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AS1.10

EDI
Tropical meteorology and Tropical Cyclones

The understanding of tropical phenomena and their representation in numerical models still raise important scientific and technical questions, particularly in the coupling between the dynamics and diabatic processes. Among these phenomena, tropical cyclones (TC) are of critical interest because of their societal impacts and because of uncertainties in how their characteristics (cyclogenesis processes, occurrence, intensity, latitudinal extension, translation speed) will change in the framework of global climate change. The monitoring of TCs, their forecasts at short to medium ranges, and the prediction of TC activity at extended range (15-30 days) and seasonal range are also of great societal interest.

The aim of the session is to promote discussions between scientists focusing on the physics and dynamics of tropical phenomena. This session is thus open to contributions on all aspects of tropical meteorology between the convective and planetary scale, such as:
- Tropical cyclones,
- Convective organisation,
- Diurnal variations,
- Local circulations (i.e. island, see-breeze, etc.),
- Monsoon depressions,
- Equatorial waves and other synoptic waves (African easterly waves, etc.),
- The Madden-Julian oscillation,
- etc.
We especially encourage contributions of observational analyses, modelling studies, and theoretical investigations of tropical cyclones and other synoptic-scale tropical disturbances including the physics and dynamics of their formation, structure, and intensity, and mechanisms of variability of these disturbances on intraseasonal to interannual and climate time scales.
Findings from recent field campaigns are also encouraged.

Convener: Enrico Scoccimarro | Co-conveners: Jean Philippe Duvel, Eric Maloney, Kevin Reed, Allison WingECSECS
Presentations
| Wed, 25 May, 08:30–11:50 (CEST), 13:20–15:55 (CEST)
 
Room F1

Wed, 25 May, 08:30–10:00

Chairperson: Enrico Scoccimarro

08:30–08:36
Session introduction

08:36–08:42
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EGU22-11988
Benoit Vanniere et al.

Although most GCMs project a decline of tropical cyclone activity in a warmer world, some recent studies have cast doubts on this consensus by suggesting that the number of tropical cyclones might increase in future. The HighResMIP experiments offer such an example of contradicting projections. Indeed, AMIP-type experiments which have been forced by transient SSTs preserving the trend simulated by CMIP6 models in scenario SSP5-8.5, predict an increase of cyclone activity in the North Atlantic, whereas experiments with the same atmospheric models coupled to an ocean model predict a decline. In this paper, we intend to explain and reconcile those results. To do so, we compare several recent and past projects including HighResMIP, CMIP6 scenario SSP5-8.5 and the time-slice experiments of the UPSCALE project. We used several different approaches to explain the future change in TC activity, including SST anomalies relative to the tropical mean, the ventilation index for tropical cyclone genesis and predictors of tropical cyclone precursors.

SST anomalies show that subtle differences in SST trends between basins in the AMIP and coupled experiments can explain the differences in TC projections. This analysis should guide the construction of SST for the transient AMIP experiments used in future HighResMIP protocols. Once the less reliable projections have been discarded from our model ensemble, we show that there exists a remarkable agreement between the projections of HighResMIP coupled, scenario SSP585 and UPSCALE. We find that the saturation deficit is the component of the ventilation index which explains the largest fraction of the change, with the potential intensity and vertical wind shear playing a secondary role. Finally, we find that there is some agreement between models on the different time of emergence of a trend in TC activity in each basin, which we attempt to link to differences in the time of emergence of the trend of saturation deficit.

How to cite: Vanniere, B., Roberts, M., Hodges, K., and Vidale, P. L.: Tropical Cyclones in Future HighResMIP Experiments : Explaining and Reconciling Projections, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11988, https://doi.org/10.5194/egusphere-egu22-11988, 2022.

08:42–08:48
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EGU22-3048
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On-site presentation
Leone Cavicchia et al.

Tropical cyclones (TCs) are regularly listed among the costliest natural disasters, due to the associated strong wind, heavy precipitation, and risk of storm surges. Therefore, being able to understand and predict TC activity at different time scales would lead to clear societal and economic benefits.

Several genesis potential indices (GPIs) have been introduced in the literature, linking TC activity to favourable conditions in a number of large-scale meteo-climatic variables. The advantage of using such indices lies in the ability to study TC occurrence in climate model simulations, which do not usually reproduce individual TC accurately due to the limited horizontal resolution.

Existing GPIs generally have good skill in reproducing the spatial pattern and seasonal cycle of historical TC activity. On the other hand, they commonly fail to reproduce TC interannual variability across different ocean basins. A further issue is found for climate projections where, for those climate models with high-enough resolution to allow for TC tracking, the trends of GPI and directly detected cyclones are often in disagreement.

Here we revisit the issue of TC genesis potential in reproducing TC activity by exploiting the last generation of climate model simulations, obtained from the CMIP6 model intercomparison project. Using data obtained from both the ScenarioMIP and HighResMIP simulations, we investigate the effect of horizontal resolution and other model features on the modelled GPI’s skill in reproducing TC interannual variability and trends.

How to cite: Cavicchia, L., Ascenso, G., Scoccimarro, E., Castelletti, A., Giuliani, M., and Gualdi, S.: Tropical cyclone genesis potential in CMIP6 climate models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3048, https://doi.org/10.5194/egusphere-egu22-3048, 2022.

08:48–08:54
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EGU22-6117
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ECS
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Alyssa Stansfield and Kevin Reed

Extreme precipitation is expected to increase with climate change at the Clausius-Clapeyron rate of approximately 7% per °C of warming; however, tropical cyclone (TC) precipitation may increase at a greater rate due to feedbacks between the storm dynamics and the thermodynamic increase in moisture. Previous modeling studies simulate increasing TC intensities with warming sea surface temperatures (SSTs), which may push the precipitation increase above the Clausius-Clapeyron rate. Recent work by the authors used the Community Atmosphere Model (CAM) in a state of radiative-convective equilibrium (RCE) with globally-uniform SSTs varying between 295 and 305 K to break down the TC precipitation response to warming into thermodynamic and dynamic contributions. Results showed that for 99th percentile TC precipitation, increases in atmospheric moisture (thermodynamics) contributed just over 66% of the precipitation increase while increases in TC intensity (dynamics) contributed about 20%. This work explores if the relationship between TC precipitation, SST, and storm intensities found in the RCE simulations holds for observations and high-resolution climate model simulations. The observations consist of TC tracks and intensities from the IBTrACS database, SSTs from the NOAA OISST dataset, and precipitation from the IMERG satellite product. The high-resolution climate model simulations are from the High Resolution Model Intercomparison Project (HighResMIP), a CMIP6-endorsed MIP that has both historical and future climate runs. The methodology involves extracting TC precipitation using an automated algorithm, binning TCs by relevant characteristics (i.e., their local-environment SSTs, intensities, and outer sizes), extracting various precipitation metrics from their precipitation fields, and calculating relationships between the precipitation metrics, TC characteristics, and SSTs. The goal is to use these relationships to project future TC precipitation changes under different future climate change scenarios using just changes in SST.

How to cite: Stansfield, A. and Reed, K.: Projecting Future Tropical Cyclone Precipitation Increases using a Hierarchical Modeling Framework, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6117, https://doi.org/10.5194/egusphere-egu22-6117, 2022.

