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AS1.12

Atmospheric Convection

This session deals with atmospheric convection, being dry, shallow, or deep convection. Contributions on these aspects resulting from the use of large-eddy simulations, convection-permitting simulations, coarser-resolution simulations using parameterised convection and observations are welcome. Studies that investigate the organization of convection, being in idealized set-ups (radiative convective equilibrium and self-aggregation) or in observations, as well as studies that investigate the importance of organization for climate are particularly welcome. Besides this, studies that investigate general aspects of convection such as processes controlling the lifecycle of convection, interactions of convection with other physical processes and representation of convection in numerical weather prediction and climate models, being for instance through the use of machine learning techniques, are also welcome.

Convener: Cathy Hohenegger | Co-conveners: Leo Donner, Adrian Tompkins, Holger Tost
Presentations
| Mon, 23 May, 15:10–18:20 (CEST)
 
Room 0.11/12

Mon, 23 May, 15:10–16:40

Chairpersons: Leo Donner, Holger Tost

15:10–15:17
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EGU22-778
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ECS
Beth Dingley et al.

The self-aggregation of convection in idealised models has been widely studied. Work has been done to identify key physical mechanisms responsible for both driving and maintaining aggregation in a range of idealised radiative-convective equilibrium (RCE) models. These idealised models are typically run without any land, rotation, variation in sea-surface temperatures (SSTs), or a diurnal cycle. Due to these idealisations, a key question in the study of convective aggregation is how these convective processes and mechanisms manifest in the real-world. Several studies have tried to tackle this question by increasing the complexity of processes in the idealised models, such as SST gradients, adding a slab ocean, adding a diurnal cycle, or adding an aerosol diabatic heating perturbation. Particularly, the inclusion of interactive ocean surfaces has been shown to strongly impact the formation of aggregated clusters.

The interactions between land surfaces and aggregation are currently less well understood. Early studies have found that convective aggregation may favour land areas over oceans, and that soil moisture feedbacks can act to oppose the aggregation altogether. Thus, in this study we investigate the relationship between land, oceans, and aggregation, addressing the following questions:

  • How does the inclusion of an idealised island into a global RCE model impact the aggregation of convection?
  • Are the physical mechanisms responsible for the aggregation similar to those seen in land-free simulations?
  • How sensitive are these results to our choice of land parameters, such as initial soil moisture, surface temperature, soil type, and land topography?

How to cite: Dingley, B., Dagan, G., Herbert, R., and Stier, P.: The impact of land-sea contrasts in the aggregation of convection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-778, https://doi.org/10.5194/egusphere-egu22-778, 2022.

15:17–15:24
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EGU22-1898
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ECS
Michael Baidu et al.

Vertical wind shear plays a key role in the organisation and intensification of Mesoscale Convective Systems (MCSs). In West Africa, the meridional temperature gradient between the hot Sahel and the humid Gulf of Guinea results in a strong easterly wind at mid-levels and south-westerlies at low-levels leading to a strong vertical wind shear. A decadal increase in vertical wind shear has recently been linked to a decadal increase in intense MCSs over the Sahel. Here, the effects of vertical wind shear on MCSs over West Africa have been investigated using a 10-year (1998 - 2007) MCS dataset. The results show that, a strong vertical wind shear is associated with long-lived, fast moving, large and cold (deep) storms with high rain-rates.  The observed cloud-top heights of storms over the oceans are closer to their level of neutral buoyancies (LNBs) compared to their land counterparts. We hypothesise that this is due to greater entrainment dilution over land compared to storms over the ocean. The difference between the observed cloud-top heights and the LNBs of land MCSs is minimised over regions of high vertical wind shear. Strong vertical wind shear results in colder brightness temperatures relative to the temperature at their LNBs. This is consistent with recent modelling work showing that shear reduces  entrainment dilution of squall-line updrafts. We conclude that, modelling the impacts of vertical shear, which are normally missed in convection parameterisations, are not only important for predictions of high impact weather, but also important for modelling the climatology of anvil heights.

How to cite: Baidu, M., Schwendike, J., Marsham, J., and Bain, C.: Observed Effects of Vertical wind shear on the intensities of Mesoscale Convective Systems in West Africa., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1898, https://doi.org/10.5194/egusphere-egu22-1898, 2022.

15:24–15:31
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EGU22-2410
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ECS
Edward Groot and Holger Tost

Upper tropospheric (UT) divergence is potentially an important mediator between convective scale error growth and advective/non-linear large scale error growth at jet stream scales (Baumgart et al. 2019). To investigate possible mechanistic links of error growth from small convective scales to the synoptic scales, but also to gain insight in convective processes and their (representation) uncertainty, we have compared UT divergence in an array of idealized LES-simulations of convective systems with different degree of organisation.

Using ensemble and physics perturbations, we have found that isolated convective systems roughly seem to obey the expected near-linear relationship between latent heating and mass divergence, but squall lines are found to be anomalous in this sense. At the same time, large intrinsic variability among squall lines with extremely similar initial conditions and unperturbed physics is explored in much detail. A link between the squall line anomaly and amount recirculated updraft air into the cold pool and its possible role in latent heat consumption is tested. Furthermore, the origin of squall line variability is investigated in depth.

