Shallow cumulus clouds (SCC) play a vital role in regulating the Earth’s energy and water cycles, yet their accurate representation in numerical weather prediction and climate models remains a significant challenge. This study employs realistic large-eddy simulations (LES) using the ICON model to analyze both instantaneous and lifetime-averaged statistics of SCC observed on three different days during the FESSTVaL campaign. The excess of virtual potential temperature within the cloud is used to categorize the clouds into active and passive states. The estimated cloud mass flux follows the Weibull distribution, with distinct shape parameters for active and passive clouds, reflecting the memory of the random process. The unity shape parameter for passive clouds indicates memorylessness, while a shape parameter less than unity for active clouds highlights the role of convective memory, where past convection influences the current convective state.

Additionally, the mass flux distribution varies significantly across different cases. These differences can be explained in terms of efficiency, which depends on energy partitioning, the Bowen ratio, and large-scale forcing, and is conceptually linked to approximating moist atmospheric convection as a moist heat engine. This further highlights the role of turbulent fluxes and boundary layer dynamics in shaping the efficiency, which governs the estimation of moist static energy under varying environmental conditions. These findings enhance our understanding of SCC dynamics and offer valuable insights for improving cloud parameterizations in weather and climate models. This study underscores the crucial role of realistic numerical simulations in addressing the challenges of atmospheric convection and turbulence, particularly at the gray-zone scale.

How to cite: Singh, J., Sakradzija, M., and Schmidli, J.: Investigating the Cloud Base Mass Flux and Its Controlling Factors in Shallow Cumulus: Insights from Realistic Large-Eddy Simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7335, https://doi.org/10.5194/egusphere-egu25-7335, 2025.

We develop a novel approach to detect cloud-subcloud coupling during the cloud life cycle and analyze a large eddy simulation of marine shallow cumulus based on the Barbados oceanographic and meteorological experiment campaign. Our results demonstrate how the activity of sub-cloud coherent updrafts (SCUs) affect the evolution of shallow cloud properties during their life cycles, from triggering to development, and through to dissipation. Most clouds (~80%) are related to SCUs during their lifetime but not every SCU (~20% for short-lived ones) leads to cloud formation. The fastest growing SCUs in a relatively moist region are most likely to initiate clouds. The evolution of cloud base mass-flux depends on cloud lifetime. Compared with short-lived clouds, longer lived clouds have longer periods of development, even normalized by the full lifetime, and tend to increase their cloud base mass-flux to a stronger maximum. This is consistent with the evolution of mass flux near the top of SCU, indicating that the development of clouds is closely related to the sub-cloud activity. When the SCUs decay and detach from the lifting condensation level, the corresponding cloud base starts to rise, signifying the start of cloud dissipation, during which the cloud top lowers to approach the rising cloud base. Previous studies have described similar conceptual pieces of this relationship but here we provide a continuous framework to cover all the stages of cloud-subcloud coupling. Our findings provide quantitative evidence to supplement the conceptual model of the shallow cloud life cycle and could help to improve the steady-state assumption in parameterization.

How to cite: Holloway, C. E., Gu, J.-F., and Plant, R. S.: Connections Between Sub-Cloud Coherent Updrafts and the Life Cycle of Maritime Shallow Cumulus Clouds in Large Eddy Simulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7055, https://doi.org/10.5194/egusphere-egu25-7055, 2025.

This work describes the study of the influence of vertical wind shear (hereafter "shear") on deep convective clouds. Using a set of high-resolution Large-Eddy Simulations (LES) produced by the research model Meso-NH with varying shear, tendencies in the relationship between shear magnitude and the organisation and intensity of the storms are drawn. Increasing shear is associated with higher precipitations, stronger ascent in the updraft, and more intense cold pools under the convective cells. When the shear becomes strong enough, the convective cells evolve into supercells, drastically changing the regime of the event and highlighting a non-linearity in the behaviour of convective systems. Turbulent quantities are affected, with higher subgrid and resolved turbulent kinetic energy (TKE) for higher intensity storms. Moreover, the upwind TKE is higher than the downwind TKE, although the ratio for all simulations is not affected. Using four different indicators of organisation, a clear trend towards increasing organisation is diagnosed, with the supercell regime diverging from the other simulations. Vertical wind shear, via its effect on the organisation of convective cells, significantly alters the effect of convective storms, and should be taken into account by parametrization schemes.

