Masoud Rostami

Heatwaves in the subtropical and mid-latitude regions, which arise from localized heating can ripple through the atmosphere and persist for weeks, yet the mechanisms behind this chain of events remain difficult to disentangle. Here we use the Aeolus 2.0 model, which is a moist-convective thermal rotating shallow-water (mcTRSW) framework of intermediate complexity, to explore how buoyancy anomalies evolve under both dry and moist conditions, innovatively including background effects to enhance realism. We find that localized heating sets off a rapid atmospheric adjustment: air converges near the surface, diverges aloft, and quickly organizes into paired cyclonic (lower layer) and anticyclonic circulations (upper layer). Earth’s rotation then distorts these structures, producing asymmetries, spiral rainbands, and rainband-driven feedbacks. Moist convection greatly amplifies this response by releasing latent heat, which fuels sustained instability and rainfall organization. Inertia–gravity waves emerge as a key pathway for redistributing heat and momentum, while Rossby waves and beta gyres gradually reshape anomalies, tilting them poleward and breaking them into smaller vortices. The simulations also reproduce well-known observational signatures, including comma-shaped water vapor patterns and mesoscale vortices. Together, these results show how a simple localized heat source can trigger a cascade of atmospheric responses that link convection, wave dynamics, and large-scale circulation. By capturing these processes, Aeolus 2.0 provides a bridge between theoretical frameworks and full climate models, offering new insight into the dynamics that sustain extreme heatwaves in a warming world.

Influence dynamique des effets diabatiques sur l'évolution des instabilités des vortex terrestres et planétaires: Le but de la thèse est de comprendre l'influence dynamique des effets diabatiques, comme la convection humide, sur les instabilités des vortex atmosphériques terrestres et planétaires. Un modèle verticalement intégré, avec les paramétrisations type relaxation des transitions de phase et de dégagement de la chaleur latente, le modèle de St-Venant avec la convection humide, a été utilisé. La version précédente du modèle a été améliorée pour inclure l'eau précipitable, sa vaporisation et son entrainement. L'approche consiste en 1) analyse détaillée de stabilité des profils idéalisés, ou extraits des données, des vortex, 2) étude de saturation non-linéaire des instabilités à l'aide de schéma numérique de haute résolution aux volumes finis. Les résultats principaux de la thèse sont : 1. Démonstration et quantification d'une forte influence des effets humides sur les instabilités des vortex synoptiques, y compris asymétrie cyclone-anticyclone des vortex de faible intensité aux latitudes moyennes, et de l'intensification des vortex type cyclones tropicaux, avec formation des nuages caractéristiques. 2. Explication de l'origine dynamique de l'hexagone au pôle Nord de Saturne, et de l'absence de structure similaire au pôle Sud, en termes d'instabilité du système vortex polaire - jet circumpolaire, et sa saturation non-linéaire. 3. Explication de la structure observée du vortex polaire hivernal sur Mars en termes d'instabilité et sa saturation en présence de réchauffement /refroidissement radiatif et de déposition de CO2 (transition de phase gaz - solide). Une nouvelle paramétrisation simple a été proposée pour ce processus, incluant l'influence des noyaux de déposition.

