple satellite observations and reanalysis, this study investigates the impacts of a) updraft, b) precipitation, c) retrieval errors, as well as (d) vertical co-location between aerosol and cloud, on the assessment of Nd-toaerosol sensitivity (S) in the context of marine warm (liquid) clouds. Our analysis suggests that S increases remarkably with both cloud base height and cloud geometric thickness (proxies for vertical velocity at cloud base), consistent with stronger aerosol-cloud interactions at larger updraft velocity. In turn, introducing the confounding effect of aerosol–precipitation interaction can artificially amplify S by an estimated 21 %, highlighting the necessity of removing precipitating clouds from analyses on the Twomey effect. It is noted that the retrieval biases in aerosol and cloud appear to underestimate S, in which cloud fraction acts as a key modulator, making it practically difficult to balance the accuracies of aerosol–cloud retrievals at aggregate scales (e.g., 1° × 1° grid). Moreover, we show that using column-integrated sulfate mass concentration (SO4C) to approximate sulfate concentration at cloud base (SO4B) can result in a degradation of correlation with Nd, along with a nearly twofold enhancement of S, mostly attributed to the inability of SO4C to capture the full spatio-temporal variability of SO4B. These findings point to several potential ways forward to account for the major influential factors practically by means of satellite observations and reanalysis, aiming at an optimal observational estimate of global radiative forcing due to the Twomey effect.

How to cite: Jia, H., Quaas, J., Gryspeerdt, E., Böhm, C., and Sourdeval, O.: Addressing the difficulties in quantifying the Twomey effect for marine warm clouds from multi-sensor satellite observations and reanalysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4896, https://doi.org/10.5194/egusphere-egu22-4896, 2022.

11:06–11:12
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EGU22-17
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ECS
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Virtual presentation
Yunfei Che et al.

Using in-situ aircraft observations from six flights over Zhejiang on Sep. 1 and Sep. 4, 2016, this study investigates differences in aerosol and cloud properties between daytime and nighttime. The samples were divided into marine type and continental type based on the backward air mass trajectories and aerosol characteristics. The results show that the aerosol number concentration (Na) near the ground during daytime is higher than that at nighttime. During daytime, Na has a significant decreasing trend near the top of the planetary boundary layer (PBL), which is not obvious during nighttime. There may be still a relative high concentration of aerosols remaining in the transition zone between the PBL and the free troposphere. Under similar liquid water content (LWC) conditions, the cloud droplet number concentration (Nc) at night is lower, and the cloud droplet effective diameter (cloud ED) is larger. The total Na of marine type aerosols is generally lower than that of continental type aerosols, but for aerosols with particle diameters greater than 1 μm, the marine type aerosols are higher. The study shows a strong negative Na-cloud ED relationship for marine type aerosols, but no obvious Na-cloud ED relationship for continental type aerosols. The number of cloud condensation nuclei (CCN) is higher under high-Na conditions; the ratio of CCN to Na reveals that the activation efficiency of marine type aerosols is higher than that of continental type aerosols. There is no obvious difference in activation efficiency between day and night.

How to cite: Che, Y., Zhang, J., Zhao, C., Zhou, X., and Duan, J.: Aerosol and cloud properties over a coastal area from aircraft observations in Zhejiang, China, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-17, https://doi.org/10.5194/egusphere-egu22-17, 2022.

11:12–11:18
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EGU22-13537
Graham Feingold et al.

High-resolution modeling of aerosol-cloud interactions typically applies aerosol perturbations for the duration of the simulation, which may be anywhere from a few hours to a few days.

In reality, however, natural and anthropogenic aerosol perturbations have characteristic durations, along with concomitant changes in meteorology and associated cloud conditions. In this talk we will explore the effect of the duration of aerosol perturbations on the cloud radiative responses using idealized large eddy simulations. We will also consider observed seasonal cycles in meteorology, clouds, and aerosol, and how they affect cloud albedo responses. These exercises will help to assess the radiative effect of aerosol-cloud interactions.

How to cite: Feingold, G., Prabhakaran, P., Zhang, J., Zhou, X., and Hoffmann, F.: Exploring the Influence of the Duration of Aerosol Perturbations on Cloud Responses, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13537, https://doi.org/10.5194/egusphere-egu22-13537, 2022.

11:18–11:24
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EGU22-7676
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ECS
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Virtual presentation
Juha Tonttila et al.

The so-called autoconversion is a key numerical process used to describe the coalescence growth of cloud droplets to drizzle and rain in atmospheric models. Together with further growth of drizzle through accretion, these processes are typically represented by relatively simple two-moment bulk parameterizations. One of the shortcomings of this approach is that most often the parameterizations do not explicitly consider the impact of coarse mode aerosol and giant cloud condensation nuclei (GCCN), even though they are known to be important in marine cloud regimes. More elaborate sectional models, such as the Sectional Aerosol Module for Large-Scale Applications (SALSA) solve the coalescence growth equations and are able to account for the effect of large aerosol particles on droplet growth and drizzle formation.

In this work, the autoconversion and accretion rates diagnosed from SALSA are compared with the process rates from bulk parameterizations run simultaneously within a large-eddy simulation model (UCLALES-SALSA). The model is used to create an ensemble of simulations comprising varying aerosol conditions in terms of the coarse mode particles in marine stratocumulus and shallow cumulus regimes. The difference between SALSA and bulk process rates is taken as the approximation error in the bulk parameterizations. The dependence of this error term on the coarse mode aerosol concentration is shown by a multivariate sensitivity analysis based on the ensemble data. Further, machine learning methods, notably the neural networks, are used to represent the approximation error term. The trained networks are shown to successfully capture the main features of the model-based approximation error. As an outlook, the machine learning-based representation of the approximation error allows to enhance the existing bulk microphysical parameterizations for drizzle formation and to introduce an explicit dependence on coarse model aerosol and GCCN.

How to cite: Tonttila, J., Romakkaniemi, S., and Kokkola, H.: Approximation error correction for drizzle formation in bulk microphysical parameterizations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7676, https://doi.org/10.5194/egusphere-egu22-7676, 2022.

11:24–11:30
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EGU22-12007
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ECS
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Highlight
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On-site presentation
Rachel Sansom et al.

The transition from stratocumulus to cumulus clouds that takes place as air is advected from the subtropics towards the equator causes a decrease in cloud radiative effect, with cloud fraction halving from start to finish. The transition is initiated by increasing sea surface temperatures, and it is widely agreed that the lower tropospheric stability plays a key role in the timing of the transition. In this work, we study the relative importance of five atmospheric initial conditions: specific humidity in the boundary layer and free troposphere, free tropospheric potential temperature, inversion height and initial aerosol distribution. We simulate a Lagrangian trajectory of a stratocumulus-to-cumulus transition, using the Met Office/NERC cloud model coupled with a bulk microphysics scheme and a radiation scheme. From this base simulation we make 60 perturbations to simulate the transition under different combinations of the atmospheric initial conditions mentioned. Additionally, we include a model parameter from the Khairoutdinov and Kogan autoconversion parameterisation from 2000. We discuss here the relative importance of these so-called parameters, in particular the role of aerosol, and we explore whether a much faster transition by drizzle takes place in simulations with lower aerosol concentrations. 

How to cite: Sansom, R., Lee, L., Johnson, J., Regayre, L., and Carslaw, K.: Exploring a Stratocumulus-to-Cumulus Transition: A Perturbed Parameter Ensemble of Large-Eddy Simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12007, https://doi.org/10.5194/egusphere-egu22-12007, 2022.

11:30–11:36
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EGU22-847
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ECS
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Highlight
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On-site presentation
Ulrike Proske et al.