08:54–09:00
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EGU22-2314
Alexander Baker et al.

In the North Atlantic, approximately half of tropical cyclones undergo extratropical transition, and landfalling systems pose risks to populous midlatitude regions. The frequency of tropical-origin storms across the midlatitudes is projected to increase under anthropogenic climate change, but multi-model studies are required to help reduce uncertainties. One key uncertainty is the role of Atlantic multidecadal variability (AMV), a robust understanding of which will help contextualise projections. We assess the impacts AMV+ and AMV– on basin-wide tropical cyclone and extratropical transition activity in an ensemble of coupled sensitivity experiments from CMIP6 HighResMIP. We used objective methods—a Lagrangian feature-tracking algorithm and cyclone phase-space analysis—to identify tropical cyclones undergoing extratropical transition and present analysis of changes in cyclogenesis, tracks, and intensity in response to AMV forcing.

How to cite: Baker, A., Vidale, P. L., Roberts, M., Hodges, K., Seddon, J., Tourigny, E., Lohmann, K., Roberts, C., and Terray, L.: Impact of Atlantic multidecadal variability on North Atlantic tropical cyclones and extratropical transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2314, https://doi.org/10.5194/egusphere-egu22-2314, 2022.

09:00–09:06
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EGU22-2395
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ECS
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Virtual presentation
Carlos Calvo-Sancho et al.

In recent years, western Europe has been threatened by anomalous tropical cyclones, developed from tropical transition (TT) processes in which a baroclinic cyclone becomes in a fully barotropic cyclone. Thirty-three tropical transition events were identified in the North Atlantic basin during the period 1979-2019 using ERA5 and HURDAT datasets. The TTs show a favored seasonality covering 70% of total between September and November.

A TT climatology is built and analyzed using large-scale storm-centered composites to study their common features and highlighting their differences respect the long-term climatology. The results reveal that TT synoptic environment is mainly characterized by a trough at 300 hPa and a strong anticyclone located north of the cyclone. In addition, a previous westerlies meridional trough with quasigeostrophic forcing acts as precursor. As the ERA5 does not accurately represent the diabatic processes due to its horizontal resolution, the deepening of mean sea level pressure is not shown in the composites. The average Potential Vorticity 300-200 hPa (PV) shows a decreasing in the upper troposphere around the cyclones as the moment of TT is approaching, while the PV is increasing in the lower troposphere. This PV conjunction promotes low-level wind speed intensification around the cyclone center that is linked with differential diabatic heat source in the low troposphere.

How to cite: Calvo-Sancho, C., González-Alemán, J. J., Bolgiani, P., Santos-Muñoz, D., Farrán, J. I., Sastre, M., and Martín, M. L.: A Climatology of Tropical Transitions in the North Atlantic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2395, https://doi.org/10.5194/egusphere-egu22-2395, 2022.

09:06–09:12
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EGU22-1045
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ECS
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Virtual presentation
Vineet Kumar Singh et al.

Tropical cyclones in the north Indian Ocean evolve differently in response to the SST changes during the pre-monsoon (April-June) and post-monsoon (October-December) cyclone season. We analyzed the north Indian Ocean cyclones for the period 1982–2019 and observed that there is a contrasting ocean-atmosphere response to cyclones in the north Indian Ocean during the two cyclone seasons. During the pre-monsoon season, anomalous large SSTs along with high near-surface moisture disequilibrium and higher winds enhance the latent heat flux exchange from the ocean to the atmosphere. This increase in the latent heat flux exchange enhances the convection during the cyclone which in turn releases a large amount of latent heat of condensation in the atmosphere resulting in anomalous warming of 3-4°C at the upper levels (300-400 hPa) of the atmosphere. However, during the post-monsoon season, the upper-level anomalous warming is only about ~1°C. Suppressed cyclone-induced upper-level warming is mainly attributed to the weaker ocean-cyclone interaction in this season. As a result, the latent heat flux exchange between the ocean and atmosphere is weak resulting in weaker convection leading to less upper-level warming as compared to the pre-monsoon season. Also, in the lower levels of the atmosphere, there is anomalous large cooling in the pre-monsoon season as compared to the post-monsoon season. This difference in the low-level anomalous cooling is attributed to the difference in the evaporative cooling due to the difference in the low-level moisture profiles in the atmosphere in the two seasons. Through this study, we highlight that both the oceanic and atmospheric response to the north Indian Ocean cyclones is significantly different during the two cyclone seasons. Also, this is for the first time that the mean cyclone-induced atmospheric heating is reported for the north Indian Ocean. The cyclone-induced atmospheric heating can significantly modulate the atmospheric circulation, thus our study will help in better understanding the atmospheric response to cyclones and its other implications.

How to cite: Kumar Singh, V., Mathew Koll, R., and Deshpande, M.: Contrasting ocean-atmosphere response to the north Indian Ocean cyclones during the pre-monsoon and the post-monsoon seasons, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1045, https://doi.org/10.5194/egusphere-egu22-1045, 2022.

09:12–09:18
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EGU22-1806
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ECS
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On-site presentation
Stella Bourdin et al.

The direct detection — or tracking — of tropical cyclones (TC) in gridded datasets outputs from reanalyses or model simulations is required to assess TC statistics. This issue has been tackled independently by many modeling centers or research groups; hence there is little homogeneity in the existing methods. The trackers – i.e., the algorithms used to perform that tracking -- generally fall into one of two categories: physics-based or dynamics-based. Physics-based trackers use sea-level pressure as their primary tracking variable, with additional warm-core and intensity criteria, whereas dynamics-based trackers use kinematic variables such as vorticity.

We compared four trackers taken from both categories and that we deem very different from one another in terms of their formulation: UZ (sometimes called TempestExtremes, Ullrich et al. 2021), OWZ (Tory et al. 2013), TRACK (Hodges et al. 2017) and CNRM (Chauvin et al. 2016). We assessed their performances by tracking TCs in ERA5 and comparing the outcome to the IBTrACS database – a collection of TC observations from several meteorological centers worldwide.

We find typical detection rates ranging from 70 to 80% and False Alarm (FA) rates ranging from 20 to 50% depending on the trackers. Based on the finding that a large proportion of these FAs are extra-tropical cyclones, we adapted an existing filtering method that relies on the relative positions of the detected tracks and the upper troposphere subtropical jet. When applied identically to the four trackers, it reduces FA rates to figures ranging from 9 to 30% while leaving detection rates unchanged.

Even though we were able to find most of the observed TCs in ERA5, we find, in agreement with several results in the recent literature, that their intensity is largely underestimated. However, and perhaps counterintuitively, there is no simple attenuation relationship between observed and reanalyzed TCs: for example, the strongest observed TCs are found in ERA5 with intensities covering almost the entire TC intensity scale.

We conclude by providing guidelines applicable when faced with the question of which tracker(s) to use depending on the research question. In particular, we show that using several trackers is not necessarily relevant for optimizing detection skills but combining them can be helpful to gain insight into different aspects of TCs in the same dataset.

Finally, we used the expertise gained above to track TCs in a set of HighResMIP simulations performed with the IPSL-CM7A model at different resolutions. In agreement with recent results, we find that the ability to simulate TCs improves significantly with resolution. Even though the intensity of simulated TCs remains too weak on average, the global statistics approach observations for simulations at a few tens of kilometers of horizontal resolution.

