Convection-permitting coupled simulations over the Northern Italy with the Regional Earth System RegCM-ES

Northern Italy is an area located in the Northern-eastern part of the Mediterranean region widely recognized as a regional hot-spot for climate change and characterized by the occurrence,  in particular in Autumn, of severe convective thunderstorms. These weather extremes, due to the presence in the area of several small and steep river catchments, often drive the occurrence of devastating flash-floods and flooding. The need for a better understanding of  past and future changes in the frequency of these extremes has produced in the last years an increasing interest in using over the area high-resolution convection-permitting models (hereafter CPMs) ​​that explicitly simulate the convection in the atmosphere instead of using parametrizations as it occurs at a coarser resolution. However, recent studies have stressed that the absence in the CPMs of an active and explicit coupling between atmosphere and ocean affects the simulation of convective events, partially explaining the observed biases between simulated and observed precipitation over the area.Here we combine a convection-permitting resolution (3 km) and an explicit coupling between atmosphere and ocean in a single modeling tool and assess its performance in simulating the precipitation regime over the Northern Italian during the ERA5 period. The modeling system, named RegCM-ES, is composed by: (i) the atmospheric module RegCM5 with an horizontal resolution of 3 km and 41 vertical levels, (ii) the ocean module MITgcm with an horizontal resolution of approximately 700 m and 59 vertical levels (non hydrostatic), and (iii) the river discharge module CHyM with an horizontal resolution of approximately 1 km. Moreover, RegCM5 has a prescribed time evolving aerosol concentration and is coupled with the CLMU urban model.The performances and the added value of the RegCM-ES with respect to the standalone atmospheric RegCM5 in simulating the precipitation regime and its extreme over the Northern Italy have been evaluated by comparing the numerical outputs of an hindcast experiment that adopt as initial and boundary conditions state-of-the-art atmospheric and ocean reanalysis, with some high-resolution observational and reanalysis datasets available for the area. The comparison, although still showing for both modelings tools significant wet/dry biases over the Alps/lowland areas of the Northern Italy, found an improvement in the representation of the precipitation regime and related extremes in particular in Autumn over the Eastern part of the domain that is the area where the coupling between ocean and atmosphere is effective. Moreover, an improvement in the simulation of the river discharge is found, spatially coherent with the improvements of the simulated precipitation. Overall, this comparison has offered valuable indications that RegCM-ES in convection-permitting configuration can be considered a suitable tool for studying the factors driving the extremes in the region, and is currently adopted to produce high resolution ocean and climate projections for the region.