Clouds are a major component of Earth's energy budget, influencing e.g. the radiative balance and precipitation formation. In turn, cloud properties are determined by the microphysical processes that occur within clouds, e.g. modifying their albedo.
Global climate models employ cloud microphysical schemes to parameterize these processes. The schemes have grown in detail and complexity, but it is doubtful whether this will help us to reduce uncertainty (Carslaw et al., 2018). In fact, cloud microphysics (CMP) and aerosol schemes have become so detailed that they are becoming difficult to constrain with observations (Reddington et al., 2017; Morrison et al., 2020) and to comprehend, while their results are more difficult to interpret. Simplification or removal of single processes within the CMP schemes might offer a remedy that reduces complexity and enhances interpretability. For such simplifications it is first necessary to determine which processes are non-influential so that less accurate descriptions could suffice.
Recently, Proske et al. (2021) applied global sensitivity analysis on an emulated perturbed parameter ensemble (PPE) of four CMP processes, perturbing their effectiveness in the global aerosol climate model ECHAM-HAM. They thereby investigated to which of the four processes the model is most sensitive. They found that accretion and self-collection of ice have a negligible influence while aggregation dominates the response to perturbations.
Here, we extend this analysis to the whole CMP scheme in ECHAM-HAM, creating a PPE of all processes, especially widening the scope to warm microphysics. With the analysis we characterize the scheme, uncover which processes drive the model response and suggest candidates for simplification to ultimately guide model development to a simplified representation of CMP.


Carslaw, Kenneth, Lindsay Lee, Leighton Regayre, and Jill Johnson. “Climate Models Are Uncertain, but We Can Do Something About It.” Eos 99, doi: 10.1029/2018EO093757, 2018.

Morrison, Hugh et al. “Confronting the Challenge of Modeling Cloud and Precipitation Microphysics.” Journal of Advances in Modeling Earth Systems 12, no. 8, doi: 10.1029/2019MS001689, 2020.

Proske, Ulrike et al. “Assessing the Potential for Simplification in Global Climate Model Cloud Microphysics.” Atmos. Chem. Phys. Discuss. [preprint], doi: 10.5194/acp-2021-801, in review, 2021.

Reddington, Carly et al. “The Global Aerosol Synthesis and Science Project (GASSP): Measurements and Modeling to Reduce Uncertainty.” Bulletin of the American Meteorological Society 98, no. 9, doi: 10.1175/BAMS-D-15-00317.1, 2017.

How to cite: Proske, U., Ferrachat, S., and Lohmann, U.: Which cloud microphysical processes are dispensable in a global aerosol climate model?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-847, https://doi.org/10.5194/egusphere-egu22-847, 2022.

11:36–11:42
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EGU22-9720
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ECS
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Virtual presentation
Eva Pauli et al.

In this contribution geographic patterns of fog and low stratus (FLS) formation and dissipation over central Europe are presented using a novel satellite-based data set. 
Formation and dissipation of FLS are the results of complex interactions of meteorological and land-surface processes. Furthermore, the timing of FLS formation and dissipation has implications for traffic and the production of solar energy. Yet, little is known about the spatial and temporal patterns of both in central Europe. To improve this situation, this study analyzes these patterns, as well as the meteorological drivers and their modifications by the land surface. The basis of the analysis is a novel FLS formation and dissipation data set, derived based on satellite FLS products and logistic regression.
Very distinct and contrasting spatial and seasonal patterns of FLS formation and dissipation are found across the study area. In large-scale river valleys, FLS forms most frequently in the morning and dissipates in the afternoon. In mountainous areas and on the coast, FLS forms during the night and dissipates in the morning. FLS persists longer in winter compared to other seasons. The quantitative analysis of meteorological drivers shows that the large-scale meteorological conditions, in particular mean surface pressure and wind speed, substantially influence the timing of FLS formation and dissipation. Local variations in topography modulate these conditions, leading to local differences in the observed patterns.

How to cite: Pauli, E., Cermak, J., and Andersen, H.: A geographic perspective on fog and low stratus formation and dissipation over central Europe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9720, https://doi.org/10.5194/egusphere-egu22-9720, 2022.

11:42–11:50
Discussion

Thu, 26 May, 13:20–14:50

Chairpersons: Annica Ekman, Anna Possner, Edward Gryspeerdt

13:20–13:30
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EGU22-7078
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solicited
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Highlight
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On-site presentation
Mingxi Yang et al.

Ship exhausts have historically been significant sources of sulfur dioxide and aerosols to the marine atmosphere and some global models suggest the emissions cause a large negative radiative forcing by modifying cloud properties. International Maritime Organisation (IMO, an agency of the UN) regulations require that ships in international waters reduce their sulfur emissions from a maximum of 3.5% to 0.5% from January 2020. The ACRUISE project, taking advantage of this unique large-scale aerosol perturbation, investigates the impacts of the IMO’s 2020 sulfur regulations on aerosols, clouds, and radiation in the North Atlantic and globally. Here I summarise our findings so far from intensive aircraft observations, high-resolution model simulations, and deep learning-based satellite cloud analysis. 

 

Aerosol-cloud interaction near shipping lanes was studied from an aircraft in the northeast Atlantic in 2019 as well as in 2021. Aerosol chemical and physical properties were markedly different between the two years, with much lower sulfur content, smaller, and less hygroscopic aerosols in 2021. A detailed analysis of the aerosol and cloud microphysics observations within/immediately outside the ship plumes will be performed to determine whether some clouds appeared to be strongly impacted by ship plumes, while other clouds were not. To help interpret the aircraft data and provide context, we ran nested regional domain simulations of the Met Office Unified Model for all flight campaigns. These high-resolution simulations (few hundred metres) show a generally diffuse pattern of perturbed trace gases and aerosols that are not apparent as individual ship tracks, suggesting that analysis of tracks alone may underestimate the climatic effects of ship emissions.

 

We have trained a deep learning model to detect ship-tracks in satellite imagery with good skill and applied it to the whole MODIS mission in order to develop a global climatology. We will discuss the spatial and temporal distribution of shiptracks relative to the underlying ship emissions, and particularly focus on the effects of the IMO regulation as well as the global COVID-19 pandemic. Ongoing work that combines airmass trajectory modelling with known positions of ships will enable us to assess the impact of ship emissions on all pixels, and not just those identified as ship tracks.

How to cite: Yang, M., Bell, T., Bower, K., Carslaw, K., Choularton, T., Christensen, M., Coe, H., Grosvenor, D., Lee, J., Watson-Parris, D., Stier, P., and Yoshioka, M.: Insights from ACRUISE (Atmospheric Composition and Radiative forcing changes due to UN International Ship Emissions regulations) from aircraft, modelling, and satellite perspectives, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7078, https://doi.org/10.5194/egusphere-egu22-7078, 2022.

13:30–13:36
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EGU22-4188
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ECS
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Highlight
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On-site presentation
Peter Manshausen et al.

Cloud reflectivity changes due to anthropogenic aerosol remain a source of significant uncertainty in our understanding of climate change. Ship tracks, long lines of polluted clouds that are visible in satellite images, have been used widely as opportunistic experiments for quantifying aerosol-cloud-interactions. However, only a small fraction of the clouds polluted by shipping show ship tracks, potentially introducing a sampling bias when extrapolating such studies to a wider range of environmental conditions.

To overcome this issue, we develop a novel method to investigate all clouds polluted by shipping, regardless of whether they exhibit visible ship tracks. While previous studies are limited to on the order of thousands of tracks, we use on the order of two million equivalent ship paths. Combined with reanalysis winds and trajectory modelling, these paths enable us to identify clouds that are exposed to pollution and compare them to unpolluted ones nearby. This way we show that formerly invisible ship emissions change cloud properties considerably: cloud droplet numbers increase even when no ship tracks are visible, with the anomaly roughly half as large as in visible tracks. These “invisible” ship tracks also show a more positive liquid water response. For the first time, we directly detect shipping-induced cloud property changes in the trade cumulus regions of the Atlantic. These regions also show stronger liquid water responses than the stratocumulus regions previously studied for ship tracks. We estimate the global radiative forcing from liquid water adjustment to be between (-1.89, -0.30) W m-2, well outside the equivalent IPCC estimate of (0.0, +0.4) W m-2. We also show that only 30 days of satellite observations are needed to confidently detect changes in cloud droplet number from known shipping, with implications for potential marine cloud brightening experiments.