How to cite: Bourdin, S., Fromang, S., Dulac, W., Cattiaux, J., and Chauvin, F.: Tracking tropical cyclones in reanalysis and simulations: guidelines from an intercomparison of four algorithms, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1806, https://doi.org/10.5194/egusphere-egu22-1806, 2022.

09:18–09:24
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EGU22-5755
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ECS
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On-site presentation
William Dulac et al.

The ERA5 dataset from the ECMWF is the first global reanalysis product to reach a horizontal resolution of 0.28125° (31km), a resolution that is thought to allow for a realistic representation of small-scale atmospheric features such as tropical cyclones.
Using the CNRM Tropical Cyclone Tracking Scheme carefully calibrated for ERA5 and a track pairing algorithm that uses the International Best Track dataset (IBTrACS) as reference, we investigate how well tropical cyclones (TC) are represented in ERA5.

First we show that the majority of IBTrACS systems are found by the ERA5 tracking, but that performances in terms of probability of detection and false alarm rate varies from one geographical basin to the other. Moreover, by comparing the intensities between tracked TCs from ERA5 and their observational counterparts, we show that TCs in the reanalysis are rather weak considering the spatial resolution – both in terms of maximum wind speed and pressure minimum. By looking at mean wind speed life cycles in several geographic basins we also show that TCs de-escalate too quickly after reaching their peak intensity. Finally, using a compositing technique we look at the internal structure of TCs and and that despite the weak intensity, they present expected features regarding radial and tangential wind speed and upper-core temperature anomaly when sorted by Saffir-Simpson categories.

How to cite: Dulac, W., Cattiaux, J., Chauvin, F., Bourdin, S., and Fromang, S.: How Realistic are Tropical Cyclones in the ERA5 Reanalysis?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5755, https://doi.org/10.5194/egusphere-egu22-5755, 2022.

09:24–09:30
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EGU22-11647
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ECS
Guido Ascenso et al.

Tropical cyclones (TCs) are one of the most devastating natural disasters worldwide, in terms of both economic damage ($1100 billion in the last 20 years) and fatalities (210,000 in the last 20 years). The accuracy of TC track and intensity forecasts has been increasing steadily since the 1980s, but it remains difficult to predict how many TCs will form each year and where, as the physical processes that lead to the formation of TCs are still poorly understood. Several Genesis Potential Indexes (GPIs) have been proposed that predict the likelihood of formation and distribution of TC genesis given large-scale factors such as sea surface temperature and air humidity. These indices are constructed as the product of a number (typically 3-5) of dynamic and thermodynamic variables, each of which is assigned a coefficient and an exponent. Such indices serve not only to improve our understanding of TC formation by isolating the variables most linked to it, but also as a guideline for insurance companies and governments of how severe a TC season will be, and to predict how climate change will affect the frequency and severity of TCs in climate model simulations. Nevertheless, current GPIs have large margins of error, especially at the local scale and in terms of interannual variability.

In this work, we explore the search space for this type of index, intended as the space of variables are relative coefficients and exponents that can be used to structure a GPI. We begin by optimizing the coefficients and exponents of the well-known GPI developed by Emanuel and Nolan (ENGPI), and show that a simple genetic algorithm can lead to substantial improvements in spatial correlation between the index and observed data. However, we also show that interannual and spatial correlation may be conflicting objectives which cannot be optimized simultaneously. We then modify the structure of the ENGPI by introducing new variables used in other similar indices, some of which seem to lead to improvements. We then repeat the above experiments using different genetic algorithms, and find that different algorithms converge to different solutions with similar performance, indicating that there are many valid ways to structure a GPI; we offer an interpretation of this finding, which we believe to be relevant for future research. Finally, we show that thermodynamic variables tend to be discarded when optimizing interannual correlation, but not when optimizing spatial correlation; we offer a possible explanation of why this may be.

How to cite: Ascenso, G., Cavicchia, L., Scoccimarro, E., and Castelletti, A.: Using Genetic Algorithms to Optimise Genesis Potential Indices for Tropical Cyclone Genesis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11647, https://doi.org/10.5194/egusphere-egu22-11647, 2022.

09:30–09:36
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EGU22-156
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Virtual presentation
Gaurav Tiwari et al.

The northern Indian Ocean (NIO) is known for tropical cyclones (TCs), likely to increase in the future. It occurs mainly in April-June (pre-monsoon) and October-December (post-monsoon) seasons, destructive for the coastal regions of India, Bangladesh, Pakistan, Oman. To provide reliable alerts and disaster warnings ahead of time, better forecasting of TC aspects (such as track, landfall, strength, rainfall, and so on) is a primary focus. The Weather Research and Forecasting (WRF) model is a mesoscale numerical weather prediction system used to forecast short and medium-range weather phenomena. The reliability of the skill of WRF prediction has been qualitatively enhanced with the successful implementation of some advanced methods and subjected to various constraints i.e. initial conditions, domain, parameterization, etc. In this study, the sensitivity of the initialization is accessed by deploying the digital filter initialization (DFI), and a stochastically perturbed physics-tendency (SPPT) based ensemble-mean techniques to anticipate the two NIO TCs, Tauktae (May 2021) and Nivar (November 2020). Compared to control simulations, the adoption of both DFI and SPPT-based ensemble-mean approaches in the model setup yields considerable gains, replicating closer to the observations, albeit with some deviations. The DFI technique significantly improved the TC's track prediction, while the SPPT-based ensemble-mean forecast approach increased the model's efficiency in predicting the TC's intensity.

How to cite: Tiwari, G., Kumar, P., Bobde, V., and Mishra, A. K.: Prediction of north Indian Ocean Tropical Cyclones  using  WRF model:  Sensitivity for perturbation and filtering on the initial condition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-156, https://doi.org/10.5194/egusphere-egu22-156, 2022.

09:36–09:42
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EGU22-566
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ECS
Beata Latos et al.

In this study, we have examined meteorological drivers that led to the genesis of tropical cyclone Seroja. Developing over the Maritime Continent and in April 2021, it brought historic flooding and landslides to southern Indonesia, East Timor and Western Australia’s Mid West region. Seroja was the first tropical cyclone to have a significant impact on Indonesian land.

We have shown that the genesis of tropical cyclone Seroja in the region of Timor and Suvu Seas was associated with enhanced Equatorial Convection on March 27, 2021 which was preceded by warm sea surface anomalies (SSTs) in that region. The Equatorial Convection was related to Madden-Julian Oscillation (MJO) mode: it developed on the leading edge of MJO where SSTs were high. We have also investigated the role of tropical waves in the development of tropical cyclone Seroja. The interaction between convectively coupled equatorial Rossby wave and three convectively coupled Kelvin waves embedded within the larger-scale envelope of the MJO, provided a supportive environment for this extreme event. The Equatorial Convection that eventually became tropical cyclone Seroja moved southwest, boosted by environmental cyclonic vorticity associated with Rossby Wave. Each of the three Kelvin Waves that arrived over the Maritime Continent had a unique contribution in this event; structuring the convection, winds and precipitation patterns.