How to cite: Groot, E. and Tost, H.: Squall line sensitivity in LES simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2410, https://doi.org/10.5194/egusphere-egu22-2410, 2022.

15:31–15:38
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EGU22-2876
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ECS
Cristian Vraciu

In this work, we present an alternative theory for the parameterization of atmospheric convection based on plumes with a realistic description. The convective source terms, which provide the feedback of the convection upon the large-scale variables are obtained by performing a sub-grid averaging, considering that the convective variables are fluctuations from the mean, large-scale, state. In this way, we do not need to consider that the large-scale is described by the environmental state, which means that we do not need to consider that the fractional area occupied by the convection is very small. Thus, our formulation can be implemented in numerical weather prediction (NWP) and climate models with high resolutions, in which case a stochastic representation for the fractional area occupied by the convection in every grid box must be considered. In our parameterization, the convective variables in each grid box are described by one round steady-state convective plume placed in the center on the box with radial profiles which respect the condition that the convective variables represent fluctuations from the mean state. Performing an integral analysis over the plume's domain in the radial direction, we obtain a system of ordinarily differential equations (ODE) which take into consideration both the buoyancy-driven and the turbulent entrainment, offering thus a more accurate description of the convective dynamics then the classic entrainment hypothesis. Moreover, the influence of the large-scale is taken into consideration at any height, not just at the cloud base as in the standard mass-flux formulation. The system of ODE that we obtain can be analytically solved between the vertical grids of the NWP or climate models if we consider that the plume radius is constant and we use the scaling argument that the buoyancy-driven entrainment scales with the inverse of the height. The radial coefficients which result from the radial integration of the plume can be obtained by prescribing the exact form of the radial profiles of the vertical velocity, scalar components and turbulent fluxes, or determined using large-eddy simulations. The closure of our model consists in prescribing the vertical velocity and the radius of the plume at the initial level, which can be considered to follow a given probability density function as in the existing stochastic parameterizations.

How to cite: Vraciu, C.: On the parameterization of atmospheric convection with a realistic plume model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2876, https://doi.org/10.5194/egusphere-egu22-2876, 2022.

15:38–15:45
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EGU22-2883
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ECS
Mostafa Hamouda and Bodo Ahrens

Recent studies have focused on the relationship between global warming and extreme precipitation events. It is consensed that the risk of flooding is increasing due to global warming, since warmer air temperatures accommodates more moisture content according to Clausius-Clapyeron relation. One of the major flood sources is known as Vb cyclones, i.e. cyclones travelling through the Mediterranean then moving northwards on the eastern flank of the Alps towards central Europe. In this study, a special focus is shed on the convection process during major Vb events. Using a convection tracking method (Purr et al. 2021) and mid-tropospheric vertical velocity and vorticity method (Poujol et al.2019) on convective-permitting simulations (3km resolution) driven by ERA5 reanalysis data, the results show that at least one third of the total amount of rainfall is produced by convection. Moreover, the diurnal cycle is found to contribute to enhancing the convective fraction, as the surface becomes warm in the afternoon, setting up suitable conditions for convection to occur. Both methods show similar patterns and comparable amplitudes. The added value of using such a computationally expensive simulation is also investigated, by comparing the results from the convection-permitting simulations to a lower resolution (11 km) downscaling with parameterized convection. Using Poujol et al. (2019) method, the the results do not show a completely accurate rainfall enhancement due to the diurnal cycle; however a comparable fraction due to convection during a Vb event is identified.

 

Purr, C, Brisson, E, Ahrens, B. Convective rain cell characteristics and scaling in climate projections for Germany. Int J Climatol. 2021; 41: 3174– 3185. https://doi.org/10.1002/joc.7012

Poujol, B, Sobolowski, S, Mooney, P, Berthou, S. A physically based precipitation separation algorithm for convection-permitting models over complex topography. Q J R Meteorol Soc. 2020; 146: 748– 761. https://doi.org/10.1002/qj.3706

How to cite: Hamouda, M. and Ahrens, B.: On The Convective Precipitation Contribution during Vb-events, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2883, https://doi.org/10.5194/egusphere-egu22-2883, 2022.

15:45–15:52
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EGU22-3629
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ECS
menno veerman et al.

The energy transfer by electromagnetic radiation drives atmospheric processes on a wide range of scales: Solar irradiance drives heat and moisture fluxes from the surface and the net radiative cooling of the upper troposphere enables deep convection. However, computing accurate radiative fluxes in atmospheric models is expensive because of the integration over the non-linear absorption spectra of the atmosphere. This integration is typically approximated using so-called correlated k-distribution or similar methods and requires a large number (~102) of spectral quadrature points. In this study, we aim to find the lowest spectral resolution that still allows for accurate cloud field properties in large-eddy simulations of convective clouds. First, we reduce the spectral resolution of the radiation parametrization RRTMGP with an optimization algorithm that repeatedly combines adjacent quadrature points while maintaining the highest possible accuracy on a set of radiative metrics. Reduced sets of quadrature points are then tested further using three distinct sets of large-eddy simulations of convective clouds: deep convection in radiative-convective equilibrium (RCEMIP), shallow convection with precipitation shallow cumulus over the ocean (RICO), and shallow convection over land with a tight connection to the surface energy balance. We find that the spectral resolution of the radiation model, and thereby its computational costs, can be reduced by a factor three to four while retaining statistically similar cloud field properties. While this could reduce the total computational costs of an atmospheric simulation, or allow for a smaller radiation time step, it may also be a crucial step in increasing the feasibility of using 3D radiative transfer in large-eddy simulations.