How to cite: Bidou, G., Ricard, D., and Lac, C.:  Influence of vertical wind shear on organisation and decametric-scale turbulence in convective clouds using large-eddy simulations., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8868, https://doi.org/10.5194/egusphere-egu25-8868, 2025.

Atmospheric convection can spontaneously cluster and confine within an envelope. These clusters of convection often propagate under the influence of a large-scale mean wind, such as the Madden-Julian oscillation (MJO). The motivation of this study is to understand how a large-scale mean wind influences the propagation of a convection cluster. To this end, we investigate the response of convective self-aggregation, a model depiction of a convection cluster in radiative-convective equilibrium (RCE), to a wind perturbation. We impose a constant mean wind on an aggregated convective system (obtained through a simulation without mean wind) and observe its evolution in a three-dimensional cloud-resolving model. We note in our modeling experiments that convection clusters exhibit a propagating behavior under a large-scale mean wind, albeit with a speed that is less than the prescribed forcing. We find that surface fluxes are critical in slowing down the convection clusters. Enthalpy and momentum fluxes slow down the convection cluster with comparable effects through different mechanisms. Enthalpy fluxes favor convection upwind through the wind-induced surface heat exchange (WISHE) feedback, opposing the convection cluster movement. Momentum flux acts as a negative feedback on surface winds in places of strongest near-surface winds.

How to cite: Goswami, B. B., Aubel, A., and Muller, C.: How does a convection cluster respond to a large-scale mean wind?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8288, https://doi.org/10.5194/egusphere-egu25-8288, 2025.

Downdrafts play an essential role in the feedback between deep convective clouds and their surrounding environment, and they must be properly accounted for in climate model parameterization schemes. Such downdrafts found near and in-cloud, such as subsiding shells and hydrometeor-loaded downdrafts, significantly contribute to downward mass flux in the lower and middle troposphere. However, environmental links to driving mechanisms and characteristics of downdrafts must be understood for proper implementation in parameterization schemes. Using CM1, simulations modeling convection were performed utilizing weakly-sheared dry and wet season composite soundings compiled during the Green Ocean Amazon Campaign, as well as similar thermodynamic soundings with a prescribed increase of vertical wind shear. The soundings in this study were adapted to isolate relative humidity and shear effects on convective downdrafts. All deep convective updrafts in the simulations that met a required vertical velocity threshold were analyzed, along with their near-cloud environments and associated downdrafts. Magnitude differences of subsidence in the matrix of environments encouraged a parcel trajectory analysis, which showed that downward accelerations were primarily driven by negative buoyancy accelerations and were aided by cloud-top pressure perturbations. Compared to other near-cloud downdrafts, subsiding shell accelerations relied heavily on strongly negative thermal buoyancy for downward accelerations but were also moderated by upward vertical perturbation pressure gradient accelerations away from cloud top, ultimately making them weaker than all other downdrafts. Future work aims to increase understanding of and improve mass transport processes found in near-cloud downdrafts and apply such understanding to climate model cumulus and convective parameterization schemes.

How to cite: Mulhern, Q. and Peters, J.: Environmental Drivers and Dynamics of Downdrafts in Simulations of Convection, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4833, https://doi.org/10.5194/egusphere-egu25-4833, 2025.

Examination of the Tropical Rainfall Measuring Mission (TRMM) satellite database (1994-2015) of 272 tropical and subtropical islands reveals a modest weakening of convective intensity with increased terrain height,h or ambient wind,U (for a given island area, A), and a strengthening with increasing A. Quasi-idealized, convection-permitting simulations broadly reproduce these sensitivities to h and A, but not that to U. In both observations and simulations, intensity increases with the island-averaged convective available potential energy (CAPE). Because CAPE generally decreases over taller islands that protrude deeper into the free troposphere, convective intensity varies inversely with h. The frequency of convective events increases with total island area over which both large CAPE and strong near-surface horizontal convergence coincide. This trend favors higher frequencies over larger islands with complex (but shallow) terrain. The model's inability to reproduce the observed decrease of convective intensity with U stems from a negative observed correlation between CAPE and U that was neglected in the simulations. Thus, as with h, the negative observed trend between intensity and U ultimately stems from the impacts of CAPE on convective intensity.

