This study constructs and analyses composite three-dimensional fields of Doppler-radar observed radial velocity and reflectivity for all precipitation events occurring in the Lago Maggiore region on the Mediterranean side of the Alps during autumn 1998 and 1999. Mean patterns for the two years are in close agreement with each other. The radar data are consistent with previous rain-gauge studies in showing that the rain was heaviest over the lower windward slopes, and decreased toward higher terrain. The three-dimensional reflectivity fields show that precipitation growth occurred mainly at low altitudes. The composite radar data show that the precipitation was most intense when the mean flow around the 2 km level was southerly or south-easterly, i.e. when the mean flow was most perpendicular to the Alpine barrier. Sounding data from Milan indicated the Froude number of the flow upstream of the Lago Maggiore region. When the Froude number was high, the flow proceeded directly up and over the terrain of the lower Alpine slopes. Under these unblocked conditions, the low-level flow (including the layer from the surface to 2 km above mean sea level) rose directly up and over the terrain, and the precipitation was greatly enhanced over the lower windward slopes and over the portions of the Po Valley just upstream of the mountains. Under unblocked conditions, the precipitation enhancement only extended a short distance (a few tens of kilometres) upstream of the Alps. When the upstream Froude number computed from the Milan sounding was low (blocked conditions), the Doppler radial velocities indicated that the low-level flow (in the layer below 2 km above mean sea level) turned cyclonically as it approached the Alpine barrier, instead of rising over the terrain. The composite radar reflectivity data showed less precipitation enhancement directly over the windward slope, but in contrast to the unblocked case showed that the precipitation was enhanced 140 km or more upstream of the terrain. Evidently, the low-level flow began rising far in advance of the barrier in blocked conditions. The 1998 and 1999 autumn data sample further indicates the relative roles of wind speed and stability, which are combined in the Froude number. When the wind speed upstream was strong (>8 m s(-1)), significant precipitation enhancement occurred on the windward slope of the Alps in the Lago Maggiore region, regardless of the static stability. However, the enhancement was far greater under unstable conditions. When the wind speed was weak (<8 m s(-1)), the precipitation was generally near or below average, except when the stability was low and weak and some patchy enhancement occurred over the Po Valley just upwind of the Alps. A diurnal precipitation maximum occurred in the early morning hours (0700-1000 LST), possibly where down-valley flow converged with synoptic-scale up-valley flow.
