Simulating a Mediterranean heavy‐precipitating event with parametrized convection: role of subgrid‐scale topography

Abstract Getting the right precipitation probability density function in the Mediterranean area is a challenge for most global and regional climate models with parametrized convection. Over land in particular, the intensity of heavy‐precipitating events is often underestimated. In the present study, we provide a process‐based analysis of the representation by the CNRM‐ALADIN63 regional climate model of one of these events, which occurred in the southeast of France on November 1–2, 2008. The CNRM‐ALADIN63 model, when run in a configuration where the large‐scale dynamics is nudged towards that of the ERA‐Interim reanalysis, is first shown to capture the location and intensity of the heavy‐precipitating event appropriately. Then, using a reference convection‐permitting simulation of the same event and a conditional sampling approach to identify and characterize convective updraughts, the ability of the model convection parametrization to capture further convective details is assessed. The model misses the occurrence of the updraught mass flux largest values, despite a significant and systematic overestimation of the updraught vertical velocity. The area covered by the convective updraught is in fact found to be severely underestimated, suggesting an inappropriate approach for convective closure: the event occurred along the foothills of the Massif Central, where the subgrid‐scale features of the topography interact strongly with the impinging large‐scale flow and thereby drive the position and large area fractions of convective updraughts, at least during the mature phase of the event. A preliminary topography‐based convective closure is proposed and implemented in the model to assess this hypothesis further. The results confirm that the parametrization deficiencies can be significantly reduced with a proper inclusion of subgrid‐scale topographic features.