How to cite: Robinson, F., Kirshbaum, D., Sherwood, S., Cahill, L., Juliano, E., and Liu, C.: Investigating the effects of orography and ambient wind on deep convection over tropical islands, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1221, https://doi.org/10.5194/egusphere-egu25-1221, 2025.

In the tropics, a significant portion of precipitation originates from deep convective systems (DCS), which are composed of convective cells organized in both space and time. These systems are characterized by upper-level cloud shields made up of high-altitude, ice-topped clouds that form cohesive and recognizable structures, easily identifiable in satellite imagery. These cloud shields vary widely in spatial and temporal scales, ranging from a few dozen to millions of square kilometers and lasting from a few hours to several days. Due to their ubiquity over tropical oceans, these cloud shields play a critical role in the Earth's radiation budget and influence related climatic feedbacks. However, their potential morphological changes in response to climate change remain poorly understood.

In this study, we analyze the sensitivity of the cloud shield morphology to environmental conditions using a comprehensive dataset spanning nine years of satellite observations over the entire tropical ocean. By combining this dataset with the recent ECMWF reanalysis, we build robust statistics to explore the relationship between cloud shield morphology and environmental factors. Our focus is on a specific dimension of this complex problem: investigating how the thermodynamic and dynamic environment influences the morphology of the cloud shield. This work advances previous studies by encompassing the full spectrum of deep convective systems (DCS), rather than focusing solely on mesoscale convective systems (MCS). Moreover, we emphasize the cloud shield characteristics of these systems, going beyond the traditional focus on precipitation features morphology. Multilinear regression between DCS morphology and environment is used in a 2D phase space linked to the life cycle of the systems, namely the time to reach the maximum extension and the associated maximum area.

In this presentation, we will show that dynamical drivers exert stronger morphological control than the thermodynamic factors. The result reveals an overwhelming role for wind shear over a deep tropospheric layer in explaining the scale dependence of cloud shield morphology. In particular, the variability of the shield growth rate is very well explained by deep layer shear. The depth of the systems is also strongly related to dynamics and secondly to water vapor loading. These results feed the debate on the relative role of deep- vs. low-level shear in influencing deep convection.

How to cite: Fiolleau, T., Roca, R., and Netz, L.: Scale-dependence of tropical oceanic deep convective systems’ cloud shield morphology to environmental conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11683, https://doi.org/10.5194/egusphere-egu25-11683, 2025.

Upper tropospheric clouds are most abundant in the tropics and often form as cirrus anvils from convective outflow, building mesoscale systems (MCS). While latent heating is released into the atmosphere by the precipitating parts of these MCSs, the long-lasting anvils play a crucial role in modulating the Earth’s energy budget and heat transport. Convective organization may change the relationship between latent and radiative heating within the MCSs.

We present a coherent long-term dataset which describes tropical UT cloud systems for process and climate studies. In order to investigate also the cirrus surrounding these anvils, we used CIRS (Clouds from IR Sounders) data, retrieved from AIRS (Atmospheric InfraRed Sounder) and IASI (Infrared Atmospheric Sounding Inferometer) measurements, together with atmospheric and surface properties from the meteorological ERA reanalyses as input to artificial neural network (ANN) models to simulate the cloud vertical structure and radiative heating rates derived from CloudSat radar – CALIPSO lidar measurements, available only along narrow nadir tracks. In this way, we could expand this sparse sampling in space and in time. Furthermore, a rain rate classification, with an accuracy of about 70%, allows us to build objects of strong precipitation to identify convective organization. This dataset is now available at https://gewex-utcc-proes.aeris-data.fr/data/.