Our results indicate that earlier studies of ship tracks may be suffering from selection biases by focusing only on visible tracks from satellite imagery. The strong liquid water path response we find translates to a larger aerosol cooling effect on the climate, potentially masking a higher climate sensitivity than observations would otherwise suggest. Further work is in progress to evaluate the dependency of aerosol effects on environmental factors such as atmospheric stability and sea surface temperature, as well as extending the analysis to cloud top height and, if possible, to cloud fraction.

How to cite: Manshausen, P., Watson-Parris, D., Christensen, M., Jalkanen, J.-P., and Stier, P.: Invisible Ship Tracks as Opportunistic Experiments for Aerosol Cloud Interactions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4188, https://doi.org/10.5194/egusphere-egu22-4188, 2022.

13:36–13:42
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EGU22-9756
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On-site presentation
Edward Gryspeerdt et al.

The sensitivity of clouds to anthropogenic aerosol perturbations remains one of the largest uncertainties in the human forcing of the climate system. A key difficulty is in isolating the impact of aerosols from large-scale covariability of aerosol and cloud properties. Natural experiments, where aerosol is produced independently of the cloud and meteorological properties, provide a pathway to address this issue. These aerosol sources often modify cloud properties, leaving linear cloud features known as shiptracks (when formed by a ship) or pollution tracks (more generally).

In this work, we use a database of point sources of aerosol over both land and ocean to identify clouds that are sensitive to aerosol and to measure their response. Using a neural network to identify when a point source is modifying the cloud, we are able to measure the sensitivity of individual clouds to aerosol at a global scale, looking at over 400 million cases.

We find the probability of track formation is strongly dependent on the background cloud and meteorological state, similar to previous regional studies. With our global database, we identify regions that are strongly susceptible to aerosol perturbations, even where aerosol sources are rare. We find that there are several regions that are highly susceptible to aerosol, but that have been previously overlooked due to a low frequency of pollution tracks.    

How to cite: Gryspeerdt, E., Louro Coelho, M., Smith, T., Suarez De La Fuente, S., Quilelli Correa Rocha Ribeiro, R., and van Reeuwijk, M.: Measuring cloud sensitivity to aerosols at a global scale using isolated aerosol sources, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9756, https://doi.org/10.5194/egusphere-egu22-9756, 2022.

13:42–13:48
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EGU22-8581
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ECS
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On-site presentation
Athulya Saiprakash and Odran Sourdeval

The understanding of aerosols and their impact on climate via their interactions with clouds remains one of the largest uncertainty to our current estimates of future climate warming. A change in the amount of aerosol in the atmosphere, whether natural or artificial, can have a direct impact on cloud particle nucleation. However, anthropogenically induced aerosol-cloud interactions (aci) are thought to counteract some of the effects of global warming, quantifying their effect on total radiative forcing is vital for future climate predictions.

To tackle this problem, recent research has focused on so-called natural laboratories in order to better understand aci, meaning experiments where aerosol emissions are relatively well localized and understood, hence removing one important aspect of the aci uncertainties. For instance, volcano eruptions, ship tracks, industrial tracks, or contrails represent such laboratories. The purpose of the study is to assess if emission restriction events can act as a natural laboratory for aci studies, almost in a reverse manner as industrial tracks. Here, we will compare regional cloud properties observed during the lockdown periods to climatologies using MODIS satellite cloud droplet number concentration (Nd) retrievals. CAMS global reanalysis and CAM-chem model simulations are used to study CCN activity and aerosol emissions during the lockdown period. This study will focus on different industrial regions to examine if there are any clear signals of aci. It is found that there is no significant decrease in Nd during lockdown compared to climatology at a regional scale. Although there is a reduction in various anthropogenic activities such as industrial emissions, motor vehicle emissions, etc but the background CCN conditions played an important role in the influence of Nd.

How to cite: Saiprakash, A. and Sourdeval, O.: Satellite-based investigation of the impact of COVID-19 restrictions on cloud properties in industrial regions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8581, https://doi.org/10.5194/egusphere-egu22-8581, 2022.

13:48–13:54
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EGU22-4783
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ECS
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On-site presentation
Mahnoosh Haghighatnasab et al.

Increased anthropogenic aerosols result in an enhancement in cloud droplet number concentration Nd, which consequently modifies cloud and precipitation process. It is unclear how exactly cloud liquid water path (LWP) and cloud fraction respond to aerosol perturbations. A volcanic eruption may help to better understand and quantify the cloud response to external perturbations, with a focus on the short-term cloud adjustments. The goal of the present study is to understand and quantify the response of clouds to a selected volcanic eruption and to thereby advance the fundamental understanding of the cloud response to external forcing. In this study we used the ICON (ICOsahedral Non-hydrostatic) model at numerical weather prediction setup  at a cloud-system-resolving resolution of 2.5 km horizontally, to simulate the region around the Holuhraun volcano for the duration of one week (1 – 7 September 2014). A pair of simulations, with and without the volcanic aerosol plume, allowed us to assess the simulated effective radiative forcing and its mechanisms, as well as its impact on adjustments of LWP and cloud fraction to the perturbations of Nd. In comparison to MODIS (Moderate Resolution Imaging Spectroradiometer) satellite retrievals, a clear enhancement of Nd due to the volcanic aerosol is detected and attributed. In contrast, no changes in either LWP or cloud fraction could be attributed. The on average almost unchanged LWP is a result of some LWP enhancement for thick, and decrease for thin clouds.

How to cite: Haghighatnasab, M., Kretzschmar, J., Block, K., and Quaas, J.: Impact of Holuhraun volcano aerosols on clouds in cloud-system resolving simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4783, https://doi.org/10.5194/egusphere-egu22-4783, 2022.

13:54–14:00
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EGU22-10341
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ECS
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Highlight
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On-site presentation
Clarissa Kroll et al.

Volcanic aerosol heating perturbs the tropical tropopause layer (TTL) and with it the stratospheric water budget. Whereas the effect of increased cold point temperatures on water slowly ascending into the TTL can be studied using general circulation models (GCM), the robustness of changes in convection after volcanic eruptions in these models is unclear as the TTL is tuned to unperturbed conditions and the simulations highly rely on parametrizations. Estimating the changes in the contributions of temperature effects, overshoots and vertical diffusion after a volcanic eruption or in a geoengineering study accordingly remains a challenge in GCM simulations. The emerging cloud resolving simulations however offer the unique possibility to gain insight into the sensitivity of the TTL to external forcings.

They allow in particular to study potential changes of convection, where two processes counteract each other after volcanic eruptions: the downwards shift of the lapse rate tropopause favoring overshooting convection in combination with increased stability in the TTL region suppressing overshooting convection.

We analyze these effects employing convection-resolving simulations for the atmosphere with the Icosahedral Nonhydrostatic Weather and Climate Model (ICON-A) at 10 km horizontal resolution in two scenarios: a control run and a volcanically perturbed run. The perturbed run has an aerosol layer in the lower stratosphere corresponding to the peak loading of an injection of 20 Tg sulfur using sea surface temperatures from the control run. In addition to a downwards shift of the lapse rate tropopause, we find that the level of neutral buoyancy, based on the temperature difference in convective areas and their surroundings, reaches the TTL more often in the volcanically perturbed simulations. This allows – contrary to previous assumptions - for more overshoots into the region above the tropical lapse rate tropopause.

How to cite: Kroll, C., Kornblueh, L., Dauhut, T., Schmidt, H., Timmreck, C., and Fueglistaler, S.: The volcanic impact on convection and stratospheric ice, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10341, https://doi.org/10.5194/egusphere-egu22-10341, 2022.

14:00–14:06
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EGU22-5862
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ECS
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Raphaël Lebrun et al.

