Processes leading to deep convection and sensitivity to sea-state representation during HyMeX IOP8 heavy precipitation event

During the first observation period of the HyMeX programme, the Mediterranean coasts of Spain were impacted by several heavy precipitating events (HPEs). The most damaging one occurred during IOP 8 resulting in cumulative rainfall amounts over 180 mm in the area of Murcia Valencia. Numerical simulations using a high-resolution atmospheric model provide a very realistic representation of the mesoscale convective systems (MCSs) at the origin of this HPE and of the associated low-level conditions, consisting in two cold sectors surrounding a warm sector. This study provides a detailed analysis of the mechanisms of deep convection initiation and maintenance between 1200 UTC 28 September and 0000 UTC 29 September 2012. On the coastal mountainous area, the conditionally unstable inflow feeding the MCS is uplifted by the relief whereas at sea, a strong low-level convergence plays the same role. At the coast, cold pools are generated and strengthened by a strong low-level jet (LLJ) carrying cold dense air parcels from the Gulf of Lion and by evaporation and cooling under the precipitating systems. These cold pools play a key role in triggering the deep convection, either by directly uplifting the air masses or by deflecting the inflow horizontally. They largely control the localisation and distribution of the heavy precipitation at sea near Valencia. A weak barrier wind over the cold pools and a secondary cyclonic circulation result in a bending of the convergence line at sea, in agreement with radar observations. A sensitivity study to a more realistic representation of the sea state in the air sea exchanges shows that the LLJ is decelerated by the increased sea-surface roughness, resulting in cold pools extending further at sea and shifting the precipitation patterns 50 km offshore.