How to cite: Latos, B., Peyrillé, P., Lefort, T., Flatau, M., Flatau, P., Baranowski, D., Florida, N., Permana, D., and Szkółka, W.: The role of tropical waves in the genesis of tropical cyclone Seroja, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-566, https://doi.org/10.5194/egusphere-egu22-566, 2022.

09:42–09:48
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EGU22-1492
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Virtual presentation
Shoujuan Shu

The vertical structure of precipitation and its evolution during mid-level dry-air intrusion (DAI) for landfalling tropical cyclones (LTCs) over China in the past 10 years were examined using several observed TRMM PR products and a reanalysis dataset. We show that in the outer region where the environmental mid-level dry air intrudes more easily, the process of environmental DAI has an important effect on the vertical structure and precipitation of LTCs through promoting substantially stratiform precipitation while inhibiting infrequent intense convective precipitation. Although the total mean rain rate does not change much during the DAI period, both the mean rain rate and area of stratiform precipitation are almost doubled, while the convective precipitation halves compared to the situation prior to the DAI period. Also, the vertical structure of precipitation relative to the vertical wind shear (VWS) is modulated by the dry air, with a clear stratiform precipitation structure in the DAI region, though the dry-air distribution of LTCs does not depend on the direction of the VWS but rather on the synoptic environmental collocation. Further analysis shows that the mid-level DAI is favorable to the generation of stratiform precipitation through producing moderate mid-level convergence and less intense low-level subsidence, which contribute to the mid-level spin-up without spinning down the low-level circulation. At the same time, it helps maintaining the uniform stratiform precipitation above the melting layer and homogenizing the low-level circulation, and thus boosts the development of stratiform precipitation in intensity and area in the outer region.

How to cite: Shu, S.: How does dry air influence the precipitation of landfalling tropical cyclones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1492, https://doi.org/10.5194/egusphere-egu22-1492, 2022.

09:48–10:00
Discussion

Wed, 25 May, 10:20–11:50

Chairpersons: Leone Cavicchia, Enrico Scoccimarro

10:20–10:26
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EGU22-1725
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ECS
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Virtual presentation
Shirin Ermis and Ralf Toumi

Tropical cyclones (TCs) are some of the most dangerous natural hazards that human civilisation is exposed to. Effective adaptation for coastal regions requires reliable forecasts of risks for the season. Natural Hazard models such as the Synthetic Tropical cyclOne genRation Model (STORM) developed by Bloemendaal et al. (2020) are a common choice to assess risks without the expense of running a full forecast model. STORM has so far only been compared to observations on a basin-wide scale. However, for useful risk assessments in coastal regions, the model is also required to be skilful on much smaller spatial scales. We examine landfall statistics in some key areas such as the Gulf of Mexico.  Numerous indices for TC genesis have been developed over the past decades that aim to derive genesis locations from meteorological variables. None of the currently operational indices however is capable of realistically modelling interannual variability in genesis numbers and locations. Here, we compare the purely statistical Poisson interannual variability to that observed. Using Poisson regression between observations and driving environmental variables such as relative sea surface temperatures and wind shear, we then produce a new index for genesis location that has better predictive skill on interannual time scales.

How to cite: Ermis, S. and Toumi, R.: Modelling interannual variability in a tropical cyclone hazard model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1725, https://doi.org/10.5194/egusphere-egu22-1725, 2022.

10:26–10:32
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EGU22-3862
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Virtual presentation
Xiaodong Tang et al.

The diurnal variation of tropical oceanic convection has been recognized for decades. Recent observational studies have also documented a diurnal cycle associated with the upper-level cirrus canopy of tropical cyclone (TC) measured using the infrared brightness temperature from satellites. However, TC canopy clouds are not always coupled tightly with deep convection. The overshooting top (OT) is an appropriate proxy for deep convection with an intense updraft that can penetrate the tropopause, which has an important influence on typhoon intensification. So far, there are no observational evidences for the relationship between diurnal intensity variation and OT occurrences in TC.

We analyze the diurnal variation of OTs within 45 western North Pacific typhoons, using 9003 Himawari-8 satellite images and a unique OT detection algorithm. We examine the distribution of OTs in different types of typhoons in terms of both intensity and intensity change and the relationship between the OTs and typhoon intensification on a diurnal scale. Our results show that a greater OT density occurs in strong typhoons and rapid intensification (RI) typhoons. Moreover, RI typhoons showed greater diurnal variation than non-RI typhoons. The diurnal cycle of OT density in RI typhoons was in phase with the intensification of the typhoon, with the maximum in the early morning. These observational results are consistent with recently published case study simulations of the diurnal radiation effects on TC in both realistic and idealized scenarios. Therefore, OT density can become a potentially effective indicator to estimate diurnal changes in typhoon intensity.

How to cite: Tang, X., Sun, L., Zhuge, X., Tan, Z.-M., and Fang, J.: Diurnal Variation of Clouds Overshooting Tops Detected by Himawari-8 Satellite and Typhoon Intensity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3862, https://doi.org/10.5194/egusphere-egu22-3862, 2022.

10:32–10:38
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EGU22-6009
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ECS
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On-site presentation
Saranya Ganesh Sudheesh et al.

Tropical storms that develop over the North Indian Ocean basin pose a major threat to the extensive peninsular coastlines teeming with overpopulated cities and vast areas of low-lying farmlands. With each year, the economic and property losses due to storm-induced gales, landslides and flash floods over the coastlines are becoming more frequent. Reliable subseasonal prediction of tropical cyclogenesis over the landlocked North Indian Ocean basin has extreme demand and requires accurate rendition of the crucial parameters that influence the storm development. While several genesis potential indices are used for climatological monitoring and prediction of cyclogenesis globally, their skill in subseasonal prediction of individual storm development is limited, especially near coastlines. This study reviews an improved genesis potential parameter, namely IGPP, that can detect cyclogenesis, evolution and storm tracks from post-processed Multi-model ensemble outputs. The IGPP is a revised version of Kotal Genesis Potential Parameter (KGPP) introduced by the India Meteorological Department for short and medium‐range operational cyclogenesis prediction over the North Indian Ocean. We analyzed and compared the cyclogenesis prediction systems when multiple storm systems of different intensities develop simultaneously. Results show that false alarms and overestimation of values present in KGPP are remarkably reduced by using IGPP for all the cases. Moreover, IGPP outperforms KGPP in distinguishing between developing and non-developing storms by accurately representing the cyclogenesis and intensity variations. The mean IGPP shows better correlation with maximum wind speeds of selected storms, with an improvement of almost 34 % compared to KGPP, which we attribute to the changes in thermodynamic and shear terms. The thermodynamic term is modified as the mean equivalent potential temperature of the surface and middle troposphere to include the effect of warm sea surface and tropospheric latent heat release whereas the vertical wind shear between 850 and 200 hPa levels is averaged over an annular region between 100 and 200 km radii from the storm centres and rescaled. IGPP has replaced KGPP operationally and is successfully implemented as one of the indices for the extended range probabilistic prediction of cyclogenesis by the India Meteorological Department. Probabilistic predictions using IGPP has been instrumental in providing early guidance on storm formation and weekly forecasts are available at https://www.tropmet.res.in/erpas/.

