Within the Mesoscale Alpine Program - Special Observing Period, several stratiform precipitation events were observed which were associated with cold frontal passages over the Lago Maggiore Target Area (LMTA).They were investigated by three ground-based Doppler radars among others: the French Ronsard radar, the Swiss Monte-Lema radar and the US S-POL (dual polarization) radar which were operated over the LMTA to provide detailed informations of the flow circulation over the complex terrain. In this study, we analyze the Doppler radar data collected during four selected intensive observation periods (IOPs) 4,8,9 and 15. The observed precipitation structures are investigated along with the Doppler-deduced airflow organization in the LTMA.

IOP_4 and IOP_9 were associated with frontal systems passing rapidly over the observational domain, producing moderate precipitation with a line organization for IOP_4. This IOP was characterized by southeasterly flow below 1 km altitude, which progessively turned to southerly and southwesterly flow aloft. Similar flow structures accompanied the frontal system of IOP_9, but the southeasterly flow had a deeper extent. In both IOPs, cyclonic deviation of the south to southeasterly component flow was observed when approaching the rugged terrain in LMTA.

IOP_8 and IOP_15 provide two examples of blocked upstream flow, due to the existence of the low level northeasterly to norhterly downslope flow against an overall warmer southerly flow. Persistent stratiform precipitation was associated with the IOP_8 frontal system, while in IOP_15 the rapid frontal passage from northwest to southeast due to a deep and vigorous cold northerly flow helped to displace rapidly the precipitation to the southeast.