Global or regional atmospheric circulation models often work with horizontal resolutions too large to be able to represent clouds, who have to be parameterized. The way clouds are parameterized and the way they overlap can have a significant impact on their radiative properties. A first objective of this work is to quantify the effect of a vertical description of cloud properties finer than that of current atmospheric models on the calculation of the radiative fluxes at the top of the atmosphere. A second objective is to propose a representation of these subgrid effects that is consistent with the representation of the cloud overlap between layers. For low-level clouds and using LES results as reference, we show the ability of the exponential-random overlap algorithm to represent the vertical distribution of the cloud fraction over a wide range of vertical scales that includes both subgrid scales and overlap between layers, with a constant value of the overlap parameter. Starting from a coarse vertical grid representative of that of atmospheric models, this algorithm is then used to construct the vertical profile of the cloud fraction with a much finer vertical resolution. This reconstruction allows us to test different simplifying hypotheses. We confirm that the frequently used maximum-random overlap leads to a significant error by underestimating the low-level clouds cover with a relative error of about 50%. We suggest some possible representations of subgrid effects and recommend to consider the vertical distribution of the cloud fraction seen from above, which depends on the volume cloud fraction but also on the cloud overlap and the subgrid vertical heterogeneity, when developing or evaluating cloud properties.

How to cite: Lebrun, R., Dufresne, J.-L., and Villefranque, N.: A consistent representation of cloud overlap and cloud subgrid vertical heterogeneity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5862, https://doi.org/10.5194/egusphere-egu22-5862, 2022.

14:06–14:12
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EGU22-5334
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ECS
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On-site presentation
Huan Liu et al.

Clouds play a critical role in the climate system by modifying incoming and outgoing radiation. In turn, clouds are affected by climatic changes. Cloud fraction (CF, the part of the sky covered by clouds) is one of the most reliable observed cloud properties and can be regarded as a good first approximation for cloud radiative effects.

Different types of clouds modify the energy budget and respond to climatic changes differently. Current climate models predict a negative trend in CF of shallow clouds (with low top height), which will further warm the climate by reducing the reflection of solar radiation. For other types of clouds, the predicted trend in CF has a net-zero radiative effect. The modeled results suffer from large variability and it contributes greatly to the uncertainty of climate projections. Therefore, observational constraints are badly needed.

Here, we divide the cloud records into three classes: total clouds (including all clouds), shallow clouds (with top pressure > 700 hPa), and free-atmosphere clouds (other than shallow). For each class, we decompose the general CF into two parts: one which is related to clouds’ horizontal size (CF under cloudy conditions) and a second part, related to the frequency of occurrence (the ratio of cloudy days to all days). 17 years (2003-2019) of satellite observations (MODIS aboard Aqua) and reanalysis data (ERA5) are used in this work. Satellite records show significant regional CF trends. They show that: (1) the trend in shallow clouds’ size dominates the trend in total CF, (2) there are opposite trends between the shallow and free-atmosphere clouds’ occurrence, and (3) the trend in shallow clouds’ occurrence compensate the trend in shallow clouds’ size and lead to a weak trend in shallow CF. It can indicate the development of shallow clouds into free-atmosphere clouds and it can relate to an overlapping problem, where MODIS cannot detect shallow clouds under other clouds. Reanalysis data reveals that after considering a correction for the overlapping problem of cloudy layers, the observed opposite trends (point no. 2 above) are still detected, but to a smaller extent, indicating that this relationship in MODIS records is impacted by the clouds overlapping problem. This means that when considering a correction for the overlapping problem, larger local trends in shallow clouds’ CF (point no. 3 above) are expected. Our findings provide new statistical relationships between clouds’ trends by high-quality observational records and shows that the overlapping problem biases systemically the trends in cloud properties.

How to cite: Liu, H., Koren, I., and Altaratz, O.: Observed Relationships between Cloud Fraction Trends of Shallow and Free-atmosphere Clouds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5334, https://doi.org/10.5194/egusphere-egu22-5334, 2022.

14:12–14:18
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EGU22-415
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ECS
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Simon Anghel et al.

Measuring the level of cloud coverage (CC) is important when analysing the absorption of solar radiation, the cloud formation process, and also to improve the forecast in general. This is becoming a more common task with the availability of digital sensors and their usage in monitoring the sky.

In this study we present a method of estimating the level of cloud coverage using Deep Learning (DL) applied to all-sky images of Meteorites Orbits Reconstruction by Optical Imaging (MOROI) network.

To label the data, a supervised validation was employed on the daytime images captured during the course of two years, recorded on Galati, Romania These were divided into three groups; CC <20%, CC between 20-80%, and CC >80%. Next, a set of DL models were trained, optimized and tested towards accurately classifying images according to each group.

We found that the classification accuracy can range between 89-95 % depending on how the cloud coverage is labeled and how the daytime is defined. This is mostly due to thin cirrus clouds, tricking the models, or poor sky illumination during sunrise or sunset. We discuss these methods and present a few strategies which circumvent classification problems, to further increase the accuracy of models.

The next step is to extend the study on the rest of the network, and also combining it with other sensors (e.g. satellite data) in order to understand the cloud-circulation coupling and its impact across climate models.

How to cite: Anghel, S., Voiculescu, M., Nedelcu, D.-A., and Birlan, M.: Cloud coverage estimation via deep learning applied to all-sky data in Romania : First Results, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-415, https://doi.org/10.5194/egusphere-egu22-415, 2022.

14:18–14:24
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EGU22-10593
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ECS
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On-site presentation
Michele Martinazzo et al.

In this study we investigate the level of accuracy of scaling methods, and analytical approximations, commonly used in fast radiative transfer routines of weather and climate models. Specifically, we focus on Chou’s approximation (Chou et al., 1999), and a simple scaling method based on the similarity principle. The former one is widely implemented in existing fast radiative codes to solve the radiative transfer problem in the infrared spectral region. At this regard, updated Chou backscatter parameters are computed based on realistic particle size distributions of liquid water and ice particles and by exploiting state of the art optical properties databases.

The assessment of the accuracy of approximate methodology all over the infrared spectrum is obtained by considering a widespread collection of atmospheric scenarios. Top of the atmosphere synthetic spectral radiances are computed for each scenario by considering alternatively an accurate and time-consuming methodology, such as the discrete ordinate solution (DISORT), or the approximate methodologies. The residuals are evaluated at far- and mid-infrared wavelengths and compared with the goal noise of the future 9th Earth Explorer FORUM satellite sensor (Palchetti et al., 2020). Results are discussed and analyzed in terms of geometrical, microphysical, and optical properties of the clouds layers (Martinazzo et al., 2021). In case of both water and ice cloud scenarios, the approximate solutions perform well in the mid infrared for most of the cases studied. When the far infrared region is considered, not negligible inaccuracies are observed.

To reduce the computational errors of basic scaling methods, a correction term is modelled and computed using the solution proposed by Tang et al. (2018) which assumes a downward radiance term not necessarily equal to the blackbody radiance, as it is done in Chou’s approximation. The Tang methodology, originally implemented for flux computations, is here adapted to the simulation of radiance fields, and refined by computing the appropriate multiplicative coefficients in the far and mid-infrared separately.  Results show that the range of validity of the new methodology is extended with respect to Chou's approximation and covers most of the cloud cases encountered in nature. It represents an accurate solution for the computation of radiance fields in presence of cirrus clouds which are one of the targets of the FORUM mission.

Finally, the whole set of radiative parameters needed to solve the radiative transfer equation using the Chou and Tang approximations is parametrized by mean of polynomial functions of the effective dimension of the cloud particle size distribution. The parametrized parameters are then implemented in the sigma-FORUM code a forward model designed for the fast calculation of radiance and its derivatives with respect to atmospheric and spectroscopic parameters of nadir-looking hyperspectral instruments. The σ-FORUM model is an updated version of sigma-IASI model (Amato et al., 2002), a monochromatic radiative transfer model based on a look-up table of optical depths parametrized as a polynomial concerning the atmospheric temperature and constituents. The strategy enables fast, accurate radiance and analytical derivatives calculations.

How to cite: Martinazzo, M., Maestri, T., Cossich, W., Serio, C., Masiello, G., and Venafra, S.: A New Cloud Properties Parameterization Implemented in the Fast Radiative Transfer Code sigma-FORUM, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10593, https://doi.org/10.5194/egusphere-egu22-10593, 2022.