We could demonstrate that this rain intensity classification is more efficient than cold brightness temperatures to detect large latent heating, the latter derived from radar measurements of the Tropical Rainfall Measuring Mission (TRMM). While TRMM provides a diurnal sampling over a month, the spatial coverage within a time window of one hour is only about 7%. Therefore, we also expanded these latent heating profiles over the whole tropics, using ANN regression. The zonal averages of vertically integrated latent heating (LP) align well with those from the full diurnal sampling of TRMM–SLH over ocean.

In combination with a cloud system analysis we found that deeper convection leads to larger heavy rain areas, with a slightly smaller thick anvil emissivity. Convective organization enhances the mean atmospheric cloud radiative effect (ACRE) of the MCSs, in particular at small rain intensity. The projection of different MCS properties in the LP-ACRE plane can be further used for a process-oriented evaluation of parameterizations in climate models.

How to cite: Chen, X., Stubenrauch, C., and Mandorli, G.: Diabatic heating of mesoscale convective cloud systems from synergistic satellite data , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6542, https://doi.org/10.5194/egusphere-egu25-6542, 2025.

Mesoscale convective systems (MCSs) are large, organised storms that threaten communities in multiple regions around the world with extreme rainfall, lightning and strong winds that can lead to flooding, mudslides, destruction of property and loss of life. Improving predictability of these storms is vital for reducing their impact on the population and requires understanding of processes that favour their growth.

Our recent observation-based analysis of thousands of MCSs across seven storm “hot-spots” (West Africa, South Africa, India, China, South America, Great Plains and Australia) revealed a new mechanism of storm enhancement by mesoscale (~500 km) soil moisture gradients via vertical wind shear, a key ingredient for MCS growth. Specifically, a 10-30% increase in extreme (90^th percentile) precipitation feature size and rainfall was observed on days with favourable surface conditions, compared to days with unfavourable conditions.

In the current work we exploit multidecadal global convection permitting high-resolution (10 km) ICON simulation to analyse surface driven MCS enhancement under climate change. For the seven regions considered in the observational analysis, in ICON we find precipitating mature storms to be favoured in the vicinity of mesoscale soil moisture gradients and a strong relationship between vertical wind shear and storm size and rainfall, consistent with the observations. For an SSP370 type scenario (7 W/m² forcing by the year 2100) we show the impact of changing surface conditions on MCS enhancement linked to our identified mechanism.

How to cite: Barton, E., Klein, C., Taylor, C., Marsham, J., Parker, D., Maybee, B., Feng, Z., Leung, L. R., and Hohenegger, C.: Strengthening of Mesoscale Convective Systems by Soil Moisture Gradients in ICON , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5932, https://doi.org/10.5194/egusphere-egu25-5932, 2025.

Resolving deep convection using a horizontal grid spacing of 10 km or finer was supposed to produce a correct representation of tropical precipitation. Global coupled or uncoupled storm-resolving simulations using the ICOsahedral Non-hydrostatic model (ICON) show a proper representation of the tropical rainbelt over land. However, the tropical rainbelt over the western Pacific shows a double structure, and the uncoupled simulation relates this bias to the lack of precipitation over the warm pool. We test three hypotheses based on an energetic framework to explain the lack of precipitation over the warm pool, 1) the radiative effect of high clouds, 2) too-frequent or efficient shallow precipitating clouds, and 3) surface heat fluxes in light near-surface winds. Experiments show that in ICON, increasing surface heat fluxes over light near-surface winds produces more precipitation over the warm pool, giving a single tropical rainbelt over the Western Pacific. An increased radiative effect of high clouds did not increase warm pool precipitation due to compensation with reduced surface heat fluxes and changes in circulation. Moreover, the representation of precipitating shallow convection does not affect warm pool precipitation. Thus, our experiments indicate the role of surface heat fluxes in light near-surface winds to trigger precipitation, as over the warm pool.

How to cite: Segura, H., Bayley, C., Fiévet, R., Glöckner, H., Günther, M., Kluft, L., Naumann, A.-K., Ortega, S., Praturi, D. S., Rixen, M., Schmidt, H., Winkler, M., Hohenegger, C., and Stevens, B.: Getting a single tropical rainbelt in a global storm-resolving model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12680, https://doi.org/10.5194/egusphere-egu25-12680, 2025.