How to cite: Sudheesh, S. G., Sahai, A. K., Sukumarapillai, A., Joseph, S., and Beucler, T.: An Improved Genesis and Evolution Parameter for Subseasonal Prediction of the North Indian Ocean Tropical Cyclones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6009, https://doi.org/10.5194/egusphere-egu22-6009, 2022.

10:38–10:44
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EGU22-6307
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ECS
Towards a process-oriented, diagnostic framework for understanding multi-scale variability among tropical cyclogenesis events
(withdrawn)
Jonathan Martinez et al.
10:44–10:50
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EGU22-6728
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ECS
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Virtual presentation
Kelly Núñez Ocasio and Rosimar Ríos-Berríos

Despite recent advancements in our understanding of tropical cyclogenesis (TCG), it remains an elusive research topic. Here, the Model for Prediction Across Scales (MPAS) is used to study the TCG case of the African easterly wave (AEW) that became Hurricane Helene (2006). This study has two main objectives: 1) evaluate MPAS high-resolution AEW hindcast capability by comparing MPAS simulations—initialized with data from both the Integrated Forecasting System (IFS) and the Global Forecast System (GFS)—with observations and, 2) analyze the role of moisture in the mechanisms that lead to Helene’s TCG. Both 15-km horizontal grid resolution simulations developed a more intense wave and ultimately tropical depression compared to observations. However, the track, intensity, and rainfall of the simulated pre-Helene when initializing with IFS were more comparable to those of observations than the simulation initialized with GFS. The simulated pre-Helene initialized with GFS was more intense than the IFS-initialized pre-Helene, with the track of the wave deviating farther east of the observed track, especially as it reached the west coast of Africa. The more intense GFS-initialized pre-Helene is associated with larger moisture availability in the boundary layer (BL), and stronger West African monsoon southwesterly winds in the mean state when compared to the IFS simulation and observations. A moisture flux convergence budget centered on the wave trough shows that during the wave’s lifetime the convergence term in the BL dominates and increases as the wave approaches TCG. However, TCG only happens when conditions are optimal—net moisture flux in the BL at the center of the wave increases in addition to increased mass convergence. These results could potentially be the link that explains the intersection between recent TCG theories. In the moisture-vortex instability (MVI), the wave-related flow advects moisture and temperature towards the synoptic-scale vortex, creating a favorable environment for convection near the vortex center, promoting TCG. The pre-genesis top-heavy profile to bottom-heavy profile during genesis, which provides vorticity convergence associated with TCG, could be a consequence of MVI attainment. It is the increase in wave-centered net moisture flux in combination with wave-centered mass flux convergence increase in the BL that ultimately could bring these theories together and help explain how TCG is achieved.

How to cite: Núñez Ocasio, K. and Ríos-Berríos, R.: African easterly wave evolution and tropical cyclogenesis in a pre-Helene (2006) hindcast using the Model for Prediction Across Scales (MPAS), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6728, https://doi.org/10.5194/egusphere-egu22-6728, 2022.

10:50–10:56
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EGU22-2727
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ECS
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Virtual presentation
Kasturi Singh and Jagabandhu Panda

Tropical cyclones (TCs) and warming climate, both possess significant importance to human life. Both of these aspects are quite interesting for several researchers as TCs are one of the deadliest systems formed over the ocean and the warming climate either enhances or suppresses their formation. Moreover, the landfalling TCs are the primary reason that causes a high death toll and property loss every year. Limited studies focused on the impact of the warming climate on the landfalling cyclonic disturbances (CDs) of the North Indian Ocean (NIO), and consequently the vulnerable states of India to TC landfall and rainfall occurrence. In order to conduct the study, the pre-warming period (PWP) is defined from 1880-1946 and the current warming period (CWP) from 1947 onwards based on the sea surface temperature (SST) variations over NIO.

             The analysis of the impact of warming climate on landfall activity of NIO CDs reveals that Bangladesh (BD), Andhra Pradesh (AP), and Tamil Nadu (TN) are more vulnerable to severe cyclones formed over the Bay of Bengal (BOB) during the CWP. Among western coastal states, Gujarat (GJ) is prone to severe cyclonic storms and Arabian Peninsula countries are vulnerable to cyclonic storms formed over the Arabian Sea (AS) during CWP. During PWP, the most vulnerable places to landfalling CDs were Odisha (OD), AP, and West Bengal (WB). Overall changes in the tracks of the CDs are noted during the CWP. Accordingly, BD and Arakan are found to be more vulnerable to landfalling CDs in the CWP pre-monsoon season, whereas in post-monsoon months, AP, TN, and BD are more prone coastal areas of BOB. The seasonal analysis of change in genesis location of CDs during PWP and CWP over both BOB and AS agrees well with the overall landfall locations. Altering in wind direction from NW to N-NW and increased meridional SST during CWP over BOB are found to be encouraging the landfall activity near AP and TN coasts. The W-SW and zonally distributed SST possibly supports landfall activity over Gujarat.

            Furthermore, the CD contributed rainfall (CDR) over India is also investigated using high-quality reliable daily rainfall data during CWP. Among eastern coastal states, the AP, TN, OD, and southern WB, and among western coastal states, Karnataka (KA) and Kerala (KL) suffer maximum rainfall from pre-monsoonal CDs. Gujarat received ~70%, and both AP and TN received up to 20-30% of CDR during pre-monsoon months. During the post-monsoon season, coastal AP, TN, OD, KA, and coastal KL received higher accumulated CDR. During the post-monsoon season, Gujarat, OD, and AP received a maximum rainfall contribution of up to 50%. Owing to the stable CDR trend along with decreasing CD frequency, the results indicate an increased rainfall contribution by CDs during the post-monsoon months. The current study would be highly beneficial for disaster management plans while India is experiencing developmental growth.

How to cite: Singh, K. and Panda, J.: The variability of landfalling cyclonic disturbances over North Indian Ocean and consequent rainfall contribution to India in warming climate scenario, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2727, https://doi.org/10.5194/egusphere-egu22-2727, 2022.

10:56–11:02
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EGU22-6808
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ECS
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Virtual presentation
Mincheol Moon and Kyung-Ja Ha

Approximately, 25.6 tropical cyclones (TCs) occur in the western North Pacific (WNP) each year, of which 3.4 TCs affect the Korean Peninsula (KP). In 2019, a record of seven TCs affected the KP. We investigated and elucidated the favorable conditions influencing the TCs approaching the KP using the Weather Research and Forecasting model version 4.0 (WRFv4). The cold Maritime Continent-warm WNP sea surface temperature (SST) was found to be a major factor. The effect of the SST gradient was examined for one representative case using WRFv4 with varying SST anomalies. This study suggests that the SST gradient-induced change in the circulation over the Maritime Continent is a main factor causing the TCs to approach the KP.

How to cite: Moon, M. and Ha, K.-J.: Abnormal Activities of Tropical Cyclones in 2019 Over theKorean Peninsula, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6808, https://doi.org/10.5194/egusphere-egu22-6808, 2022.

11:02–11:08
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EGU22-6921
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ECS
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Frederick Iat-Hin Tam et al.