14:24–14:30
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EGU22-2754
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ECS
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On-site presentation
Romain Pilon et al.

Saharan dust represents more than 50% of the total desert dust emitted around the globe and its radiative effect significantly affects the atmospheric circulation at a continental scale. Atmospheric models often fail to represent the dust vertical distribution and the Saharan Air Layer. They underestimate the effects of deep convection on the vertical transport and of the role of scavenging on the confinement of dust aerosols in this layer. Using multi-year simulations performed with a variable-resolution climate model and processed-based analysis, we show that scavenging in deep convection and further re-evaporation of dusty rainfall in the lower troposphere are critical processes for explaining the vertical distribution of desert dust. They play a key role in maintaining a well-defined dust layer with sharp transition at the top of the SAL and in establishing the seasonal cycle of dust distribution.

How to cite: Pilon, R., Senghor, H., Diallo, B., Escribano, J., Hourdin, F., Grandpeix, J.-Y., Boucher, O., Gaye, A. T., and Machu, E.: Saharan dust vertical distribution is controlled by convection and scavenging. Why do models miss this?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2754, https://doi.org/10.5194/egusphere-egu22-2754, 2022.

14:30–14:36
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EGU22-3657
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ECS
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On-site presentation
Ove Haugvaldstad et al.

Mineral dust aerosols play an important role in the Earth’s radiative budget. Dust aerosols interact with radiation in both the longwave and shortwave spectrum through direct radiative effects by absorption and scattering, and indirect effects through influencing cloud microphysical properties. Understanding dust-climate interactions are becoming increasingly important as effective air quality measures are reducing anthropogenic aerosol emissions and desertification in arid and semi-arid regions of the world are projected to increase in the face of climate change. 

In this work, we aim to better understand dust-climate interactions in the CMIP6 models by diagnosing the dust direct and indirect effects in 9 CMIP6 models participating in the piClim-2xdust experiment under AerChemMIP. The piClim-2xdust experiment doubles the dust emission in the model while keeping the other aerosols at pre-industrial levels. This means that any changes to the clouds and clear sky top of the atmosphere energy balance can be attributed to dust-cloud or dust-radiation interactions. We assess the robustness of the dust radiation and cloud response in the CMIP6 models and discuss the impact of differences in the representation of dust aerosols between the models.

How to cite: Haugvaldstad, O., Olivié, D., Schulz, M., and Storelvmo, T.: Decomposing the direct and indirect radiative effects by mineral dust aerosols in CMIP6, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3657, https://doi.org/10.5194/egusphere-egu22-3657, 2022.

14:36–14:50
Discussion

Thu, 26 May, 15:10–16:40

Chairpersons: Edward Gryspeerdt, Geeta Persad

15:10–15:20
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EGU22-8024
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ECS
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solicited
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On-site presentation
Julie Thérèse Pasquier et al.

The cloud radiative feedbacks are particularly complex and uncertain in the Arctic, which is a region of amplified warming. Within mixed-phase clouds (MPCs), the radiation fluxes are influenced by the thermodynamic phase and cloud particle concentrations. However, the processes responsible for ice crystal formation, particularly secondary ice production (SIP) processes, are still poorly understood.

We conducted in-situ cloud microphysical measurements in Ny-Ålesund, Svalbard, as part of the Ny-Ålesund AeroSol Cloud ExperimeNT campaign (NASCENT, Pasquier et al., BAMS, in revision). The main instrument used was a holographic cloud probe mounted on a tethered balloon system to image cloud particles. Additionally, ambient ice nucleating particles (INPs) and cloud condensation nuclei (CCN) were measured at ground level, remote sensing instruments (e.g. cloud radar) profiled the entire troposphere, and radiosondes were launched to determine the in-cloud temperature profiles.

Here we discuss the SIP occurrence in Arctic MPCs measured in autumn 2019 and spring 2020. We defined local SIP as occurring when the concentration of pristine ice crystals smaller than 100 µm in diameter is larger than the INP concentration. During the six days of measurements in MPCs, regions with local SIP were observed in 40% of the in-cloud measurements. Regions with high concentrations of small pristine ice crystals (>10 L-1) coincided with the presence of large frozen and broken drops, providing evidence for SIP during the freezing of drizzle drops. We suggest that the formation of drizzle drops initiating high SIP upon freezing was determined by the low CCN concentration in the pristine Arctic environment. Furthermore, SIP occurred at all observed temperatures (-24 °C to -2 °C). The frequency of SIP occurrence was highest in the temperature range between -24 °C and -18 °C (up to 96%), whereas the concentration of small pristine ice crystal peaked between -5 °C and -3 °C (up to 95 L-1). Our observations demonstrate the high importance of SIP for ice crystal formation in Arctic MPCs over a larger temperature range. Thereby, SIP influences the radiative properties of Arctic MPCs and hence the surface radiative energy budget.

How to cite: Pasquier, J. T., Ramelli, F., David, R. O., Wieder, J., Li, G., Lauber, A., Lohmann, U., Carlsen, T., Gierens, R., Maturilli, M., and Henneberger, J.: Importance of secondary ice production over a large temperature range in Arctic mixed-phase clouds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8024, https://doi.org/10.5194/egusphere-egu22-8024, 2022.

15:20–15:26
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EGU22-6344
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ECS
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Highlight
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Virtual presentation
Lucas Sterzinger et al.

Aerosol concentrations in the Arctic can get quite low, and recent work has shown that low aerosol concentrations could affect Arctic cloud formation and structure. Arctic mixed-phase clouds have been observed to persist for days at a time and dissipate suddenly, and it has been hypothesized that some instances of cloud dissipation are caused by aerosol concentrations falling below some critical value required to sustain the cloud.

We found three cases - from a Department of Energy ARM site on the north slope of Alaska, the ICECAPS-ACE project at the NSF Summit Station in Greenland, and the ASCOS field campaign - where clouds are observed to dissipate coincidentally with a drop of surface aerosol concentration. These cases were used to initialize idealized large eddy simulations in which aerosol concentrations were held constant at observed values before being immediately removed. The resulting simulations are considered to be the fastest possible aerosol-limited dissipation. Comparing simulated liquid water path (LWP) to observations, we find that the ARM case dissipated much faster than our simulations, indicating that the observed dissipation was not driven by lack of available aerosol. The Summit Station and ASCOS simulations dissipate (with respect to LWP) at approximately the same rate as observations, which suggests aerosol-limited dissipation may indeed be occurring in these cases.

Furthermore, we find that the microphysical response to aerosol removal varies between the specific cases we simulate. Simulations where the cloud produces constant liquid drizzle dissipate, within 3-4 hours,  via an acceleration of precipitation once aerosols are removed. Conversely, the case with a non-precipitating liquid layer dissipates more quickly (< 2 hours), possibly by glaciation via the Wegener-Bergeron-Findeisen (WBF) in which ice grows and precipitates at the expense of liquid droplets. The simulations suggest that aerosol-limited dissipation in the Arctic is plausible, and we present two microphysical pathways by which this dissipation can occur.

How to cite: Sterzinger, L., Sedlar, J., Guy, H., Neely III, R., and Igel, A.: Arctic Mixed-Phase Clouds Sometimes Dissipate Due to Insufficient Aerosol - Evidence from Idealized Large Eddy Simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6344, https://doi.org/10.5194/egusphere-egu22-6344, 2022.

15:26–15:32
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EGU22-5547
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Virtual presentation
Paul Zieger et al.

The physical and chemical properties of aerosol particles are important for the formation of cloud droplets and ice crystals. This is especially true for pristine regions such as the Arctic, where particle number concentrations are often very low. Observations from these regions are still sparse due to the technical challenges involved.