Idealised simulations under the assumption of radiative-convective equilibrium (RCE) demonstrate that the spatial aggregation of convection can significantly influence the domain-mean climate. One notable implication is the warming of the free troposphere with increased convective organisation, resulting in greater atmospheric stability. However, atmospheric temperature is also closely tied to surface temperature in regions of deep convection. The interplay between convective organisation and surface temperature in modulating free-tropospheric temperature remains unclear.

To address this question, we conduct idealised cloud-resolving simulations incorporating a diurnal cycle and prescribed sea surface temperatures (SSTs). The SST is spatially fixed with temperature gradients: a warmer ocean hotspot surrounded by cooler ocean regions. We vary the temperature of the ocean hotspot to modify the temperature gradients between the hotspot and the surrounding oceans. Additionally, we introduce an island away from the hotspot by coupling the atmosphere to a 0.05-meter deep slab ocean model. The latent heat flux calculation in the slab ocean model is rescaled by a factor of 0.1 to represent the reduced latent heat fluxes typically observed over land. The presence of temperature gradients enables continuous convection over the hotspot, whereas convection over land occurs only in the afternoon, after being heated by incoming radiation. Consequently, the model successfully simulates a diurnal cycle, characterised by enhanced precipitation over land in the late afternoon and early evening, and increased precipitation over the ocean in the early morning.

We find that daily variations in atmospheric temperature are closely related to the daily evolution of convective organisation. Additionally, enhanced temperature gradients between the hotspot and the surrounding ocean further promote convective organisation. Consequently, convection is most organised, and the free troposphere is warmest, in the simulation with the highest hotspot temperature (and the largest temperature gradients). We test the modelling results with ERA5 reanalysis data and confirm that the degree of organisation plays a crucial role in modulating the tropical free-tropospheric temperature. However, organisation appears to be primarily important for daily variations in atmospheric temperature on timescales shorter than 20 days, while surface temperature in the deep convective region becomes more significant on longer timescales (greater than 20 days).

How to cite: Bao, J., Muller, C., and Singh, M.: The influence of convective organization on tropical free-tropospheric temperature, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15829, https://doi.org/10.5194/egusphere-egu25-15829, 2025.

A fundamental quantity in tropical dynamics is the `convective mass flux', which measures the rate at which mass is transported upwards, per unit area, in convective updrafts. Convective mass flux encodes information about the frequency and intensity of thunderstorms, and has been linked to the strength of the large-scale tropical circulation. Changes in convective mass flux under warming are thus an important, but uncertain, aspect of climate change. Here we build off recent work linking changes in mass flux to the clear-sky energy budget to show that convective mass fluxes decrease along isotherms at around 3-5 \% K$^{-1}$ under warming. We show that this constraint holds throughout the free-troposphere and across a hierarchy of models; from idealized radiative-convective equilibrium simulations to CMIP6 models. This decrease in convective mass flux with warming is driven by a stabilization of the lapse rate and can be captured with a simple analytical model. We also revisit previous work by Held and Soden (2006), who proposed a scaling for changes in the convective mass flux with warming. We show that the Held and Soden scaling does not capture inter-model spread in cloud-base mass flux changes under warming in cloud-resolving models, and that their original verification is not robust across GCMs. Altogether, this work provides a quantitative constraint on changes in convective mass flux throughout the troposphere which can be derived from first principles, and which is verified across a hierarchy of models. 

How to cite: Williams, A. I. L. and Jeevanjee, N.: A robust constraint on the response of convective mass fluxes to warming, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1228, https://doi.org/10.5194/egusphere-egu25-1228, 2025.

