Analysis of Extreme Precipitation During the Mediterranean Event Associated with the Alex Storm in The Alpes-Maritimes: Atmospheric Mechanisms and Resulting Rainfall

On October 2, 2020, the Alpes-Maritimes was hit by precipitations of tremendous intensity, particularly in the inlands of Nice in the Tinee, Vesubie and Roya valleys, in the foothills of the Mercantour massif. The fury of this rainfall is the result of a well-known phenomenon of a Mediterranean event: a convective multicell cluster with retrograde formation (back-building thunderstorms), which means new cells continuously take place of previous ones. This leads to heavy rainfalls over small areas and long durations, which can trigger flash-floods. The spatial distribution of rainfall as shown by radar images, suggests several "corridors" crossing the department in a south-west to north-east axis, in accordance with synoptic winds. A meteorological analysis of the atmospheric mechanisms is proposed, supported by the outputs of the AROME model of Meteo-France. In addition, a cartography of the data collected by rain gauges in the area and from different networks is compared to the images obtained by radars (Meteo-France products PANTHERE and ANTI-LOPE). The contribution of non-conventional rain gauges (outside of state networks) in this type of post-crisis feedback can be further evaluated. We demonstrate that the AROME model has correctly anticipated the phenomenon, which allowed the authorities to emit a weather warning of highest level on the previous day. During the event, as telecommunication networks are broken and prevent transmission of surface reports, the radar images are the only available tool for decision makers. We also show that, in those mountainous lands, correcting radar imagery with rain gauges is essential for correct use of accumulation products, and finally river flows.