Quantifiable assessment of how different physical processes promote tropical cyclone (TC) development is paramount in improving basic understanding of TC genesis and TC intensification forecasts. This assessment can be made via Eulerian budgets or by linearizing the equations of motion. For instance, the Sawyer-Eliassen equation gives the secondary circulation driven by a steady thermodynamic forcing. However, existing diagnostic frameworks often make implicit assumptions such as axisymmetry and temporally-averaged forcing, precluding discussions on how spatially heterogeneous or transient forcing may affect TC intensity. 

In this work, we combine principal component analysis with multiple linear regression to build a linear framework that predicts the evolution of three-dimensional wind fields at different forecast windows, based on current heating and wind conditions. We apply this model to ensembles of WRF simulations on Hurricane Maria (2017) and Typhoon Haiyan (2013). Uniquely, the simulations include cloud radiative feedback denial experiments, which enables us to quantify the extent to which radiative processes drive TC intensification. Given their simplicity, our models are reasonably accurate, with coefficients of determination exceeding 0.8 for forecast windows longer than six hours. The linear nature of our model allows us to cleanly decompose the contributions of different physical processes to three-dimensional TC kinematic changes. Using radiative heating as an example, preliminary results suggest that this heating creates outward-propagating diurnal variability in wind perturbations during critical intensification periods of Hurricane Maria. These wind perturbations resemble a shallow lower-tropospheric secondary circulation; implications of this circulation to TC intensification are explored. 

More generally, our framework can map thermodynamic forcing to kinematic changes without relying on axisymmetric assumptions, which opens the door to data-driven discovery of the leading physical pathways to TC intensification.

How to cite: Tam, F. I.-H., Beucler, T., and Ruppert, J.: A Data-Driven Approach to Isolate the Role of Radiative Heating in Tropical Cyclone Intensification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6921, https://doi.org/10.5194/egusphere-egu22-6921, 2022.

11:08–11:14
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EGU22-9805
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Virtual presentation
Allison Wing et al.

Process-oriented diagnostics of tropical cyclones (TCs) facilitate a comparison of models to observations with respect to the physical processes relevant to TCs, informing which processes to target for model improvement. Here we use diagnostics based on the column-integrated moist static energy (MSE) variance budget, which focuses on how convection, moisture, clouds, and related processes are coupled. We use five different reanalysis datasets to provide an 'observation'-based reference against which high-resolution global climate models can be evaluated: ERA-Interim, MERRA-2, CFSR, ERA-5, and JRA-55. We calculate the budget in 10 x 10 degree boxes following tracked TCs composites over storm snapshots of the same intensity. The composites are qualitatively similar to prior work, with radiative feedbacks contributing most to MSE variance growth in the early stages of TC development and in weaker storms, and with surface flux feedbacks increasing strongly with intensity. Reanalyses that have a stronger radiative feedback, normalized by the box-mean MSE variance, in a given intensity bin exhibit a higher percentage of storms that intensify to the next bin, which emphasizes the value of the MSE variance budget as a process-oriented diagnostic for understanding model simulation of TCs. However, there is a large spread in MSE variance and the radiative and surface flux feedback contributions to MSE variance growth across reanalyses, even when considering composites over storms of the same intensity. The spread across reanalyses is comparable to the spread across the high-resolution climate models considered by Wing et al. (2019). This suggests that the data assimilation present in reanalyses does little to constrain the TC-MSE variance budget and indicates that caution must be taken when evaluating climate models against reanalysis. Ongoing work continues to evaluate climate model simulations, including those from the HighResMIP ensemble, against the reanalysis-based reference, and examine the large-scale environments associated with TC formation in reanalyses.

How to cite: Wing, A., Dirkes, C., Camargo, S., Kim, D., and Moon, Y.: Process-oriented diagnosis of tropical cyclones based on the moist static energy variance budget in reanalyses and high-resolution climate models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9805, https://doi.org/10.5194/egusphere-egu22-9805, 2022.

11:14–11:20
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EGU22-1254
Ii Lin et al.

Devastating Japan in October 2019, Supertyphoon (STY) Hagibis was an important typhoon in the history of the Pacific. A striking feature of Hagibis was its explosive RI (rapid intensification). In 24 h, Hagibis intensified by 100 kt, making it one of the fastest-intensifying typhoons ever observed. After RI, Hagibis’s intensification stalled. Using the current typhoon intensity record holder, i.e., STY Haiyan (2013), as a benchmark, this work explores the intensity evolution differences of these 2 high-impact STYs.

We found that the extremely high pre-storm sea surface temperature reaching 30.5∘C, deep/warm pre-storm ocean heat content reaching 160 kJ cm-2, fast forward storm motion of ~8 m s-1, small during-storm ocean cooling effect of ~ 0.5∘C, significant thunderstorm activity at its center, and rapid eyewall contraction were all important contributors to Hagibis’s impressive intensification. There was 36% more air-sea flux for Hagibis’s RI than for Haiyan’s.

After its spectacular RI, Hagibis’s intensification stopped, despite favorable environments. Haiyan, by contrast, continued to intensify, reaching its record-breaking intensity of 170 kt. A key finding here is the multiple pathways that storm size affected the intensity evolution for both typhoons. After RI, Hagibis experienced a major size expansion, becoming the largest typhoon on record in the Pacific. This size enlargement, combined with a reduction in storm translational speed, induced stronger ocean cooling that reduced ocean flux and hindered intensification. The large storm size also contributed to slower eyewall replacement cycles (ERCs), which prolonged the negative impact of the ERC on intensification.

How to cite: Lin, I., Rogers, R. F., Huang, H.-C., Liao, Y.-C., Herndon, D., Yu, J.-Y., Chang, Y.-T., Zhang, J. A., Patricola, C. M., Pun, I.-F., and Lien, C.-C.: Ocean Interaction and the Intensity Evolution of Two High-Impact Super Typhoons: Hagibis (2019) and Haiyan (2013), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1254, https://doi.org/10.5194/egusphere-egu22-1254, 2022.

11:20–11:26
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EGU22-7760
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ECS
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Virtual presentation
Qiuyun Wang and Jianping Li

Understanding of the El Niño phenomenon is improving and several studies have considered the dynamics of El Niño diversity, however, the important role of tropical cyclones has not been reported. Here we show a clear influence of tropical cyclones over the western North Pacific on the spatial pattern of El Niño: By changing the Walker circulation and equatorial thermocline, strong (weak) accumulated cyclone energy helps to shift the center of strongest sea surface temperature anomalies three months later to the equatorial eastern (central) Pacific. The greater number of central-Pacific El Niño events after 1999/2000 may be associated with weaker accumulated cyclone energy in this period. A modified physically based empirical model (ACE+SST model) for predicting El Niño spatial patterns is constructed that captures well the spatiotemporal characteristics of El Niño events. Taking into account the key influence of western North Pacific tropical cyclones on El Niño diversity will improve our understanding and prediction of El Niño.

How to cite: Wang, Q. and Li, J.: Feedback of tropical cyclones on El Niño diversity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7760, https://doi.org/10.5194/egusphere-egu22-7760, 2022.

