Several 12 hour fine scale simulations of the main precipitating episode of the MAP (Mesoscale Alpine Programme) IOP2A, 17 Sept. 1999, have been performed at a resolution of 2 km using the two-way, triple nested non-hydrostatic mesoscale model MesoNH. A first series of numerical experiments suggests a strong sensitivity of the simulation to the initial analyse and to the lateral boundary conditions. We show that the 4DVAR analyse from ECMWF is fully able to initiate a first line of convection over the nord-west foothills of the Pô valley in Northern Italy after integrating MesoNH for 6 hours. As the system further develops and produces a copious amount of precipitation during its move to the east during the next 6 hours, we can explore the sensitivity of the precipitation to the bulk microphysical scheme of MesoNH by using 3 or 4 ice species, so considering or not hail as a full ice category. The results show that a separate treatment of hail-like hydrometeores leads to a more active and organized convective system with an increase of the precipitation. The numerical results are directly evaluated against a composite of reflectivity obtained from three aligned Doppler radars including S-Pol (deployed by NCAR). It is shown that the explicit treatment of hail, suggested by polarimetric informations extracted from S-Pol, provides also the best location of the rainfall in the 2 km scale simulation.