Microphysical description of a mid-latitude squall line deduced from polarimetric measurements

The Mesoscale Alpine Programme (MAP) is an international programme devoted to the study of mesoscale atmospheric and hydrologic processes over complex terrain. A field phase took place all over the Alps from 7 September 1999 until 15 November 1999. In order to document the orographic precipitation mechanisms, research radars, such as the French C-band Doppler Ronsard radar (CETP) and the American S-band Doppler / polarimetric S-POL radar (NCAR), were deployed in the southern flank of the Alps, in the so-called Lago Maggiore Target Area. Those radars, along with the existing operational ones, such as the Swiss C-band Doppler Monte Lema (SMI) radar, permitted to constitute a unique data set, in terms of spatial and temporal resolution, on alpine precipitating systems.

For the present study, we focus on IOP2a (17-18 September 1999) of the MAP field phase. IOP2a corresponds to the passage of an intense squall line over the Multiple-Doppler coverage area. The squall line was triggered by the orography in a conditionally unstable environment and, once formed and active, generated frequent lightning and locally intense rain rates. Using polarimetric data of the S-POL radar, a microphysical description of the mesoscale convective system at several states of its evolution (triggering phase, organized phase, decaying phase) will be given and interpreted with the simultaneous lightning impacts, the 3D wind and reflectivity fields.