11:26–11:32
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EGU22-12721
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Virtual presentation
Shuyi Chen and Edoardo mazza

This study focuses on the influence of the North Atlantic Oscillation (NAO) on interannual tropical cyclone (TCs) activity and rainfall using observational and reanalysis products. Using Poisson regression models, we show that the low-frequency NAO variability is associated with a distinct pattern of TC activity across the North Atlantic basin. Across the western Atlantic, the Caribbean Basin and the Gulf of Mexico, TC activity increases as the NAO decreases: an interquartile range decrease in the NAO corresponds to a 30-40 % increase in TC track density. While the NAO is known to affect the weather regimes of the mid-latitudes, we show that its low-frequency component influences the large-scale environment across Main Development Region. The negative NAO phase is associated with significantly higher Sea-Surface Temperature (SST) and lower tropospheric wind shear. Finally, we investigate whether the NAO influence on TC activity can be detected in the basin-scale variations of TC rainfall. By building monthly rainfall composites from satellite and reanalysis products, we show that TC rainfall is indeed strongly enhanced in the Caribbean and in the Gulf of Mexico during the negative phase of the NAO. Such modulation is particularly evident during neutral or La Niña conditions.

How to cite: Chen, S. and mazza, E.: Modulation of Tropical Cyclone Activity and Rainfall by the North Atlantic Oscillation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12721, https://doi.org/10.5194/egusphere-egu22-12721, 2022.

11:32–11:38
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EGU22-2080
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ECS
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Jeffrey Thayer and Deanna Hence

The Madden-Julian Oscillation (MJO) can create favorable conditions for tropical cyclone (TC) genesis in the Indian Ocean, but past work has not thoroughly investigated how TCs after genesis may influence MJO convection development. This work utilizes long-term composite analysis to broadly establish the relationship between Indian Ocean TCs and each MJO phase over the Indian basin, and then seeks to isolate direct impacts of the TCs on MJO convection coverage and intensity.

We first examine Indian Ocean TC interactions with MJO convection using daily-mean ERA5 reanalysis and TRMM precipitation products from 1998-2018 for TC and non-TC days per MJO phase, excluding 3 days before and after Best-Track TC lifespans to reduce contamination of non-TC composites. Preliminary analysis suggests that TC periods are associated with stronger MJOs, with an anomalously stronger MJO large-scale circulation and associated subsidence over the equatorial Indian Ocean. We find higher CAPE and increased TRMM rainfall during convectively-active MJO phases over the eastern Indian Ocean when TCs are present, but increased dry-air advection, greater CIN, and decreased TRMM rainfall over the western Indian Ocean during the same phases. These findings allude to suppression of MJO convection development during TC periods in the western MJO convective envelope, with coincident enhancement of MJO convection in the eastern MJO convective envelope. While these broad conclusions are consistent during non-convectively-active MJO phases, changing MJO strength during TC periods for convectively-active MJO phases limit our ability to quantify TC impacts on MJO convection using only composite analysis.

To better quantify TC influences, we next isolate direct TC impacts on MJO convection using metrics for the TC range of influence, likelihood of interaction with MJO convection, and strength of TC-MJO convection interactions. Since a TC’s influence likely extends beyond the 34-kt wind radii provided by Best-Track, we determine an “outer wind radius” by integrating a radial wind model outward from each TC eye to ~5 m/s. We next quantify the overlapping area between each TC outer wind radius and the coincident MJO convection by using the MJO precipitation boundary determined from a large-scale precipitation tracking dataset, with the size of the overlapping area providing the likelihood of interaction. For time periods when TC outer wind radii and MJO convection overlap, we compare the convection coverage and intensity observed by TRMM between MJO convection sectors with and without TC wind overlap. The strength of a TC-MJO convection interaction is finally quantified by comparing the convection coverage and intensity between these sectors. With climatological statistics on the likelihood and strength of TC-MJO interactions, future MJO prediction and Maritime Continent rainfall forecasts could be adjusted according to the presence or absence of Indian Ocean TCs.

How to cite: Thayer, J. and Hence, D.: Tropical Cyclone Interactions with Madden-Julian Oscillation Convection in the Indian Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2080, https://doi.org/10.5194/egusphere-egu22-2080, 2022.

11:38–11:50
Discussion

Wed, 25 May, 13:20–14:50

Chairpersons: Eric Maloney, Enrico Scoccimarro

13:20–13:26
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EGU22-271
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ECS
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On-site presentation
Eliza Karlowska et al.

Forecasting the Madden-Julian Oscillation (MJO) is challenging for many numerical weather prediction (NWP) systems and climate models. Models tend to simulate slower MJO propagation than in observations, impacting other weather and climate patterns across the world through its teleconnections. Observations show that sea surface temperatures (SST), and subsequently sea surface fluxes influence MJO convection in the tropics. Coupled ocean-atmosphere models, which dynamically predict SST, tend to perform better in forecasting the MJO than atmosphere-only models that use persisted SST. Lower resolution coupled climate models are routinely used by forecasting centres, however, there are only a few operational weather forecasts utilising high resolution coupled NWP systems. The Met Office has developed a coupled NWP system running in near real-time since May 2016, alongside their operational, atmosphere-only NWP system. Comparison between the models using the Real-time Multivariate MJO index reveals that both are similarly skillful within 7 and 10 forecast days for operational and coupled models, respectively. The coupled model produces faster MJO propagation than the operational model. Consistent with this faster propagation, coupled model forecasts initiated during active MJO convection over the Indian Ocean (RMM phase 1), show enhanced convection by lead day 7 in the Sulawesi-Banda Sea region located ahead (to the east) of the convective envelope. Warm SST anomalies of order 0.1°C in that region are simulated in the coupled model composites from lead day 1, consistent with observations. When the coupled model is initialised with active MJO convection over the Maritime Continent (RMM phase 4), the model suppresses convection faster in the equatorial Indian Ocean region, which is behind (to the west) of the MJO convection. Cold SST anomalies are created in the coupled model from lead day 1 in that region, stronger than observations suggest and leading to excessive suppression of convection in the coupled model here. We explain the differences between the coupled and atmosphere-only simulations through a combination of upwelling diagnostics, analysis of moisture budget terms and targeted numerical experiments for SST-convection feedback.

How to cite: Karlowska, E., Matthews, A., Webber, B., Graham, T., and Xavier, P.: Sea surface temperature impact on Madden-Julian Oscillation convection in the Met Office coupled and atmosphere-only forecast models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-271, https://doi.org/10.5194/egusphere-egu22-271, 2022.

13:26–13:32
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EGU22-11037
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ECS
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Virtual presentation
Seung-Yoon Back et al.

The Madden-Julian oscillation (MJO) events can be categorized into four types based on their propagation characteristics: standing, jumping, slow-propagating, and fast-propagating types. While the characteristics of each MJO type have been documented in the literature, it remains unknown whether such diversity is realistically represented in the state-of-art climate models. This study evaluates the MJO diversity in 28 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. We find that many CMIP6 models reasonably reproduce the MJO diversity although the relative frequency of propagating types tends to be underestimated. When individual models are grouped into the GOOD and POOR models by considering the performance in capturing propagation pattern of the four MJO types, the GOOD models show a much stronger relationship between MJO type and underlying sea surface temperature (SST) anomalies, especially for standing and fast-propagating types. We find a systematic difference in the model biases in the climatological mean SST and column water vapor between the GOOD and POOR models, with the POOR models exhibiting much stronger cold and dry biases over the equatorial western Pacific. Our results suggest that the MJO diversity can be improved by reducing model mean bias.

