Understanding the Role of Sea Surface Temperature and Urbanization on Severe Thunderstorms Dynamics: A Case Study in Surabaya, Indonesia.

Within the period 2014–2017, five hail events were reported in the city of Surabaya in Indonesia. Although deep convection commonly develops over the Maritime Continent, severe thunderstorms triggering hail events develop less frequently as specific atmospheric conditions are required. The rapid urbanization in Surabaya might have led to increased heat release to the atmosphere and to the deepening of convection, which raises the question of whether urbanization is the culprit of the recent hail events in Surabaya. Hence, for a selected hail event, we used the high‐resolution Weather Research and Forecasting model to understand the storm dynamics and to explore the role of urbanization, sea surface temperature, and aerosol concentration on the storm dynamics with a total of 11 scenarios. The control simulation reveals that low‐level convergence induced by a sea breeze creates instability. At the same time, the urban heat release enhances the energy supply to induce hail formation and retain the storm's lifetime over the city. A factor separation method revealed that the urbanization (added anthropogenic heat flux, urban aerosol, and the rise in building height) and the sea surface temperature increase contribute to the storm enhancement over Surabaya, producing two times higher updraft velocity, doubling the maximum graupel mass mixing ratio and finally resulting in 15%–30% higher accumulated precipitation over Surabaya, compared to the control simulation. Plain Language Summary: Urban dwellers are vulnerable to intense precipitation through the risk of casualties and damaged infrastructure by flooding, hail and wind gusts. The city of Surabaya was recently hit by relatively many hail storms, while the underlying mechanism remains unclear. This study uses the Weather Research and Forecasting model to explore whether the storms are affected by sea surface temperature, amount of cloud condensation nuclei, and urbanization (building height and anthropogenic heat flux). We find the sea surface temperature is the main driver of the system through the supply of heat and moisture. Next to that urbanization effects enhances the precipitation as well. Key Points: Recently, Surabaya was hit by relatively many severe thunderstorms producing hail events, but their underlying mechanisms are unknownUsing the Weather Research and Forecasting model, we explore the role of sea surface temperature (SST), urbanization, and aerosol content on the dynamics of such hail eventsFactor separation analysis reveals that the combined effect of increased SST and urbanization explains the enhanced precipitation [ABSTRACT FROM AUTHOR]