A numerical study to investigate the roles of former hurricane
Leslie, orography, and evaporative cooling in the 2018 Aude heavy
precipitation event
Abstract. In southeastern France, the Mediterranean coast is regularly affected by heavy precipitation events. On 14–15 October 2018, in the Aude department, a back-building quasi-stationary mesoscale convective system produced up to about 300 mm of rain in 11 h. The synoptic situation was perturbed by the former hurricane Leslie, involved in the formation of a Mediterranean surface low that focused the convective activity. At mesoscale, convective cells focused west of a quasi-stationary cold front and downwind of the terrain. To investigate the roles of Leslie, orography and evaporative cooling in the processes that led to the observed rainfall, numerical simulations are run and evaluated with near-surface analyses comprising standard and personal weather stations. Simulations show that, in a first part of the event, low-level conditionally unstable air parcels found inside strong updrafts mainly originate from the Mediterranean Sea, east of 4.5° E, whereas in a second part, an increasing number originates from Leslie's remnants. Air masses from east of 4.5° E appear as the first supplier of moisture over the entire event. Still, Leslie contributed to substantially moisten mid-levels over the Aude department, diminishing evaporation processes. Thus, the evaporative cooling over the Aude department does not play any substantial role in the stationarity of the cold front. Regarding lifting mechanisms, most of the air parcels found inside strong updrafts near the location of the maximum rainfall are lifted above the cold front, attesting its key role in focusing convection. Downwind of the Albera Massif, mountains bordering the Mediterranean Sea, cells formed by orographic lifting seem to be maintained by low-level leeward convergence, mountain lee waves and a favourable directional wind shear; when terrain is flattened, rainfall is substantially reduced. The location of the exceptional precipitation appears to be driven primarily by the location of the quasi-stationary cold front and secondarily by the location of convective bands downwind of the orography.