How to cite: Back, S.-Y., Kim, D., and Son, S.-W.: MJO diversity in CMIP6 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11037, https://doi.org/10.5194/egusphere-egu22-11037, 2022.

13:32–13:38
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EGU22-9672
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On-site presentation
Eric Maloney and Michael Natoli

The diurnal cycle in tropical islands is critical for water supplies and agriculture, and potentially has important feedbacks onto longer timescale tropical disturbances. However, the conditions that regulate the diurnal cycle of rainfall on such islands remain poorly understood.  Here, we use observations and a cloud resolving model to understand intraseasonal variability of the diurnal cycle in and near Luzon during boreal summer. 

The boreal summer intraseasonal oscillation (BSISO) and quasi biweekly oscillation (QBWO) modulate the Luzon diurnal cycle in a similar manner during their lifecycles. In particular, the diurnal cycle of rainfall and offshore propagation of rainfall into the South China Sea are maximized during phases of the BSISO and QBWO characterized by increasing tropospheric moisture, sufficient insolation, and weak offshore easterly flow. These conditions occur in advance of the large-scale convective envelope of the BSISO and QBWO. Such a phase relationship suggests that enhanced diurnal rainfall may aid northward propagation of intraseasonal disturbances in the South China Sea.

The Cloud Model 1 (CM1) integrated at 1 km horizontal grid spacing is used to isolate the relative importance of background wind, humidity, and insolation for the diurnal cycle in and near an idealized tropical island resembling Luzon. By prescribing daily mean BSISO background wind variations through nudging, it is shown that the wind direction and speed are major regulators of offshore propagation of diurnal disturbances on the west side of the island. In particular, offshore propagation is maximized during periods of offshore easterly flow at low levels. Other experiments that test sensitivity to the vertical profile of wind are discussed. We also show that BSISO humidity and surface shortwave radiation variations are major contributors to diurnal cycle variability.  These results not only have implications for the diurnal cycle in the Philippines, but also other tropical islands such as Sumatra with prominent diurnal cycles modulated by intraseasonal variability.

 

How to cite: Maloney, E. and Natoli, M.: Intraseasonal Variability of the Philippines Diurnal Cycle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9672, https://doi.org/10.5194/egusphere-egu22-9672, 2022.

13:38–13:44
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EGU22-212
Quasi-biweekly oscillation over South-West Indian Ocean during boreal winter: features, propagation dynamics and its impact on Tropical Cyclone genesis and tracks
(withdrawn)
Sambrita Ghatak and Jai Sukhatme
13:44–13:50
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EGU22-1083
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ECS
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Virtual presentation
Hagar Bartana et al.

The simulation of the Madden-Julian Oscillation (MJO) and convectively coupled equatorial waves (CCEWs) is considered in 13 state-of-the-art models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). We use frequency-wavenumber power spectra of the models and observations for Outgoing Longwave Radiation (OLR) and zonal velocity at 250 hPa (U250), and consider the historical and end-of-century projections for the SSP245 and SSP585 scenarios. The models simulate a spectrum quantitatively resembling that observed, though systematic biases exist. MJO and Kelvin waves (KW) are mostly underestimated, while equatorial Rossby waves (ER) are overestimated. The models project a moderate future increase in power for the MJO, a robust increase for Kelvin waves (KW) and weaker power values for most other wavenumber-frequency combinations, including higher wavenumber ER. In addition to strengthening, KW also shift toward higher phase speeds (or equivalent depths). Models with a more realistic MJO in their control climate tend to simulate a stronger intensification, and models with a more realistic KW in their control climate tend to simulate a weaker intensification. 

 

How to cite: Bartana, H., Garfinkel, C., Shamir, O., and Rao, J.: Projected Future Changes in Equatorial Wave Spectrum in CMIP6, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1083, https://doi.org/10.5194/egusphere-egu22-1083, 2022.

13:50–13:56
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EGU22-1500
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ECS
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On-site presentation
Namendra Kumar Shahi et al.

The dominant modes of variability at the sub-seasonal to seasonal (S2S) time scales in the tropical atmosphere, with particular emphasis on the Madden-Julian oscillation (MJO), is investigated using the observed NOAA outgoing longwave radiation (OLR). This study focuses on the boreal winter season (November to April) of the period 1993-2016. The multi-channel singular spectral analysis (MSSA) method is introduced and used to isolate the dominant modes associated with the tropical boreal winter daily OLR anomalies. The results show that the dominant MSSA mode consists of an intraseasonal oscillation with a period of around 35-day and two seasonally persistent modes. The 35-day oscillation is related to the intraseasonal convective activity of the tropics with little contribution to the seasonal mean value, and the phase composites and propagation characteristics of the 35-day mode are almost identical to the MJO (referred to as the MJO mode). The seasonally persistent modes are characterized by large-scale patterns that prevail over most of the tropics with the same sign anomalies throughout the boreal winter season, which represent inter-annual variations and are related to the El Niño-Southern Oscillation (ENSO) pattern. In this work, the MSSA based analysis is applied to the fully coupled (atmosphere-ocean-land-cryosphere) Euro-Mediterranean Center on Climate Change (CMCC) seasonal prediction system to evaluate the model's performance in simulating observed dominant modes of the boreal winter tropical variability.

How to cite: Shahi, N. K., Scoccimarro, E., Gualdi, S., Peano, D., and Navarra, A.: Fidelity of the CMCC SPS model in simulating the dominant mode of tropical variability in boreal winter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1500, https://doi.org/10.5194/egusphere-egu22-1500, 2022.

13:56–14:02
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EGU22-2538
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On-site presentation
Adrian Matthews

Convectively coupled equatorial Kelvin waves (CCKWs) are tropical weather systems that can lead to extreme precipitation. A vorticity budget for CCKWs over the Indian Ocean is presented, to identify the basic mechanisms of eastward propagation and growth. The budget is well closed, with a small residual. In the lower troposphere, CCKWs behave like strongly modified theoretical equatorial Kelvin waves. Vortex stretching, from the divergence of the Kelvin wave acting on planetary vorticity (the −f D term), is the sole mechanism by which the vorticity structure of a theoretical Kelvin wave propagates eastward. In the lower and middle troposphere, this term is also the key mechanism for the eastward propagation of CCKWs but, due to subtleties in its structure and phasing linked to a combination of modal structures, it also contributes to growth. Unlike in the theoretical Kelvin wave, other vorticity source terms also play a role in the propagation and growth of CCKWs. In particular, vortex stretching from relative vorticity (the - zeta D term) is the largest source term, and this leads strongly to growth, through interactions between the background and perturbation vorticity and divergence. Horizontal vorticity advection by the background flow contributes to propagation, and also acts to retard the growth of the CCKW. The sum of the source terms in this complex vorticity budget leads to eastward propagation and growth of CCKWs. The structure and vorticity budget of CCKWs in the upper troposphere is quite unlike that of a Kelvin wave, and appears to arise as a forced response to the lower-tropospheric structure.

How to cite: Matthews, A.: Dynamical propagation and growth mechanisms for convectively coupled equatorial Kelvin waves over the Indian Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2538, https://doi.org/10.5194/egusphere-egu22-2538, 2022.