Here, we present recent results of detailed in-situ observations of aerosols and clouds performed on board the Swedish icebreaker Oden over the central Arctic Ocean in 2018. We show that Aitken-mode particles, i.e. particles below 70 nm diameter, contribute significantly to cloud-forming particles (here termed cloud residuals), especially towards autumn with the start of the freeze-up of the sea ice. These cloud-forming Aitken-mode particles coincided with air that spent more time over the ice, while accumulation-mode dominated cloud residuals showed more of an oceanic influence, as shown using air back trajectory analysis. At the same time, the Aitken-mode dominated cloud residuals were associated with changes in the average chemical composition of the accumulation mode showing an increased organic contribution, in contrast to the accumulation-mode dominated cloud residuals, which showed an increased sulfate contribution. The Hoppel-minima in both whole-air and cloud residual size distributions was almost unchanged, suggesting only little addition of aerosol mass due to aqueous-phase cloud processing. Our highly detailed observations of aerosol-cloud interactions over the central Arctic Ocean close to the North Pole provide valuable insights into the properties and the origin of particles that are relevant for cloud formation in this remote region of our planet.

This work is currently in review at the Journal of Geophysical Research (JGR).

How to cite: Zieger, P., Karlsson, L., Baccarini, A., Duplessis, P., Baumgardner, D., Brooks, I. M., Chang, R., Dada, L., Dällenbach, K. R., Krejci, R., Leaitch, W. R., Leck, C., Salter, M. E., Wernli, H., Wheeler, M. J., and Schmale, J.: Aerosol-cloud interactions over the central Arctic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5547, https://doi.org/10.5194/egusphere-egu22-5547, 2022.

15:32–15:38
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EGU22-2956
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ECS
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Highlight
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On-site presentation
Rebecca Murray-Watson and Ed Gryspeerdt

The response of cloud properties to aerosol perturbations is one of the greatest uncertainties associated with anthropogenic climate forcing. Clouds play a central role in the Arctic surface energy budget, so understanding how aerosols influence their radiative properties is important for projecting future changes in the region. This is especially relevant as increases in temperature and reductions in sea ice extent allow for more industrial activity in the region, thereby introducing fresh sources of aerosol.  

As it is difficult to retrieve reliable satellite observations in polar regions due to issues such as high solar zenith angles, the impact of aerosols on Arctic clouds is particularly uncertain. However, by carefully filtering the satellite data to remove cases associated with retrieval biases, this work uses multiple satellite and reanalysis datasets to develop new constraints for the influence of aerosols on cloud properties.

The factors which influence the droplet number concentration-liquid water path (Nd-LWP) relationship, a key component of the aerosol-liquid water path relationship, are investigated. The Nd-LWP relationship varies significantly geographically, with increases in LWP with Nd observed at high latitudes. A range of meteorological factors are investigated, and it is shown that the lower tropospheric stability (LTS) is the driving force behind the variability in the Nd-LWP relationship in the Arctic. At high stability, the relationship is significantly more positive, producing LWP increases in more polluted environments, in contrast to the response of clouds to aerosol perturbations seen at lower latitudes.

As the Arctic warms, the boundary layer stability is projected to decrease. Additionally, industrial activity is expected to increase in the region, which may increase the aerosol burden. When considered individually, these two effects would lead to increases in LWP in marine Arctic clouds. However, when working together they may produce clouds with lower water paths, leading to a weaker negative cloud feedback in a more polluted environment. 

How to cite: Murray-Watson, R. and Gryspeerdt, E.: Stability dependent increases in liquid water with droplet number in the Arctic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2956, https://doi.org/10.5194/egusphere-egu22-2956, 2022.

15:38–15:44
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EGU22-3796
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Highlight
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Virtual presentation
Heike Wex et al.

Ice nucleating particles (INPs) are rare but important atmospheric particles as they induce the formation of primary ice in mixed phase clouds and also in some cirrus clouds. A plethora of substances which can be found in atmospheric particles can induce ice nucleation. The most important ice active particle types in the atmosphere are assumed to be mineral dust and biological particles, which can originate from a large number of sources. It is hence not surprising that INP concentrations vary over several orders of magnitude at any ice nucleation temperature, with concentrations being typically larger in continental than in marine environments. Although research concerning INP and their global occurrence has seen a steep rise in the past years, global INP concentrations are still not well known, and not all important INP sources are clear, neither are there good parameterizations for describing INP concentrations in models.

To increase the knowledge of typical global INP concentrations and to draw conclusions about INP sources, we examined INP concentrations in the Antarctic region, namely at the German research Station Neumayer III, located at shelf ice in close proximity (only some 10 km) to the ice edge, at the Belgian research Station Princess Elisabeth, located roughly 200 km inland and at an altitude of 1390 m, and during a campaign including ship- and land-based data at the Antarctic peninsula. We used our well-established methods of filter collection and off-line analysis with cold-stages to derive INP concentrations in these locations.

For Neumayer, two years of data are available. INP concentrations there were generally lower than values derived, e.g., for northern mid latitudes, and they were similar to values published for the Southern Ocean in literature. No pronounced annual cycle was observed. Around and on the Antarctic peninsula, INP concentrations were roughly similar to those observed at Neumayer. However, the Princess Elisabeth station, for which only data obtained during two austral summers are available, showed the lowest values detected in this study. Our results suggest that sources of INP in the Antarctic region are rare, and particularly so on Antarctica itself. 

How to cite: Wex, H., Henning, S., Mangold, A., Van Overmeiren, P., Zeppenfeld, S., van Pinxteren, M., Herrmann, H., Dallosto, M., and Stratmann, F.: Ice Nucleating Particles in the Antarctic region, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3796, https://doi.org/10.5194/egusphere-egu22-3796, 2022.

15:44–15:50
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EGU22-8066
Anna Possner et al.

Extratropical low-level mixed-phase clouds are difficult to represent in global climate models and generate substantial uncertainty in global climate projections (Zelinka et al. 2020). In this study we evaluate the simulated properties of Southern Ocean (SO) boundary layer mixed-phase clouds for August 2016 in the ICOsahedral Nonhydrostatic (ICON) model (Zängl et al. 2015). The bulk of the simulations are part of the DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domain (DYAMOND) initiative (Stevens et al. 2020). Within DYAMOND, ICON was run with the German Weather Service (DWD) physics packages at resolutions ranging from the global climate scale to the convection-permitting scale. All simulations are evaluated with respect to their radiative and cloud properties using Clouds and the Earth’s Radiant Energy System (CERES, Su et al. 2015), Moderate Resolution Imaging Spectroradiometer (MODIS), and the raDAR-liDAR (DARDAR) version 2 (Ceccaldi et al. 2013) retrievals.

The analysis shows that previous and current versions of ICON overestimate cloud ice occurrence in low-level clouds across all latitudes in the SO. Furthermore, ICON, like many other global climate models, underestimates the reflectivity of SO boundary layer clouds. We can show that this effect is resolution dependent and largely due to an underestimation in cloud occurrence, rather than optical depth. Additional sensitivity experiments with respect to temporal model resolution and convection scheme assumptions were performed. We find a stronger model sensitivity with respect to spatial versus temporal resolution. Assumptions made within the convection scheme with respect to detrained cloud ice were found to impact the simulated total ice water content, but had a marginal impact on cloud-radiative properties.

In summary, while increases in model resolution increase cloud water content, cloud occurrence and cloud optical depth, considerable radiative biases remain in SO clouds in ICON at the convection-permitting scale. Furthermore, cloud ice forms to readily in state-of-the-art DWD ICON simulations at all spatio-temporal resolutions analysed.

How to cite: Possner, A., Danker, J., and Ramadoss, V.: Resolution Dependence of Southern Ocean Mixed-Phase Clouds in ICON, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8066, https://doi.org/10.5194/egusphere-egu22-8066, 2022.

15:50–15:56
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EGU22-8037
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ECS
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On-site presentation
Franziska Hellmuth et al.

Cloud feedbacks are a major contributor to the spread of climate sensitivity in global climate models (GCMs) [1]. Among the most poorly understood cloud feedbacks is the one associated with the cloud phase, which is expected to be modified with climate change [2]. Cloud phase bias, in addition, has significant implications for the simulation of radiative properties and glacier and ice sheet mass balances in climate models. 

In this context, this work aims to expand our knowledge on how the representation of the cloud phase affects snow formation in GCMs. Better understanding this aspect is necessary to develop climate models further and improve future climate predictions. 