How to cite: veerman, M., Pincus, R., and van Heerwaarden, C.: Accelerating simulations of convective clouds by reducing the spectral resolution of radiative transfer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3629, https://doi.org/10.5194/egusphere-egu22-3629, 2022.

15:52–15:59
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EGU22-4722
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ECS
Hans Segura et al.

State-of-the-art Global circulation models (GCMs) are characterized by the use of parameterization of convection and low spatial resolution, resulting in persistent biases in the representation of tropical precipitation. Now, Storm Resolving Models (SRM), a new generation of global climates models which, due to their high spatial resolution (<10 km) do not rely on a convective parameterization, may allow bypass some of these well-known precipitation biases. In this study, we present results of coupled SRM simulations conducted with the ICON model and integrated on seasonal time scales (SRM-ICON). We consider three different versions of the model (SRM-ICON16, SRM-ICON29, SRM-ICON52). From SRM-ICON16 to SRM-ICON29, the scheme of the vertical mixing was changed (from TEE to Smagorinsky), along with its vertical length. In addition, a problem in the ocean-atmosphere coupling regarding momentum transfer was fixed. SRM-ICON52 differs from SRM-ICON29 in the vertical coordinates in the ocean (different discretization), land initialization, and a bug in the sensible heat flux calculation was solved. Using these three versions of SRM-ICON, we aim to understand which aspects of tropical precipitation are robust, even to model bugs, and are directly improved just by using an explicit representation of convection. We find that precipitation over land is well reproduced compared to observations and robust across the three versions.  Monsoon areas in the longest run of SRM-ICON are well represented when compared with GPM for the year 2020. Moreover, the meridional pattern of precipitation during the wet season of the North American, the South African, and the Australian monsoon systems, as well as the Maritime Continent in SRM-ICON, show similar characteristics to the observed in GPM. Also, the diurnal cycle of precipitation over land and ocean can be reproduced by SRM-ICON and is robust among the three versions. In contrast, the ITCZ structure over the ocean is highly sensitive to the model version and not necessarily improved compared to low-resolution simulation. Finally, we verified that changes in total tropical precipitation amounts among the three versions of SRM-ICON are consistent with differences in atmospheric radiative cooling, and can be mainly explained by the net longwave flux divergence. 

How to cite: Segura, H., Hohenegger, C., Wengel, C., and Steven, B.: Challenges in Storm Resolving Models: Biases and consistency in the representation of tropical precipitation in the coupled ICON model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4722, https://doi.org/10.5194/egusphere-egu22-4722, 2022.

15:59–16:06
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EGU22-5352
Juraj Palenik and Thomas Spengler

Atmospheric convection is responsible for turbulent transport of heat, moisture, and momentum and fuels the convective cloud formation. Due to lack of observations, numerical prediction models treat convection using sometimes inadequately constrained parametrization schemes. Observations of atmospheric convection to further constrain these parameterisations are notoriously difficult to obtain due to the intermittent, localized,  and turbulent character of convection. However, every day, hundreds of paragliding, hang gliding, and gliding pilots probe the convective boundary layer in hope of finding the best convective thermals. They spend years learning the art of finding and flying in the convective air, while they proudly share their flight tracks online. In this presentation we show how tracks of these engineless aircrafts can be used to sample atmospheric convection. We showcase a dataset from a paragliding championship to classify convection. We elaborate on how the international databases can be used to characterize atmospheric convection and aid building parametrizations based on machine learning.

How to cite: Palenik, J. and Spengler, T.: THERMAL: Sampling Atmospheric Convection Using Paragliders, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5352, https://doi.org/10.5194/egusphere-egu22-5352, 2022.

16:06–16:13
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EGU22-5473
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ECS
Yannick Burchart et al.

Measurements of shallow cumulus cloud properties at meteorological sites are of crucial importance to evaluate Large-Eddy Simulations (LES) of this cloud regime, as well as its parameterized representation in numerical weather and climate models. However, to this date, these datasets have mainly consisted of vertical, one-dimensional profile data, often sampled with remote sensing equipment such as lidar or radar. A recently explored new method for adding multi-dimensional information is to use hemispheric images from a network of multiple cameras. Such networks observe shallow cumuli in unprecedented spatial detail and at ultra-high frequency. Fisheye cameras provide a large field of view, which enables the observation of complete shallow cumulus life cycles. Camera networks can thus strongly complement the existing instruments at a site, yielding unprecedented new insights into cumulus cloud field geometry, dynamics, and evolution. One possible way to independently assess the accuracy of camera networks is to apply it to virtual cloud fields as generated with LES, acting as truth in an Observation System Simulation Experiment (OSSE). However, for this purpose virtual hemispheric camera projections of the LES cloud fields are needed.