Extreme convective wind gusts (≥ 25 m/s) primarily occur when a thunderstorm downdraft sinks with high momentum to the ground level and diverges. Rising global temperatures and increased atmospheric moisture (as per the Clausius-Clapeyron relation) are expected to alter convective processes in a future climate. Atmospheric instability diagnostics (MUCAPE, DCP, K-Index, and Total-totals index) demonstrate some, but limited, skill in predicting extreme convective winds; Idealized model studies indicate that convection and severe weather will likely intensify due to higher CAPE, possibly intensifying extreme gusts. We employ a Pseudo-Global Warming (PGW) approach to investigate how an observed warm-season extreme wind gust event in New South Wales (NSW), Australia would have evolved if it occurred in a warmer climate. The event was simulated using the Weather Research and Forecasting (WRF) model run in a three-nested domain configuration ranging from 5 kilometers to 200 meters horizontal grid resolution, using initial and lateral boundary conditions from ERA-5 reanalysis. An ensemble of 13 Coupled Model Intercomparison Project Phase 6 (CMIP6) Global Climate Models (GCMs) was used to calculate the climate delta considering the SSP370 scenario, between the future (2070–2100) and historical (1984–2014) period, which is then added to the ERA-5 data to produce the PGW perturbed simulations. This presentation will explore whether the gust is indeed stronger in warmer climates, and what thermodynamic and dynamical mechanisms are at play.

How to cite: Surendran, G., Isaza Uribe, A., Sherwood, S., Evans, J., El Rafei, M., Dowdy, A., and Ji, F.: Investigating the Future Evolution of Extreme Convective Wind Gusts Using Pseudo-Global Warming Experiments. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12076, https://doi.org/10.5194/egusphere-egu25-12076, 2025.

Extreme precipitation intensities in the tropics depend strongly on the spatiotemporal scale at which they are calculated, potentially introducing biases when assessing their physical drivers, impacts, and climate sensitivities. Furthermore, the contribution of Mesoscale Convective Systems (MCSs) to these extremes remains loosely constrained, especially on kilometer scales. Here, we use a new analysis framework for the co-occurrence of oceanic precipitation extremes at both convective (km) and mesoscale levels, and we compare their regional prevalence and rainfall morphology. We apply a storm tracking algorithm to ten global storm-resolving models (GSRMs) and one multi-year geostationary satellite product, focusing on various convective system types.

Our results reveal that the two scales of precipitation extremes are largely statistically independent, occurring in distinct regions with large model disagreement. Heavy km-scale events predominantly appear at the edges of convective zones, with 40% of such extremes in the satellite observations produced by MCSs. Their peak intensity is not correlated with the total area of precipitation features. In contrast, intense mesoscale events scale with the precipitating area, and are generated by MCSs in about a third of cases. We also observe a continuum of extreme precipitation features, spanning deep (DCS), very-deep (vDCS), and mesoscale convective systems.

We finally discuss the relative importance of cloud and rain morphology and life cycle parameters for understanding rain extremes on multiple scales, and we comment on relationships between environmental conditions and extreme-contributing DCS that emerge in our new multiscale analysis framework.When compared to observations, the models typically underestimate the precipitating surface and show substantial variability in the fraction of extreme rainfall attributable to different convective systems. These diagnostics highlight the need for further refining GSRMs to more accurately capture the relationship between convective organization and heavy rainfall.

How to cite: Carenso, M., Fildier, B., Roca, R., and Fiolleau, T.: Multiscale oceanic precipitation extremes are determined by the morphology of rain events throughout the lifecycle of deep convective systems., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10136, https://doi.org/10.5194/egusphere-egu25-10136, 2025.

Twenty-year satellite observations of rainfall have shown offshore propagation of diurnal rainfall signals in northern coastal areas of New Guinea, with propagation speed varying from 8 m s^-1 to 12 m s^-1 even under similar weak offshore background wind conditions. This study investigates the mechanisms behind this variability in propagation speed using the Maritime Continent Austral summer climatology v1.0 (MCASClimate), a 10-year high resolution model simulations dataset. By calculating the rainfall propagation speed on days with pronounced propagation, we classify the top 30% and bottom 30% of propagation speeds as faster and slower groups, respectively.

The faster group exhibits a more widespread rainfall pattern, suggesting that inertial-gravity waves driven by land-sea thermal contrast is the dominant factor. Conversely, the slower group displays more concentrated rainfall, indicating the dominance of cold pool dynamics far offshore. The faster group is associated with clearer skies, allowing more shortwave radiation to be absorbed during the daytime, which enhances land-based convection and cold pools in the evening. This results in stronger land-sea temperature contrasts, driving more intense inertial-gravity waves that govern the rainfall propagation. In contrast, the slower group is influenced by stronger low level wind shear, which leads to convection initiation primarily at the cold pool leading edges, yielding slower propagation speeds. An interesting finding of this study is that, either cold pools or inertial-gravity waves can govern rainfall propagation over distances greater than 600 km in New Guinea, albeit with different propagation speed.