This study aims to improve the understanding of the link between the representation of cloud phase and surface snowfall in historical simulations, comparing them to a combination of satellite remote sensing and reanalysis data. We use the cloud and snowfall products from CloudSat satellite and the European Centre for Medium-Range Weather Forecast Re-Analysis 5 (ERA5), producing a global surface snowfall rate climatology for each dataset. 

We compare the outputs from the Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models to the snowfall rate climatology produced by CloudSat and ERA5. Comparing historical simulations from climate models with CloudSat will relate the already identified cloud phase biases with snowfall biases in specific regions for the past decade. Statistical analysis is carried out to determine cloud phase and precipitation (liquid and solid) biases and their potential connection to each other in the climate models. 

The results show that, globally, CMIP6 models can reproduce some of the characteristics of liquid water content and snowfall, as seen in ERA5. In addition, a comparison between ERA5 and the CMIP6 models shows that CMIP6 models underestimate the ice water path. Then, we look at the regional differences of ice water path, liquid water path, and surface snowfall to better understand how the individual CMIP6 models perform in different regions—showing an overestimation of ice water path in the southern and northern hemisphere extratropics. At the same time, surface snowfall is within the ERA5 standard deviation. When comparing the ERA5 ice water path to surface snowfall, we identify a positive relationship between the two, independent of the latitudes. On the other hand, the CMIP6 ensemble mean or CMIP6 models individually do not indicate a positive relationship. The next step of this ongoing research is to investigate the relationship between cloud phase (ice, liquid water path) and surface snowfall rate based on CloudSat retrievals. 

 

[1] Zelinka, M. D., Myers, T. A., McCoy, D. T., Po-Chedley, S., Caldwell, P. M., Ceppi, P., et al. (2020). Causes of higher climate sensitivity in CMIP6 models. Geophysical Research Letters, 47, e2019GL085782. https://doi-org.ezproxy.uio.no/10.1029/2019GL085782 

[2] Bjordal, J., Storelvmo, T., Alterskjær, K. et al. Equilibrium climate sensitivity above 5 °C plausible due to state-dependent cloud feedback. Nat. Geosci. 13, 718–721 (2020). https://doi-org.ezproxy.uio.no/10.1038/s41561-020-00649-1 

How to cite: Hellmuth, F., Storelvmo, T., and Daloz, A. S.: Assessment of surface snowfall rates and their connection to cloud microphysics in reanalysis data and global climate models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8037, https://doi.org/10.5194/egusphere-egu22-8037, 2022.

15:56–16:02
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EGU22-8098
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ECS
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On-site presentation
Huiying Zhang et al.

Ice crystals are an important component of clouds due to their strong impact on cloud radiative properties and precipitation formation. The shape of an ice crystal impacts its radiative effect, riming efficiency, and fall speed.  The ice crystal shapes are dependent on (i) the environment (temperature and humidity) that they grow in and (ii) the microphysical processes (i.e. riming, aggregation) that they have experienced. These connections offer a great opportunity to trace back the previous in-cloud conditions and the microphysical processes in clouds. Thus, ice crystal shape classification is crucial to better understand radiation properties and precipitation formation of clouds.

Scientists have explored and developed various algorithms to automatically classify ice crystal shapes in the past decades. Among those, the machine learning algorithm Convolutional Neural Network (CNN), shows a good performance due to its ability to catch the main features that describe ice crystal habits and recognize patterns between images. However, the existing classification methods all show an overlap between physical process categories (i.e. rimed) and basic habit categories (i.e. column) of ice crystals due to the existence of compound ice (i.e. column-rimed), especially in situations conducive to light riming.

A CNN was trained using over 10’000 images of pristine and complex ice crystals recorded by a holographic imager during the NASCENT campaign  (Pasquier et al., BAMS, in revision) in Fall 2019, in Ny-Ålesund, Svalbard. To avoid the overlap between physical process and basic habit categories of ice crystals, each ice label contains two properties; one of 7 basic habits property (i.e. column) and up to 3 physical process properties (aggregate, rimed, aged). The trained model gives us both the basic habit and the physical processes information, which helps us to better understand the microphysical processes in clouds.

How to cite: Zhang, H., Binder, A., Pasquier, J., Krummenacher, B., Ramelli, F., Storelvmo, T., David, R. O., and Henneberger, J.: Deep-learning based classification of ice crystals: habits and microphysical processes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8098, https://doi.org/10.5194/egusphere-egu22-8098, 2022.

16:02–16:08
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EGU22-10270
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ECS
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Virtual presentation
Mohit Singh et al.

Recent observations in the field measurements and laboratory studies suggest that organic aerosol particles may exist as highly viscous semi-solids or amorphous glassy solids under certain conditions, with important implications for atmospheric chemistry, climate, and air quality. A number of complementary techniques have been developed to probe the viscosity of aerosol particles in the last ten years. However, none of the available techniques is sufficiently versatile to determine aerosol viscosity at atmospherically relevant conditions for a range of particle sizes, chemical compositions, and sample sizes. Here we present a novel way to measure the viscosity of levitated droplets suspended in an electrodynamic balance under atmospheric conditions. Capillary oscillations are induced in a levitated droplet by the application of an external AC field. These oscillations are monitored using a high-speed camera or light scattering, and the viscosity is determined by fitting these oscillations using a suitable theoretical model. The model is based on the asymptotic analysis of surface oscillations of a charged drop, carried out using the viscous-potential flow theory.

How to cite: Singh, M., Jones, S., Duft, D., Kiselev, A., and Leisner, T.: Measurement of viscosity at low temperature from resonating droplet levitated in an electrodynamic balance, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10270, https://doi.org/10.5194/egusphere-egu22-10270, 2022.

16:08–16:14
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EGU22-4368
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On-site presentation
Annele Virtanen et al.

The susceptibility of cloud droplet number concentration (CDNC) to cloud condensation nuclei (CCN) number concentration is one of the major factors controlling the aerosol indirect forcing. In this study we investigate the sensitivity of CDNC to CCN concentrations using long term in-situ observations from three stations (Puijo, Pallas, Zeppelin) locating in Finland and Arctic. These stations represent semiurban, remote and Arctic remote environments with differences in typical updraft velocity conditions as well as in aerosol number concentrations. We compare the in-situ observations with three large scale models (ECHAM-M7, ECHAM-SALSA and NorESM) having differences in aerosol presentation while the activation parametrization is the same in all three model setups. In the comparison we use CDNC and CCN model outputs of the gridbox corresponding to the location and the height for each station. In addition, we compare the updraft velocities from the models and stations when they are available. Our current observational results show very high susceptibility of CDNC and CCN in all investigated stations. The agreement between the large scale models and observations was very good for Puijo and Pallas stations, but for the Arctic station (Zeppelin) the modelled CDNC susceptibility to CCN was much lower than the observed. This might be related to the recent results demonstrating that Aitken mode particles can active to cloud droplets at Zeppelin station (Bulatovic et al., 2021; Karlsson et al., 2021). In addition, at Zeppelin CDNC exhibits very low values which are below the lower bound imposed by ECHAM.

References:

Bulatovic, I., Igel, A. L., Leck, C., Heintzenberg, J., Riipinen, I., and Ekman, A. M. L.: The importance of Aitken mode aerosol particles for cloud sustenance in the summertime high Arctic – a simulation study supported by observational data, Atmos. Chem. Phys., 21, 3871–3897, https://doi.org/10.5194/acp-21-3871-2021, 2021.

Karlsson, L., Krejci, R., Koike, M., Ebell, K., and Zieger, P.: A long-term study of cloud residuals from low-level Arctic clouds, Atmos. Chem. Phys., 21, 8933–8959, 2021.

How to cite: Virtanen, A., Joutsensaari, J., Kokkola, H., Seland, Ø., Zieger, P., Karlsson, L., Riipinen, I., Krejci, R., Hyvärinen, A., Lihavainen, H., and Romakkaniemi, S.: Cloud droplet number susceptibility to CCN concentrations in low level boundary layer clouds: comparison of in-situ observations and large-scale models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4368, https://doi.org/10.5194/egusphere-egu22-4368, 2022.