In this study, we combine Beer-Lambert radiative transfer with an open-source Monte Carlo path-tracing code to generate such projections. The method is applied to simulate a network of multiple stereo cameras as currently installed at the Jülich Observatory for Cloud Evolution (JOYCE), Germany as part of the ongoing SOCLES project. Three-dimensional LES cloud fields for selected days are used as input. Hemispheric projections are then generated and provided to the existing algorithm for generating three-dimensional fields from actual stereo camera images. Comparing the latter to the input LES fields then allows precise error estimation. One goal is to, thus, find out how much of an arbitrary three-dimensional cloud field can, in theory, be reliably detected by a stereo camera setup. A second goal is to use this information to optimize the configuration of the stereo camera network at the site.

We find that the hemispheric path tracing projections can function well in this workflow. For the selected days we find that 81% of the reconstructable cloudy grid boxes in an LES cloud field is reconstructed by the stereo camera algorithm. Modest dependence on reconstruction tolerance is reported, while dependence on camera distance is also investigated.

How to cite: Burchart, Y., Beekmans, C., and Neggers, R.: Using atmospheric path-tracing as a simple hemispheric simulator to test stereo camera reconstructions of 3-dimensional boundary layer cloud fields, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5473, https://doi.org/10.5194/egusphere-egu22-5473, 2022.

16:13–16:23
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EGU22-5874
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ECS
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solicited
Romain Fiévet et al.

It is well-recognized that triggering of convective cells through cold pools is key to the organization of convection, as reviewed in Zuidema et al. (2017). Yet, numerous studies have found that both the characterization and parameterization of these effects in numerical models is cumbersome - in part due to the lack of numerical convergence (Δx→ 0) achieved in typical cloud-resolving simulators. Through a comprehensive numerical convergence study, we systematically approach the Δx→ 0 limit in a set of idealized large-eddy simulations capturing key cold pool processes: propagation, merging and collision of gust fronts. We characterize at which Δx convergence is achieved for physically relevant quantities, namely accumulated upwards water mass fluxes, integrated vortical rates and gust front's group velocity.

The gust front vortical size and strength achieves convergence at Δx=100m horizontal resolution (70% drop at Δx=800m), while the probability distribution of updraft fluxes upon frontal collision, ƒ(w), appears satisfactorily resolved at Δx=50m. Interestingly, ƒ(w) exhibits self-similarity as a function of Δx, down to the coarsest case of Δx=800m. A rescaling function is derived that successfully collapses all distributions onto a common solution. Further, the positive water mass perturbation caused upon propagation and collision of the gust front appears well-captured at Δx=200m (35% drop at Δx=800m). Finally, the incidental merging of several cold pools results in a large gust front, as often found in MCS. The group velocity of this merged front is only mildly dependent on resolution (10% drop at Δx=800m), suggesting that the numerical dissipation dominates over dispersion.

The understanding gained from this analysis lays the groundwork to develop robust subgrid models for CP dynamics able to sustain their growth and combat artificial numerical dissipation and dispersion.

How to cite: Fiévet, R., Meyer, B., and Haerter, J.: When is numerical resolution high enough to resolve cold pool organization?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5874, https://doi.org/10.5194/egusphere-egu22-5874, 2022.

16:23–16:30
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EGU22-6493
Tompkins Adrian Mark and Michie Vianca De Vera

Cloud resolving models run in idealized conditions of radiative convective equilibrium often show convective switching a state of random convection to a state in which the convection is clustered, leading to a much drier mean state. These simulations have shown that feedbacks with the ocean over thin mixed layers can delay or prevent clustering onset. Understanding convective aggregation is critical as it could alter our assessment of tropical climate sensitivity, and yet it has been difficult so far to even assess aggregation in observations, due to the lack of ability to observe convective core location from space until Doppler radar is available. Here, using a novel analysis method to examine a combination of state-of-the-art retrievals of clouds, water vapour, precipitation and sea surface temperature available since 2016, we present observations that demonstrate convective aggregation operates in the tropical western Pacific region on the sub-1000 km scale, in a region with very weak spatial gradients in sea surface temperature. Convection is generally seen to be in a highly aggregated state, but intermittently and rapidly flips to a random state when low wind conditions prevail, associated with thin mixed ocean layers which oppose aggregation. These events generally persist for a few days to a week or more before convection transitions back to a clustered state. We believe this to be the first direct evidence of this "switching" of clustering state occurring on the meso-scale in the tropics. Summary statistics of these random-convection episodes will be presented.

How to cite: Adrian Mark, T. and De Vera, M. V.: Observational evidence for ocean feedback causing episodes of convective aggregation breakup in the tropical Pacific., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6493, https://doi.org/10.5194/egusphere-egu22-6493, 2022.

16:30–16:37
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EGU22-7221
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ECS
Lena Frey et al.