How to cite: Chen, Z., Du, Y., Vincent, C., Short, E., and Yang, H.: What causes faster and slower diurnal offshore rainfall propagation in New Guinea?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13678, https://doi.org/10.5194/egusphere-egu25-13678, 2025.

Predicting the diurnal cycle of deep convection is valuable for applications ranging from day-to-day weather forecasting and aviation safety to climate modelling and resource management. However, current weather and climate models often struggle to accurately capture the timing of deep convective events, frequently predicting the peak of convective precipitation and the onset of storm formation too early. This study suggests that these timing inaccuracies stem from the absence of cloud-convection interactions in many models. Such interactions represent rapid feedback mechanisms with timescales similar to the transition from shallow to deep convection within a diurnal cycle (Vraciu et al., 2024). By contrast, the typical convective parameterization schemes used by the weather prediction and climate models only incorporate interactions between convection and a uniform environment, which produces feedback mechanisms too slow to align with the diurnal cycle's timing.

To address this gap, this work introduces a unified cloud-convection model that includes both cloud-convection and convection-environment interactions, applicable to both shallow and deep convection. The proposed model comprises a set of prognostic equations for the fractional areas of different cloud types and the convective updraft velocity at varying levels. In addition, following the framework of Arakawa and Schubert (1974), a prognostic equation is included to account for the cloud feedback on the large-scale environment for each cloud type. The model is tested using idealized large-eddy simulations of the shallow-to-deep transition in a diurnal cycle, yielding promising results. Furthermore, the role of cold pools is discussed in the new proposed model, based on simulations where cold pool effects are suppressed. The prognostic model presented here may form the basis for a new class of cumulus parameterization schemes with unified cloud-convection representation and unified shallow and deep treatment.

References:

Arakawa, A., & Schubert, W. H. (1974). Interaction of a cumulus cloud ensemble with the large-scale environment, Part I. Journal of the Atmospheric Sciences, 31(3), 674-701.

Vraciu, C. V., Savre, J., & Colin, M. (2024). The rapid transition from shallow to precipitating convection as a predator-prey process. ESS Open Archive, DOI: 10.22541/au.170964875.54219458/v2.

How to cite: Vraciu, C.-V.: A unified cloud-convection prognostic model for the diurnal cycle of deep convection, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-42, https://doi.org/10.5194/egusphere-egu25-42, 2025.

Convective cold pools—regions of cooled, dense air formed by evaporating rainfall—play a pivotal role in modulating atmospheric convection, yet their influence on hurricane dynamics remains insufficiently explored, especially in real-world simulations. In this study, we investigate the role of convective cold pools in the evolution of Hurricane Helene (2024) using a modified version of the Hurricane Weather Research and Forecasting model (HWRFxUT). Hurricane Helene formed in the Caribbean and intensified to become one of the deadliest hurricanes in recent history, offering a unique opportunity to study cold pool–hurricane interactions. The model setup includes nested domains at 9 km, 3 km, and 1 km resolution over the contiguous United States and employs a set of sensitivity experiments. Specifically, the rainfall evaporation rate in the Ferrier–Aligo microphysics scheme is altered by 20%, 50%, 150%, and 180% relative to a control run to assess how changes in cold pool characteristics affect the storm.

Cold pools are identified using a watershed algorithm, enabling systematic comparisons of their spatial extent and thermodynamic properties across all experiments. Analyses show that modifications to the rainfall evaporation rate significantly influence the development and distribution of cold pools in the vicinity of Hurricane Helene, with consequent impacts on storm rainfall, intensity, and track. The results underscore how changes in cold pool strength can yield marked differences in hurricane structure and evolution. These findings highlight the importance of accurately representing cold pool processes in numerical models to enhance tropical cyclone forecasts and underscore the need for continued research into this critical yet under examined aspect of hurricane physics.