We present Aeolus 2.0, an open-source numerical atmospheric model of intermediate complexity. This iteration features a self-contained dynamical core based on a multi-layer pseudo-spectral moist-convective Thermal Rotating Shallow Water (mcTRSW) model, employing the Dedalus algorithm with spin-weighted spherical harmonics for efficient problem-solving. Aeolus 2.0 simulates the temporal and spatial evolution of vertically integrated potential temperature, thickness, water vapor, precipitation, velocity fields, and bottom topography effects, across a range of resolutions from smooth to coarse.In this study, we demonstrate the dynamical core and applicability of Aeolus 2.0, particularly its bulk aerodynamic and moist-convective schemes, in assessing the impacts of increased radiative forcing on zonal winds and extreme heatwaves. Additionally, by applying the same dynamical core to the equatorial region, we refine the theory of equatorial dipolar excited systems, offering insights into the formation of MJO-like structures.Specifically, we show that an eastward-propagating MJO-like structure can emerge spontaneously through the nonlinear relaxation of a large-scale buoyancy anomaly—either positive, negative, or a combination of both. This process is triggered when the anomaly reaches a critical threshold in the presence of moist convection near the Equator. The resulting MJO-like episode manifests as a convectively coupled dipolar system, featuring an enhanced vortex pair that induces counter-rotating anticyclones, accompanied by weaker opposite-sign counterparts in the upper layer. This system interacts with a convectively coupled baroclinic Kelvin wave (BKW), which propagates at a higher phase speed than the dipolar system and operates on an intraseasonal timescale.References[1] Rostami, M., Petri, S., Fallah, B., Fazel-Rastgar, F., 2025. Aeolus 2.0's thermal rotating shallow water model: A new paradigm for simulating extreme heatwaves, westerly jet intensification, and more. Physics of Fluids 37 (1), 016604. https://doi.org/10.1063/5.0244908.[2] Rostami, M., Petri, S., Fallah, B., & Fazel-Rastgar, F., 2025. A novel sea surface evaporation scheme assessed by the thermal rotating shallow water model. Atmospheric Science Letters. 26(1), e1287. https://doi.org/10.1002/asl.1287.[3] Rostami, M., Zhao, B., Petri, S., 2022. On the genesis and dynamics of Madden-Julian oscillation-like structure formed by equatorial adjustment of localized heating. Quarterly Journal of the Royal Meteorological Society, 148(749), 3788– 3813, https://doi.org/10.1002/qj.4388.

We consider the two-layer moist-convective thermal rotating shallow water equations and design a flux globalization-based, well-balanced, path-conservative central-upwind numerical scheme for the studied model. We use the developed scheme to conduct a series of numerical simulations and report the observation of eastward-propagating excited dipolar systems. These systems are characterized by one or more convectively coupled, poorly isolated dipolar fronts, primarily driven by the equatorial adjustment of large-scale localized positive buoyancy or potential temperature anomalies on the equatorial beta plane. A formation of these dynamic structures is triggered when disturbances exceed a critical threshold in a moist-convective environment. Notably, during the evolution of cyclones, secondary counter-rotating anticyclones develop in the lower layer, while oppositely signed structures emerge in the upper layer, highlighting the system's vertical coupling. A significant finding of our experiments is the identification of a time lag mechanism, observable even under weaker moist-convective conditions, between the initial state and the system reaching the excited threshold required for eastward propagation. This time lag underscores a critical build-up phase, during which the system accumulates the necessary energy and momentum to transition into a dynamically active state.

This study investigates the impact of combined drought and extreme heatwave events on air quality in South Africa during October 2022 (study period). An analysis of meteorological data revealed a sharp increase in surface air temperature and a significant decrease in precipitation, resulting in an extreme heatwave and drought period. The Standardized Precipitation Evaporation Index (SPEI) for October 2022 showed an extreme drought value of -2.01, reflecting increased dryness over the past two decades. Meteorological analysis highlighted the influence of the intensified stationary Angola heat low and Botswana upper ridge subtropical high, facilitating dry and warm air advection into the region, exacerbating an extreme climatic situation with heatwave conditions. Ozone levels exceeded 80 ppb (unhealthy) in northern, central, and eastern regions, particularly on October 13, with concentrations ranging between 88.2 and 93 ppb. Long-term data (1980–2022) showed rising trends in PM2.5 and black carbon concentrations, peaking at 1.48 × 10⁻⁸ kg/m³ and 8 × 10⁻¹⁰ kg/m³ in October 2022, with the highest values in Free State Province. Additionally, the UV Aerosol Index (UVAI) showed significant growth over the last decade, with high values in October 2022, signaling worsening air quality and environmental health concerns. The findings emphasize the important need for mitigation measures to address the compounding effects of extreme weather on air quality and public health.