The prediction of hailstorms is highly uncertain, not least since various processes involved act on different scales. We investigate the representation of hail events in the numerical weather prediction model ICON coupled with the aerosol module ART (ICON-ART). Furthermore, we evaluate the relative contributions from aerosols, microphysical parameters and environmental conditions to the uncertainty in cloud-, precipitation- and hail parameters for severe hail events.

Our case study investigates the Andreas storm on the 28th of July over Southern Germany, causing severe damage. We perform model simulations on cloud-resolving scale and compare the model output with satellite and radar observations. The focus of our analysis is on the representation of storm tracks, total precipitation and the hail production rate in the model. 

Also, we are in the process of developing a statistical emulator. In order to identify possible input parameters for the emulator, sensitivity simulations with the model have been performed. Five input parameters have been selected, namely the CCN and IN concentrations, the riming efficiency, the CAPE and wind shear. The model response to changes of these sensitivity parameters will be presented. Further, we will present first results from the ensemble model simulations, which will be used to build the emulator.

How to cite: Frey, L., Hoose, C., Kunz, M., Miltenberger, A., and Kuntze, P.: Sensitivity experiments with ICON-ART for the Andreas hail storm on the Swabian Jura, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7221, https://doi.org/10.5194/egusphere-egu22-7221, 2022.

Mon, 23 May, 17:00–18:30

Chairpersons: Cathy Hohenegger, Adrian Tompkins, Leo Donner

17:00–17:07
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EGU22-7329
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ECS
Blaž Gasparini and Aiko Voigt

Large uncertainties persist with respect to the role of microphysical and other small-scale processes in high clouds and their interactions with circulation and convective processes. Moreover, the uncertainty in tropical high cloud feedback is the dominant contributor to the total cloud feedback uncertainty and is believed to be connected to the description of microphysics. A large part of changes in anvil cloud properties is due to changes in ice crystal number and their size, which in turn depend on the background amount of cloud droplet number and ice nucleating particles and the description of ice nucleation.

In this work, we use idealized radiative-convective-equilibrium and tropical limited area experiments with SAM and ICON-NWP models to explore the effect of the number of ice nucleating particles and the number of cloud droplet number concentration on the cloud feedback and climate sensitivity. Moreover, we show that cloud radiative properties are strongly dependent on ice crystal number and size, which can be adequately represented only by two-moment microphysical schemes with an interactive simulation of both hydrometeor mass and number.

How to cite: Gasparini, B. and Voigt, A.: The importance of ice nucleation for climate sensitivity in radiative-convective equilibrium, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7329, https://doi.org/10.5194/egusphere-egu22-7329, 2022.

17:07–17:14
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EGU22-7712
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ECS
Giovanni Biagioli and Adrian Mark Tompkins

Numerical simulations of radiative-convective equilibrium in high-resolution cloud-resolving models (CRMs) revealed the tendency of atmospheric convection to self-aggregate on periods of several weeks when the domain is sufficiently large. Nevertheless, even though CRM simulations manage to identify some of the physical mechanisms driving convective clustering, the occurrence of organization seems to be dependent on the model setup, physics and parameterizations. Robust findings from simpler, idealized models, which may reproduce some of the features of the full-physics systems, are thus beneficial to better understand the differences existing between CRMs.

To this end, we have developed a simplified two dimensional stochastic model able to predict the evolution of column total water relative humidity (CRH) in the tropical free troposphere. The model prognostic equation includes a convective moistening term, diffusive lateral transport and subsidence drying, similar to model of Craig and Mack (2013), but one novelty of the new model is that, instead of the convective moistening term as a smooth deterministic function of the background humidity, we treat convection as a point process and account for stochastic variability in the convective moistening process. Therefore the model allows experiments to use domain sizes and grid resolutions similar to those used for the idealized CRM experiments.

It is found that, depending on the chosen parameter settings, the simple model can reproduce equilibrium states of strong convective aggregation and also randomly distributed states, analogous to the CRM results. A sensitivity of the occurrence of self-organization to the initial conditions, i.e., a modest hysteresis, is also found, which also agrees with the full physics CRMs. Large ensembles of numerical experiments were performed for different values of the subsidence timescale, the moisture diffusion coefficient and the parameter that determines convective sensitivity to background humidity, as well as for a range of domain sizes and horizontal grid spacings. Using dimensional arguments, combined with empirical fits from numerical data, we define a dimensionless parameter whose value indicates whether a clustered state is likely to emerge for a given set of parameter values and experimental configurations. This quantity contains dependencies on all the model processes, while also explicitly including the domain size and resolution in an attempt to explain these latter sensitivities observed in the full-physics CRM experiments.

How to cite: Biagioli, G. and Tompkins, A. M.: Convective aggregation in idealized stochastic models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7712, https://doi.org/10.5194/egusphere-egu22-7712, 2022.

17:14–17:21
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EGU22-7887
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ECS
Alejandro Casallas et al.