How to cite: Talukdar, S., Casallas, A., Gopalakrishnan, S., Muller, C., and Niyogi, D.: Convective Cold Pools and Their Influence on Hurricane Intensification: A Case Study of Hurricane Helene (2024), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15340, https://doi.org/10.5194/egusphere-egu25-15340, 2025.

In 1987, Neelin & Held introduced the concept of the "gross moist stability" (GMS) to quantify how efficiently the tropical circulation transports energy. They constructed a simple model in which the spatial pattern of the GMS plays a leading role in determining the time-mean distribution of precipitation in the tropics. Since then, further work has revealed the importance of the GMS in theories of the Hadley Cell, the width of the intertropical convergence zone, and convectively coupled circulations, but a theory for the GMS itself remains elusive.

Here, I show that the atmospheric energy balance places strong constraints on the spatial distribution of the GMS, specifically, that the GMS must be uncorrelated with large-scale upward motion. This is contrary to the conventional view that convergence zones coincide with minima in the GMS. The importance of this result for convectively coupled circulations is explored using a series of convection-permitting simulations of a Mock-Walker cell in an idealised channel geometry. By varying an imposed radiative cooling profile, the vertical structure of the circulation is changed, allowing for large variations in the GMS. The results are then interpreted through a modified version of the theory of slow convectively coupled processes of Emanuel (2019).

How to cite: Singh, M.: The energy efficiency of tropical circulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19471, https://doi.org/10.5194/egusphere-egu25-19471, 2025.

We propose a simple, piecewise linear model for self-aggregation based on primitive equations. In this model, each atmospheric column is in one with two possible convective regimes: deep-convective or convectively inhibited, and the thermodynamics in each regime is linearised. The model simulates aggregated and non-aggregated stationary states, reproducing many properties of self-aggregation as simulated by kilometre-resolution models, in particular an hysteresis with multiple equilibria, aggregated and non-aggregated, and a similar sensitivity to convective triggering, domain size, and boundary-layer radiative cooling in the convectively-inhibited region. These results suggest that a self-aggregated state can be considered as a gravity wave with two phases: one convective and one convectively inhibited. 

How to cite: Bellon, G., Ribes, A., Meyssignac, B., and Geoffroy, O.: Convective self-aggregation as a dual-phase damped gravity wave, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20304, https://doi.org/10.5194/egusphere-egu25-20304, 2025.

Atmospheric convection over the subtropical western North Pacific (SWNP) varies on time scales around 2 weeks with significant effects on local and remote circulation. Among unknown effects, coupling between the SWNP convection variability and equatorial wave circulation is poorly understood. This paper quantifies equatorial wave perturbations using a global, wave space regression between the 43-year outgoing longwave radiation data over the SWNP region and spectral expansion coefficients of tropospheric circulation from ERA5 reanalyses. The resulting tropical wave flow is divided between the Rossby and Kelvin waves, which constitute the Gill pattern of tropical wave response to heating, and mixed Rossby-gravity (MRG) and inertia-gravity (IG) waves, which are named non-Gill pattern. The non-Gill part in the upper tropical troposphere is shown to have as large amplitude as the Gill part of the response. In particular, the IG and MRG waves contribute most of the cross-equatorial circulation and the MRG wave signals have about 25% greater amplitude than the IG wave signals. As SWNP convection intensifies, the MRG wave northerly winds across the equator strengthen whereas the IG waves represent strengthening outflow over the SWNP region. The combined effect of the Kelvin and Rossby waves enhance the circulation on the equatorward side of the anticyclone over the SWNP region, with the three times stronger Rossby wave than Kelvin wave easterlies in the upper troposphere. In the weakening phase of the SWNP convection, the northerly IG flow in the southern Indian ocean is coupled with developing anticylonic circulation of Rossby waves, suggesting the effects on extratropics in austral winter. The results suggest a caution when using Gill solution for the interpretation of circulation associated with asymmetric heating sources in real atmosphere or its models.

How to cite: Chen, P., Žagar, N., Lunkeit, F., Holube, K., Zhao, Y.-B., and Lu, R.: The Gill and non-Gill equatorial wave circulations associated with convective variability over the subtropical western North Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2257, https://doi.org/10.5194/egusphere-egu25-2257, 2025.