Mesoscale environment associated with Mediterranean heavy precipitating events

The Mediterranean region is regularly affected by heavy precipitating events (HPEs) potentially associated with devastating flash floods. The southern France is particularly prone to this risk. Indeed, the configuration of this area is propitious for triggering these HPEs. The Mediterranean sea acts as a vast heat and moisture reservoir from which these systems pump their energy. Moreover, the steep orography of this area (Massif Central, Pyrenees, South Alps) induces low-level convergence and favours upward motion.

In this study, first, the mesoscale environment in which these systems grow is characterized for a set of past events. Then, idealized high-resolution simulations are carried out to assess the sensitivity to the mesoscale environment features of the trigger and evolution of the HPEs.

3D-Var mesoscale data analyses (VARPACK ALADIN) with a resolution of around 10 km, have been realized for heavy precipitating events over a 5-year period (2002-2006) considering cases with daily precipitation greater than 150 mm between 1 August and 31 January (55 days exceeded this threshold over this period). The analysed data are surface mesonet (2m temperature and relative humidity, 10m wind), radiosounding and satellite data (QuikSCAT and SEVERI from 2005). The guess is provided by a 6-hour ALADIN forecast run from a ARPEGE large scale analysis.

Then, different diagnostics have been computed on the mesoscale analyses for these past events in order to document the time evolution of mesoscale features associated with the HPEs such as low-level jets (intensity, orientation,... ) and other key ingredients (CAPE, Precipitable Water, moisture convergence, moisture flux...). This is performed for the different phases of the life cycle of the HPEs which are determine from the observed hourly precipitation and lightning data. It is found that there is a great variability of these diagnostics for the different HPEs.

To underline differences on these diagnostics according to the location of precipitation, four domains are considered (Languedoc-Roussillon, Cévennes-Vivarais, South Alps and Corsica areas). Then, composite analyses are obtained by averaging these diagnostics for each domain and for each phase, they highlight location dependent characteristics for the diagnostics. Mesoscale features will be also related to synoptic patterns.

Then, to investigate the role of the different ingredients identified with mesoscale analyses, high resolution simulations (2.5 km) are performed with the research non-hydrostatic model MESO-NH. In particular, the structure of the moist low-level jet is studied. The full orography is used. A low-level wind and humidity anomaly is superimposed to a horizontally uniform southerly and unstable flow. Convective systems develop in different locations (over relief or over the sea) depending on these conditions.