16:14–16:20
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EGU22-4978
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ECS
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On-site presentation
Matthias Schwarz et al.

As large-eddy simulations (LES), which explicitly simulate aerosol-cloud interactions, are often considered as benchmark simulations in climate science, it is necessary to critically evaluate if these high-resolution models can skillfully represent expected physical phenomena.

Here, we focus on the first aerosol indirect aerosol effect in a warm stratocumulus cloud. We investigate if the MIMICA LES (Savre et al., 2014) with a widely used bulk two-moment microphysical scheme (Seifert and Beheng, 2006) can reproduce the susceptibility regimes identified by Reutter et al., (2009). Using a parcel model, Reutter et al. (2009) showed that the cloud droplet number (Nd) responds differently to an increase in aerosol number (Na) depending on ambient updraft strength (w). In the aerosol-limited regime, enough supersaturation can be generated by the updraft motions in the atmosphere so that increasing Na leads to an increase in Nd. Conversely, in the updraft-limited regime, adding aerosol will not increase as activation is limited by the updraft strength and only increasing w will lead to an increase in Na.

In the standard setup, the LES cannot simulate the transition from the aerosol- to the updraft-limited regime. Only when implementing a renormalization procedure following Reisin et al., (1996) and, at the same time, increasing the initial droplet radius of newly activated droplets (rdi) to values large than rdi>1µm, a regime transition emerges. However, a clear recommendation for the choice of rdi cannot be made upon physical arguments at this point. Interestingly, the “arbitrarily chosen” droplet mass by Seifert and Beheng (2006) of 1*10-12kg, which corresponds to rdi≈6µm, seems to agree quite well with the expectations from parcel model simulations. The choice is, however, still arbitrary and therefore physically questionable.

A potential way to avoid this problem, which mainly occurs at high aerosol concentrations, would be to run the LES with a small enough temporal resolution (Δt≈0.1s) to explicitly resolve all relevant microphysical processes.

 

References

Reisin, T., Levin, Z., Tzivion, S., 1996. Rain Production in Convective Clouds As Simulated in an Axisymmetric Model with Detailed Microphysics. Part I: Description of the Model. Journal of Atmospheric Sciences 53, 497–520.

Reutter, P., Su, H., Trentmann, J., Simmel, M., Rose, D., Gunthe, S.S., Wernli, H., Andreae, M.O., Pöschl, U., 2009. Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN). Atmospheric Chemistry and Physics 9, 7067–7080.

Savre, J., Ekman, A.M.L., Svensson, G., 2014. Technical note: Introduction to MIMICA, a large-eddy simulation solver for cloudy planetary boundary layers. Journal of Advances in Modeling Earth Systems 6, 630–649.

Seifert, A., Beheng, K.D., 2006. A two-moment cloud microphysics parameterization for mixed-phase clouds. Part 1: Model description. Meteorol. Atmos. Phys. 92, 45–66.

How to cite: Schwarz, M., Savre, J., Sudhakar, D., Quaas, J., and Ekman, A. M. L.: Modeling the transition from the aerosol- to the updraft-limited cloud droplet susceptibility regime in large-eddy simulations with bulk microphysics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4978, https://doi.org/10.5194/egusphere-egu22-4978, 2022.

16:20–16:26
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EGU22-13190
Wojciech W. Grabowski et al.

Scaled-up DNS and implicit LES simulations are used to study turbulent cloud base CCN activation and early growth of cloud droplets. The simulation framework includes a triply periodic computational domain ~1,000 cubic meters filled with inertial-range homogeneous isotropic turbulence. The domain experiences decrease of the mean air temperature and reduction of the mean pressure, both mimicking the rise of an adiabatic air parcel through the cloud base. Results of turbulent simulations are compared to CCN activation and droplet growth within a classical nonturbulent rising parcel. The key difference is a blurriness of the separation between activated and nonactivated (haze) CCN, especially for weak mean ascent rates, when CCN activate and subsequently some deactivate instead of becoming cloud droplets above the cloud base. This leads to significantly larger spectral widths in turbulent parcel simulations compared to the adiabatic nonturbulent parcel once CCN activation is completed. Further increase of the spectral width in the turbulent parcel is similar to that for the initially-monodisperse droplets in the inertial-range homogeneous isotropic turbulence that we and others studied previously, with the standard deviation of the radius squared increasing approximately as the square root of time. This contrasts with the classical nonturbulent parcel framework for which the radius squared standard deviation above the cloud base remains constant because of the parabolic growth of cloud droplets once surface tension and dilute effects can be neglected.

How to cite: Grabowski, W. W., Thomas, L., and Kumar, B.: Impact of turbulence on CCN activation and early growth of cloud droplets , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13190, https://doi.org/10.5194/egusphere-egu22-13190, 2022.

16:26–16:32
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EGU22-13218
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ECS
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Highlight
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On-site presentation
Miguel Garrido Zornoza et al.

On Earth, atmospheric instabilities drive the aerosolization of distinct microorganisms directly from the biosphere. Particle diameters of these (bio)aerosols typically range from ∼1nm to ∼100μm, and their residence times in the atmosphere have been suggested to range from a few days to weeks or even months. Found even beyond the troposphere, they have been shown to, in some cases, be metabolically active as well as involved in cloud processing mechanisms, depending on their surface properties, by acting as cloud condensation nuclei (CCN) or ice nuclei (IN), and thus effectively changing cloud life time and optical properties via a change in the droplet size distribution within the cloud.

Even though the immediate sample we can observe from the Solar System already suggests a wide variety of planetary climates and that any particular planetary atmosphere exhibits a high degree of complexity, it is reasonable to assume that a finite set of physical and chemical processes are the major agents governing the climate. One of these processes is the phase change of volatiles in the atmosphere and its inhomogeneous impact on the radiation budget of the planet, altering the radiatively-induced temperature gradients and thus the general circulation of the atmosphere.

With the help of a Global Climate Model (GCM), we will study the radiative forcing caused by selected bioaerosols, under different hypothesized behaviours, in a simplified model atmosphere representing a plausible exoplanetary environment. In the end, a synthetic spectrum will be generated and compared to a “bioaerosol-free’’ sample in search for discrepancies that might, or not, be regarded as biosignatures. Furthermore, with the launch of the James Webb space telescope towards L2 we shall have access to new empirical data of exoplanetary atmospheres that constitute the ideal playground for this way of hypothesis testing. 

How to cite: Garrido Zornoza, M., Mitarai, N., and Haerter, J. O.: Observational implications on the presence of selected bioaerosols in exoplanetary atmospheres, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13218, https://doi.org/10.5194/egusphere-egu22-13218, 2022.

16:32–16:38
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EGU22-3391
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On-site presentation
Diana Francis et al.

This study highlights the occurrence of atmospheric rivers (ARs) over northwest Africa towards Europe, which were accompanied by intense episodes of Saharan dust transport all the way to Scandinavia, in the winter season. Using a combination of observational and reanalysis data, we investigate two extreme dusty AR events in February 2021 and assess their impact on snow melt in the Alps. The warm, moist, and dusty air mass (spatially-averaged 2-meter temperature and water vapour mixing ratio anomalies of up to 8 K and 3 g kg−1, and aerosol optical depths and dust loadings of up to 0.85 and 11 g m−2, respectively) led to a 50% and 40% decrease in snow depth and surface albedo, respectively, in less than one month during the winter season. ARs over northwest Africa show increasing trends over the past 4 decades, with 78% of AR events associated with severe dust episodes over Europe. 
This study was published recently in Atmospheric Research (https://doi.org/10.1016/j.atmosres.2021.105959).

How to cite: Francis, D., Fonseca, R., Nelli, N., Bozkurt, D., Picard, G., and Guan, B.: Atmospheric rivers drive exceptional Saharan dust transport towards Europe and subsequent snow melt in the Alps., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3391, https://doi.org/10.5194/egusphere-egu22-3391, 2022.

16:38–16:40
Discussion