Convection-Permitting RegCM Simulations of the September 2024 Czechia Floods: Sensitivity to Microphysics and Soil Moisture

The extreme precipitation event of September 2024 over Central Europe caused widespread flooding in Czechia. Around 200 rivers were reported to have crossed their banks, and life in several cities came to a standstill. Representing such high-impact extreme events accurately remains a challenge for regional climate models. In the present study, we aim to explore the ability of the latest version of RegCM in representing such extreme rainfall events with different types of microphysical parameterizations and soil moisture representation at convection-permitting levels. A triple-nested domain framework has been adopted with a 27 km outer domain (EURO-CORDEX) nested to 9 km (covering central Europe) and further to 3 km (focused on Czechia). The 27 km and 9 km simulations use the Tiedtke convective parameterization, while the convection-permitting mode is chosen in the 3 km run to explicitly resolve the deep convection. Three microphysics schemes, namely WSM5, WSM7, and the Nogherotto–Tompkins scheme (NOG), are examined with soil moisture initialization switched on and off. This experimental design allows a systematic assessment of scale interactions and physical process sensitivities across resolutions. All these simulations are carried out with the 6-hourly initial and boundary conditions derived from ERA5 reanalysis data sets. Preliminary analysis indicates that RegCM is able to capture the heavy rainfall accumulation over the highly affected locations in the region of interest. The influence of soil moisture initialization becomes increasingly pronounced at convection-permitting scales, emphasizing the role of land surface conditions during extreme rainfall events. Among all the considered combinations, the simulations with WSM5 with soil moisture initialization seem to be closest to the observations with 3 km resolution. This study demonstrates the sensitivity of state-of-the-art RegCM to the microphysics parameterization, soil moisture initialization, and convection-permitting resolution, which are critical for improving the simulation of extreme precipitation and flood events over the European region.