Convective self-aggregation can spontaneously appear in radiative-convective equilibrium (RCE) simulations using idealized experiments with cloud-resolving models and it has been suggested that cold pools could play an important role in the development of organization, by delaying its onset when the cold pools have larger radii. Cold pool radius is determined by the amount of precipitation produced by microphysical schemes (precipitation efficiency) and the strength of the evaporation. We demonstrate this using idealized RCE experiments with the WRF model that convective cold pool characteristics can differ dramatically between 5 of the standard schemes commonly used in the model. We then systematically increase/decrease the cold pool size by changing the evaporation of rain in the 5 microphysics schemes to observe the impact on convective aggregation. One complication in interpreting the results of such experiments is that a change in the evaporation of rain also produces a change in the profile of net convective heating that could also impact organization. To isolate this effect,  a second set of experiments is performed by artificially increasing (decreasing) the horizontal wind speed used in the surface flux calculation for all grid points determined to lie within cold pool interiors to produce a faster (slower) cold pool recovery and impact their ultimate radii. The ensembles of the experiments show that the larger the cold pool radii, the larger the spatial variance of the water vapor path is in the equilibrium state and they also demonstrate how the cold pool size impacts the strength and even the sign of the surface latent heat contribution to aggregation. Nonetheless, the strong forcing of aggregation by radiation feedbacks in these experiments means that the cold pool changes do not produce large modifications to the aggregation onset time. Thus the aggregation onset may be more strongly impacted by the microphysical processes that determine the convective anvil size and low-level cloud cover, and thus ultimately the cloud-radiative forcing. This is under investigation in ongoing experiments that modify the ice fall speed and the autoconversion of cloud water to rain in the 5 microphysical schemes, which will also be reported in the presentation.

How to cite: Casallas, A., Tompkins, A., and Thompson, G.: ​​The role of cold pools and microphysics schemes in the organization of convection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7887, https://doi.org/10.5194/egusphere-egu22-7887, 2022.

17:21–17:28
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EGU22-9649
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ECS
Sophie Abramian et al.

Squall lines are the consequence of the interaction of low-level shear with cold pools associated with convective downdrafts, and beyond a critical shear, squall lines tend to orient themselves. It has been shown that this orientation has the effect of reducing the incoming wind shear to the squall line and maintains equilibrium between wind shear and cold pool spreading (Abramian et al 2021).

While the mechanisms behind squall line orientation seem to be increasingly well understood, few studies have focused on the implications of this organization. Yet, Roca and Fiolleau 2020 shows that mesoscale convective systems, including squall lines, are disproportionately involved in rainfall extremes in the tropics. One may then question whether the orientation of squall lines has an impact on the rainfall extremes, and if so, how and why.

Using a CRM, we perform simulations of tropical squall lines by imposing a vertical wind shear in radiative convective equilibrium. Our results show that the extreme precipitation in the squall lines is more intense in the critical organized case. It seems that when the condensation rate increases with the shear, the precipitation efficiency decreases strongly. The critical case appears to be the most favorable compromise between these two contributions, a hypothesis that we further investigate here.

How to cite: Abramian, S., Muller, C., and Risi, C.: Investigating extreme precipitation in tropical squall lines, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9649, https://doi.org/10.5194/egusphere-egu22-9649, 2022.

17:28–17:35
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EGU22-10117
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ECS
Jian-Feng Gu et al.

The halo region, defined as the moist buffering region without cloud liquid water, is critical for the interplay between the cloud and the environment and also has non-negligible impact on radiation, but yet lacks enough attention. This study systematically investigates how the relative humidity in the halo region decays outward to match the environmental relative humidity using high resolution large eddy simulations. A noval algorithm is developed to examine the composite structure outside the clouds. It is found that, whatever the horizontal resolution is, the distribution of relative humidity in the halo region does not depend on the size of cloud, but on the real distance away from the cloud boundary. With finer horizontal resolution, the relative humidity decays outward much more quickly. The halo size converges when the horizontal resolution is no larger than 50 m. The buoyancy length scale can explain the dependency of halo size on model resolution.  Sensitivity simulations indicate that these findings are not sensitive to the details of the sub-grid turbulence scheme and the advection schemes. Analyses of autocorrelation length scales and Lagrangian trajectories further shed light on how the halo regions at different vertical levels are connected. Our results can help improve the definition of near cloud environment in the bulk plume model in convection parameterization and also provide evidence to improve the understanding of both dynamical and radiative impact from the cloud-aerosol-environment interactions.

How to cite: Gu, J.-F., Plant, B., Holloway, C., and Clark, P.: Halo Size Around Shallow Cumulus Clouds in the Large Eddy simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10117, https://doi.org/10.5194/egusphere-egu22-10117, 2022.

17:35–17:42
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EGU22-11294
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ECS
Vijit Maithel and Larissa Back

Organization of convection in tropics exhibits a wide range of spatial and temporal scales on account of a complex maze of interactions between clouds, circulation, radiation, and moisture. Somewhere in that maze are also present the mechanisms responsible for the phenomenon of convective self-aggregation in idealized simulations. However, the exact role played by these self-aggregation mechanisms on organization of convection in the real world remains unclear including understanding when convection will aggregate further and when convection will disaggregate. Cloud radiative feedbacks have been found to be important for initiation and maintenance of self-aggregation in idealized modelling studies. In observations too, it has been shown that radiation varies linearly with convection across different regions in the tropics. This implies that radiative feedbacks add moist static energy (MSE) to the already moist columns (that is it favors aggregation). However, the question comes up then, why does convection disaggregate in observations despite the support from radiative feedbacks? We hypothesize that advection of moisture and moist static energy (MSE) instead is important for determining when convection aggregates or disaggregates in the real world.

