Spatial and temporal variability of rainfall in the Alps-Mediterranean Euroregion

Abstract : This study describes the main patterns of rainfall distribution in the Alps-Mediterranean Euroregion using a ground radar and characterizes the associated processes using model outputs. The radar dataset spans the period 2009-2012 with fine spatial (1 km) and temporal (5 min) resolutions. The most significant rain accumulations were observed in 2009 and 2010, while the most intense extreme events occurred in 2010. Conversely, 2012 was a rather dry year. Model outputs revealed that the wind shear, the pressure and the meridional wind at low level were the three main factors explaining the rainfall variability between 2009 and 2012. At the monthly scale, the maximum of rain accumulation was observed in November along coast. Results also showed that the most intense rain rates were observed during early summer and fall in the Pre-Alps. The monthly variability was characterized by a displacement of extreme rain events from land to sea from late spring to winter. Correlation analyses showed that this displacement was essentially controlled by the CAPE (Convective Available Potential Energy). Rainfall showed a diurnal variability from April to August for land areas of the Alps-Mediterranean Euroregion. The diurnal variability was significant during the spring and summer months with maximal rain intensity between 16:00 and 18:00 UTC. The correlation of the rainfall with CAPE showed this cycle was related to atmospheric instability. A secondary peak of average rain rate was observed during the early morning and was likely triggered by land breezes. The results highlighted that rainfall characteristics are extremely diverse in terms of intensity and distribution in this relatively small region.