Caractéristiques de l'écoulement et des précipitations observés durant MAP: une analyse par radar et simulation numérique
In autumn, the Alpine massif is regularly subjected to warm and wet Mediterranean airstream generated by the eastward progress of a cold front associated to a pressure low, generally located North of Europe. Due to the shape and the topography of the Alps, this phenomenon can locally lead to heavy precipitation which can be harmful for people (flash-floods, landslides ). One of the objectives of the international experiment MAP (Mesoscale Alpine Program) is the study of dynamical and microphysical mechanisms of these precipitations, in order to improve their forecasting. The campaign took place during autumn, 1999 and gave 17 intensive Observing Periods (IOP). The research works made during this thesis are based on the analysis of some of them, by means of ground-based and airborne Doppler radar data, and of the French non-hydrostatic numerical mesoscale model Méso-NH. Various types of precipitating event were studied: 2 cases of convective precipitation, the first one with strong rain accumulations in the Lago Maggiore region (western Alps, IOP 2B), and the other one in the Friuli region (eastern Alps, IOP 5); then cases with more moderate precipitation on the Lago Maggiore region ( IOP 4, 8, 9, and 15). These last 4 IOPs allowed us to study an another mechanism connected or not to the relief: the blocking process. For the 6 IOPs, the duration of precipitation seems to be the most important factor which explains rain accumulations: the persistence of precipitation due to the thalweg slowing down over the Alps led to the strongest rain rates. Maxima were recorded for IOPs 2B and 8, that are convective and stratiform events, respectively. In IOP 2B, the slowing down was caused by quasi-stationary synoptic ridge, while for the IOP 8 it was the stable character of the incidental airstream on the alpine slopes, which was blocked by the relief. The IOP 9 is similar to the IOP 8, although the blocking and so the movement of precipitating cells away of the Alps slopes, was due to upper-level airstream descent. IOPs 4 and 15, characterized also by the stability of the airstream feeding precipitation, illustrate cases for which the cold front arrival, and consequently arrival of post-frontal flow opposed to the feeding flow, was responsible for the precipitation movement. The IOP 5 is a situation characterizing fast cold front passage, and evolution of the associated line of convective cells. The relief effect was put in evidence, in stratiform and convective precipitating episodes. In every cases, it contributes to strengthen/initiate precipitation. It is responsible for the westward deflection of the stable incidental airstream. The impact of the small scale orography was also demonstrated by means of numerical tests, in particular the relief irregularities at 1-km scale played an important role in the progress of IOP 5 precipitating system.