In this study, we investigate the roles instabilities played in the initiation of the heavy orographic precipitation that occurred in the Lago Maggiore Target Area during the MAP IOP-2B event, which occurred on 19-21 September 1999. Various types of instabilities, such as conditional instability (CI), conditional symmetric instability (CSI), potential instability (PI), and potential symmetric instability (PSI), which could initiate moist convections, are considered. Various instabilities associated with the interaction of a trough-frontal system and the Alps are discussed.

Data collected during the MAP IOP-2B event and that simulated by PSU/NCAR MM5 model are used. In order to obtain detailed structure of the upstream flow, 5-km grid resolution has been used to simulate this case. The spatial and temporal rainfall distributions were realistically simulated by the model. It appears that the rain was initiated over the windward slopes during the period of 12Z September 19 to 00Z September 20. Analysis of model data suggest that both PI (d(theta_e)/d(z) < 0) and CI (d(theta_e*)/d(z) < 0, where theta_e* is the saturated equivalent potential temperature) co-existed in the impinging southerly and southeasterly flow upstream of the southern Alpine slopes and the Lago Maggiore Target Area (LMTA). The parcel trajectory analysis along a north-south cross section indicated that the vertical motion below 1.5 km was very strong and varied significantly horizontally upstream of LMTA. This was due to effects of orographic blocking and the planetary boundary layer. Above 1.5 km, however, the parcel trajectories clearly suggested that the layer-lifting initiated the convective clouds, which may have produced light rain over the southern Alpine slopes from 12Z September 19 to 00Z September 20. This implies that PI may play a significant role in initiating the convection. On the other hand, the simulated convective available potential energy (CAPE) was lower than 50 J/kg in this area, which was too weak to initiate strong convection. Thus, CI might not play an essential role in the initiation of the convection, which led to the heavy orographic rainfall in IOP-2B.

Preliminary diagnosis of the PSI and the CSI, showed that these two types of instabilities could occur since the impinging southerly moist flow flows upward along a slantwise path on the steep southern slopes of the Alps. In addition, we need to consider the three-dimensional trajectories. In this study, we will also perform some sensitivity tests in order to distinguish the roles played by different types of instabilities in initiating the moist convection responsible for generating heavy orographic rainfall.