Proceedings with Extended Abstracts (single PDF file) - Radio ...

STUDY OF A MESOSCALE LAND-TO-SEA LOW-LEVEL JET WITH ANETWORK OF UHF WIND PROFILERS: CASES OF THE MISTRALWINDV. Guénard 1 , J-L. Caccia 1 , B. Benech 2 , B. Campistron 2 , and P. Drobinski 31 LSEET, CNRS/Université de Toulon, BP132, 83957 La Garde, France2 CRA/LA, CNRS/Obs. Midi-Pyrénées, Campistrous, 65300 Lannemezan, France3 IPSL/SA, CNRS/Université de Paris VI, Jussieu, 75252 Paris Cedex 05, FranceIntroductionThe Mistral is a northerly low-level, orography-induced, cold-air out-break over theGulf of Lions blowing offshore the south-eastern region of France at any season. The climateof this area is under the influence of the Mistral which brings clear sky. It is frequentlyobserved to extend as far as a few hundreds of kilometers from the coast and is one of theprimary cause of storms over the Mediterranean.When a westerly to northerly synoptic flow impinges on the Alpine range, it isdeflected westward by the Coriolis force as well as the pressure build-up on the upstreamedge of the range. As the flow experiences channelling in the Rhône valley separating theFrench Alps, to the east, from the Massif Central, to the west, by a gap of 200 km long and 40km width (see Fig. 1), it is substantially accelerated, giving birth to the Mistral (Pettré, 1982).Although some of the large-scale features of the Mistral are well understood, thus wellforecasted, the mesoscale aspects such as the temporal, vertical and horizontal variabilities,the onset and cessation times are still to be investigated. During MAP (Mesoscale AlpineProgram, fall 1999, see Bougeault et al, 2001) and ESCOMPTE (Expérience sur Site pourCOntraindre les Modèles de Pollution atmosphériques et de Transport d’Emissions, summer2001, see Cros et al, 2003) field experiments, a UHF-wind profiler network has beendeployed in the coastal region of south-eastern France to document the spatial and temporalstructure of the tropospheric flow and in particular some Mistral events (e.g. Drobinski et al,2002). The networking approach combined with the high vertical and time resolutions of theUHF-wind profilers enables the study of the inhomogeneity and unsteadiness aspects of theMistral as well as its interactions with the ABL (Atmospheric Boundary Layer). In this paper,we have chosen to present and analyse wind and turbulence data obtained by two UHF-windprofilers during the two extreme Mistral cases (the strongest and the weakest ones, in fall andsummer, respectively) among the seven cases having occurred during MAP (three cases) andESCOMPTE (four cases). Those data are used to: (i) evidence and interpret the temporalvariability, sometimes very fast, of its vertical structure, (ii) describe the role of the upstreamorography, in the fall case, one radar being installed near the Rhône-valley exit and the secondone in the lee of the Alps, and (iii) analyse interactions between the Mistral and thermalcirculations such as land/sea breezes, in the summer case.Brief description of the Mistral meteorological aspectsWesterly to northerly upper-level large-scale flow patterns may lead to Mistral events.They must be associated with a Genoa Gulf cyclone in the lee of the Alps that results, atground level, in a north-south negative pressure gradient component allowing the air massesto be channeled and accelerated along the Rhône valley. The Mistral depth observed near thecoast depends on the upstream conditions (wind direction, Froude number, inversion layerheight) determining if the flow passes over or is blocked by the mountains along the lateralsides of the valley. In the second case, the air flow comes from the Rhône valley exit only.294