This study investigates the local circulation associated with a heavy orographic rainfall event occurred during 19-21 September 1999 (MAP IOP-2B). The near-surface flow was dominated by an easterly jet originally from the Adriatic Sea and a southerly jet from the Gulf of Genoa. A significant westward turning occurred when the southeasterly and southerly flow approached the south side of the Alps. Precipitation was mainly concentrated on the windward slopes, especially near the Lago Maggiore area. This event was simulated by MM5 model with a 5-km horizontal grid spacing in the inner nested domain. The MM5 simulation reproduced the basic features such as the timing and location of the deep trough and the associated precipitation evolution, though the total amount of precipitation is slightly higher than that measured by rain gauges.

Sensitivity experiments have been conducted to investigate the effects of upstream orography, western flank of the Alps, earth's rotation, boundary layer friction, and model horizontal resolution. The results demonstrate that the low-level convergence and the orographic uplifting of a potentially unstable impinging flow were the major causes of the heavy rainfall. The westward deflection of southeasterly flow was caused by boundary friction, rotational as well as orographic blocking. The boundary layer friction reduced the total amount of the rainfall and altered its distribution by weakening the wind near the surface. The precipitation distribution and amount over the southern upslopes of the Alps were not directly related to either coastal Apennines Mountains or the west flank of the Alps. The 1.67 km horizontal grid spacing simulation indicates that heavy rainfall tended to concentrate in the vicinity of individual mountain peaks. The total amount of rainfall was over-predicted along the windward slopes due to the strong upward motion that occurred over the upslopes. The results imply that the dynamical forcing manifested as vertical motion increases rapidly as resolution increases. It is speculated that the rainfall over-prediction problem might be caused by the inaccurate or unrealistic microphysical processes over complex topography.