We utilize a moist static energy (MSE) variance budget-based phase plane as a process oriented diagnostic tool to test our hypothesis. The phase plane is formed by taking the variance of MSE on the x-axis and time tendency of variance of MSE on the y-axis. Then, cycles of aggregation and disaggregation show up as elliptical orbits on this plane. Contributions to the MSE variance tendency from the advective terms and radiative terms can be explicitly analyzed on this phase plane. Data from idealized simulations is used to understand how self-aggregation mechanisms show up on the phase plane. The results are compared with reanalysis data to understand the differences between self-aggregation mechanisms in models and mechanisms that favor aggregation in the real world. Data from different regions is also analyzed to determine whether it’s the variations in advective terms or the radiative term that govern when convection aggregates or disaggregates in observations.

How to cite: Maithel, V. and Back, L.: Role of Moist Static Energy Advection in Evolution of Convective Aggregation in Observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11294, https://doi.org/10.5194/egusphere-egu22-11294, 2022.

17:42–17:49
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EGU22-12095
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ECS
Edward Engelbrecht and Jan Haerter

Convective organization from the interaction between cold pools and thermally induced mesoscale circulations

Land surface conditions can influence the onset and strength of convective systems. This is because sharp gradients in land surface properties result in the development of mesoscale atmospheric circulations that initiate convection, similar to the circulations present in land-sea breezes (Dirmeyer, 1995). The influence of these mesoscale circulations on convection has been well established, however, moist convection can also alter the circulations that led to its initiation - an aspect which has largely been neglected in the literature (Rieck et al., (2015)). An implication that follows from the effect of moist convection on the circulations is that it is possible for the surface temperature gradient (which triggers the initial mesoscale circulation) to reverse under cold pool effects, thus reversing the direction of the mesoscale circulation in order to bring rain to conditionally unstable regions (such as wet soils). Under these conditions, how does the mesoscale circulation tie in with cold pool outflow to organize convection across a domain with heterogeneous surface properties?

Motivated by the modeling framework from previous studies (e.g. Huang and Margulis, 2012; Rieck et al., 2015; Schneider et al., 2019)  we employ a checkerboard of alternating extremely dry vs. wet soil moisture patches in an idealized cloud resolving model coupled to a land surface model. The checkerboard approach has previously been used to show how the PBL, cloud size, precipitation duration and amount are all influenced by different strength and length scales of the land-surface gradients. Using this setup, mesoscale circulations induced by surface heterogeneities are therefore overlaid with cold pool circulations from subsequent afternoon convection. Our aim is to investigate the role of the interactions between thermal mesoscale and cold pool circulations to organize diurnal convection in a way that allows upscale growth of scattered convection in an environment where background wind is absent. Originating from this we aim to understand 1) whether convection over wet patches is determined by convection first triggering over dry patches, 2) What are the conditions required so that the static thermal circulation (controlled by the soil moisture discontinuity) will change direction? 3) To what extent does the interaction between cold pool outflow and mesoscale circulations maintain the initial soil moisture gradient field in the absence of background wind? These questions may help answer how land use changes (due to deforestation, agriculture etc.), and the resultant circulation changes focus convection so that rainfall becomes locally more extreme.

How to cite: Engelbrecht, E. and Haerter, J.: Convective organization from the interaction between cold pools and thermally induced mesoscale circulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12095, https://doi.org/10.5194/egusphere-egu22-12095, 2022.

17:49–17:56
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EGU22-12304
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ECS
Alex Doyle et al.

Operational ground-based weather radar data provide a unique opportunity to evaluate simulations of convection in high-resolution models. This is especially advantageous for tropical regions such as India where convection is frequent and interacts with the large-scale circulation. Here, storms derived from 15 Indian operational radars are directly compared to modelled storms at two convection-permitting resolutions, 1.5 and 4.4 km, for a period of 3 weeks during the peak 2016 monsoon season. We objectively identify different morphological properties of storms for 6 regions of India, that is to say their heights, sizes, and intensities. Both model resolutions are found to simulate too much shallow convection compared to radars for all 6 regions. Modelled convection is also frequently too wide and intense, but the 1.5 km model performs noticeably better. Modelled storms also exhibit a maximum area around the freezing level, higher than observed by radars, especially at 4.4 km resolution. We discuss various potential microphysical and dynamical reasons for the major differences seen, thus demonstrating the power of radar-based evaluation of monsoon convection for the Indian region.

How to cite: Doyle, A., Stein, T., and Turner, A.: Evaluating characteristics of Indian monsoon convection in high-resolution models with ground-based weather radars, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12304, https://doi.org/10.5194/egusphere-egu22-12304, 2022.

17:56–18:03
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EGU22-12318
Nicolas Da Silva et al.

