Hectometric scale simulations of a Mediterranean heavy precipitation event during HyMeX SOP1

Abstract. Offshore convection occurred over the Mediterranean sea on 26 October 2012 and was well documented during the first Special Observation Period (SOP1) of the Hydrological cycle in the Mediterranean Experiment (HyMeX). This paper analyses the triggering and organizing factors involved in this convection case study, and examines how they are simulated and represented at hectometric resolutions. For that purpose, a Large Eddy Simulation (LES) of this real case study is carried out with a 150 m horizontal resolution over a large domain encompassing the convective systems, as well as the low level flow feeding convection over the sea. This LES is then compared to a reference simulation performed with a 450 m grid spacing in the heart of the so-called grey zone of turbulence modelling. Increase of horizontal resolution from 450 m down to 150 m is unable to reduce significantly, for this case study, deficiencies of the simulation, more related to an issue of initial and lateral boundary conditions. Indeed, some of the triggering factors, such as a converging low level flow driven by a surface low pressure system, are simulated quite similarly for both simulations. However, differences for other mechanisms still exist since larger surface precipitation amounts are simulated at 450 m. It is found that the lack of a good representation of entrainment with a 450 m grid spacing along the edge of clouds causes a too quick triggering of deep convection at this resolution, associated with fast-track microphysical processes and enhanced dynamics. Furthermore, this first LES of a real Mediterranean precipitating case study highlights a convective organization with very fine scale features within the converging low level flow, features that are definitively out of range of models with kilometric horizontal resolutions.