Enviro-HIRLAM model simulations of aerosol–cloud interactions during two cases of heavy rain in Italy and Ukraine

Heavy rain episodes in the midlatitudes turn often into hazardous natural disasters, causing flooding events, infrastructure damage, and environmental repercussions. While the synoptic processes responsible for heavy rain are usually well understood, the aerosol–meteorology feedbacks sometimes remain uncertain despite their tremendous role in such episodes. The modeling study is conducted to identify key aerosol–cloud interactions during two heavy rain episodes that occurred in Europe in 2023: one over the Italy region in May 2023 and another over the Black Sea – Ukraine region in November 2023. The simulations were performed employing the Environment – HIgh-Resolution Limited Area Model (Enviro-HIRLAM) run at 15, 5 and 2 km horizontal resolutions for two model configurations: a reference run without aerosol effects (REF) and a run including indirect aerosol effects (IDAE).Simulations with the IDAE configuration showed a significant increase in specific cloud ice and liquid water, a higher fraction of low-tropospheric cloud cover (but a reduction above 5 km), and an overall increase in total cloud condensate. Despite these changes, accumulated precipitation generally decreased by up to 3–4 mm per 6 h intervals when the aerosol effects were included. At finer spatial resolutions, localized areas with enhanced precipitation were identified, although the patterns differed regionally. In particular, in Italy, higher precipitation occurred mainly over marine areas, while in Ukraine it appeared predominantly over the land surface compared with the Black Sea aquatoria. Spatial correlation analysis between the aerosol fields and the differences between REF and IDAE configurations indicated that the dominant drivers of increased cloud water, ice content, and low-level cloud cover are soluble sulfate and sea-salt particles in the accumulation and coarse modes. Their influence was most pronounced within the 2–4 km layer. Less strong, although still significant, positive correlations were identified between the cloud cover and water/ice content and the presence of soluble coarse-mode dust, black carbon, and organic carbon. At finer resolution, these correlations weaken substantially, although the relationships for sulfate and sea-salt particles in the accumulation and coarse modes remained detectable. At the same time, no direct relationship was detected between aerosol fields and total precipitation, nor with the general decrease in rainfall in the IDAE relative to REF run. This suggests that complex atmospheric feedbacks govern precipitation formation, and that increased cloud water content does not necessarily translate into increased surface rainfall.This study was conducted within the Horizon Europe programme under Grant Agreement No 101137680 CERTAINTY project (Cloud-aERosol inTeractions & their impActs IN The earth sYstem). The required infrastructure, computing and storage resources, and technical support were provided by the CSC – IT Center for Science (Finland) under PEEX Modelling Platform research and development through CSC HPC research projects (PEEX-MP-at-CSC). Initial and boundary conditions for meteorology, chemistry, aerosols, observations for data assimilation, other input required for the Enviro-HIRLAM simulations were provided by the European Centre for Medium-range weather Forecasting (ECMWF).