Mechanisms initiating heavy precipitation over Italy during HyMeX Special Observation Period 1: a numerical case study using two mesoscale models

This study focuses on the initiation of deep convection during a heavy precipitation episode, which occurred during the first Special Observation Period (SOP 1) of the Hydrological cycle in the Mediterranean Experiment (HyMeX). In the course of 14 and 15 October 2012 (Intensive Observation Period 13), intense convective events affected southern France, Corsica and several regions of Italy. Numerical simulations are performed with two state- of-the-art numerical weather prediction models (COSMO and MOLOCH), driven by the same initial and boundary data and operated on an identical domain. With this set-up, the sensitivity of the model results to horizontal grid spacing and terrain elevation is assessed. Furthermore, model outputs are compared with observations from rain-gauges, radars and radiosondes made during the campaign. Although the higher resolution runs show a higher correlation with observed precipitation, the influence of model grid spacing on total precipitation amount or timing is rather weak. Since the overall performance of both mesoscale models is fairly good, they are used together to investigate the physical processes characterizing IOP 13. In particular, the differences in the location and timing of convection between the simulations are used to identify and explore those processes that need to be well represented in order to reproduce the mechanisms initiating heavy precipitation in the Mediterranean region adequately.