Convective organization has been associated with extreme precipitation in the tropics. Here we investigate the impact of convective self-aggregation on extreme rainfall rates. We find that convective self-aggregation significantly increases precipitation extremes, for 3-hourly accumulations(+70%) consistent with earlier studies, but also for instantaneous rates (+30%). We show that this latter enhanced instantaneous precipitation is mainly due to the local increase in relative humidity which drives larger accretion rates and lower re-evaporation and thus a higher precipitation efficiency.

An in-depth analysis based on an adapted scaling of precipitation extremes, reveals that the dynamic contribution decreases (-25%) while the thermodynamic is slightly enhanced (+5%) with convective aggregation, leading to lower condensation rates (-20%). When the atmosphere is more organized into a moist convecting region, and a dry convection-free region, deep convective updrafts are surrounded by a warmer environment which reduces convective instability and thus the dynamic contribution. The moister boundary-layer explains the positive thermodynamic contribution. The microphysic contribution is increased by +50% with aggregation. The latter is partly due to reduced evaporation of rain falling through a moister near-cloud environment (+30%), but also to resulted larger accretion efficiency (+20%).

Thus, the change of convective organization regimes in a warming climate could lead to a significantly different evolution of tropical precipitation extremes than expected from thermodynamical considerations. Improved fundamental understanding of convective organization and its sensitivity to warming, as well as its impact on precipitation extremes, is hence crucial to achieve accurate rainfall projections in a warming climate.

How to cite: Da Silva, N., Muller, C., Shamekh, S., and Fildier, B.: Significant amplification of instantaneous extreme precipitation with convective self-aggregation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12318, https://doi.org/10.5194/egusphere-egu22-12318, 2022.

18:03–18:13
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EGU22-12348
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ECS
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solicited
Jiawei Bao et al.

We show with ERA5 reanalysis data that substantial zonal virtual temperature gradients, on monthly time scales, exist in the tropical free troposphere. The strength of the temperature gradients changes seasonally: the Equatorial Western Pacific Ocean (EWP) is usually much warmer than the Equatorial Central Pacific Ocean (ECP) during December-January-February (DJF), while ECP becomes slightly warmer than EWP during June-July-August (JJA). During DJF, the virtual temperature gradients in the Pacific prevail throughout the entire free troposphere, and are most pronounced in the upper troposphere near 300hPa. We find that the free-tropospheric temperature gradients are related to the pressure gradients, while the pressure gradient force is mainly balanced by the nonlinear terms in the momentum equation---zonal wind advection. Strong zonal winds occur near the equator in January, transporting momentum zonally and balancing the pressure gradient force. The reason that strong zonal winds occur is that vigorous large-scale equatorial waves are excited due to the heating pattern being more symmetric to the equator. In July, the large-scale equatorial waves are less active in the Pacific Ocean. No strong zonal wind exists to sustain the pressure gradient as well as temperature gradient to develop. As a result, the virtual temperature distributions are much more uniform. The results point out the important role of the nonlinear terms in the tropical balanced dynamics on monthly time scales, stressing the need of an improved theoretical understanding and modeling framework of the tropical atmosphere by including the nonlinear terms in the dynamical balance.

How to cite: Bao, J., Dixit, V., and Sherwood, S.: Substantial zonal temperature gradients in the tropical free troposphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12348, https://doi.org/10.5194/egusphere-egu22-12348, 2022.

18:13–18:20
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EGU22-13159
Brian Mapes and Wei-ming Willy Tsai

Multicellular conglomerations of deep convection are commonly observed, but what is the larger-scale importance of this nonrandom spatial structure? We hypothesize (based on the common meaning of the word “organization”) that more-agglomerated convection will out-compete spotty convection for scarce resources of convection’s ecology: instability and moisture. But how exactly? Can we quantify organization's advantages in scalar metrics, and further understand that issue in the vertical domain of convection’s depth? To address these questions, idealized simulations were devised in large-domain simulations with Cloud Model 1 (CM1). A double-periodic domain is uniformly destabilized with a homogeneous cooling of -4K/day, with corresponding surface fluxes to compensate for the cooling. To generate and separate isolated and agglomerated deep convection, we first modulate the “organization gradient” across the simulated domain with nondivergent nudging of a zonal wind shear belt. Other approaches for manipulating organization gradients are also tried, including the latent to sensible ratio of surface fluxes at constant buoyancy flux, and autoconversion rates in the microphyiscs (a crude proxy for aerosol effects). Domain-scale circulations are generated, roughly in proportion to the organization gradient. Measures of that gradient by in some conventional indices of horizontal pattern clumpiness are checked or calibrated against uniform-domain simulations with and without shear, suggesting that these measures are adequate to characterize the organization gradient. Vertical-plane streamfunctions of the ‘zonal' (belt) averaged overturning exhibit multiple vertical cells and counter-cells with interesting dependences on the shear profile or other org-gradient producing mechanisms.

How to cite: Mapes, B. and Tsai, W.-W.: Circulations driven by organization gradients in a cloud-resolving model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13159, https://doi.org/10.5194/egusphere-egu22-13159, 2022.