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Heavy precipitating events over southern France: mesoscale environment


Ricard Didier

The southern France is regularly affected by heavy precipitating events (HPEs) potentially associated with devastating flash floods. A climatological approach is used to characterize the mesoscale environment in which these systems develop.
Firstly, seven ground-based GPS stations located over the Western Mediterranean region (Southern France) are used to provide a climatology of tropospheric water vapour over a 4-year period (2001-2004). This GPS regional network has a high temporal and spatial resolution, allowing to document the mesoscale variability of the water vapour field.
So, integrated water vapour (IWV) computed at GPS station locations are interrelated with rain gauge data selected into the area enclosing each GPS station. We focus on the favourable period for heavy rainfall (15 August-31 December). On average, the precipitation amounts are correlated with IWV values. An approximated low-level moisture flux is also derived from GPS data and 10-m wind data: the heaviest daily precipitation correspond with the most intense moisture fluxes.
Secondly, a climatology based on 3D-var mesoscale analyses (VARPACK ALADIN) is realized for heavy precipitating events from 2002. The analysed data are surface mesonet, radiosounding and satellite data (SEVERI and Quikscatt from 2005). The guess is provided by a 6-hour ALADIN forecast. Then, different diagnostics are used to document the mesoscale features associated with the HPEs such as low-level jets (intensity, orientation, moisture transport...) and other key ingredients (CAPE, Precipitable Water, moisture convergence...) To underline differences on these diagnostics according to the localisation of precipitation, four domains are considered (Languedoc-Roussillon, Cévennes-Vivarais, South Alps and Corsica). Hourly precipitation and lightening data are used to define the starting, mature or ending phases of these HPEs. Composite analyses are realized for each domain and for each phase.