Radar observations of precipitation and airflow on the Mediterranean side of the Alps: Autumn 1998 and 1999

This study constructs and analyses composite three-dimensional fields of Do ppler-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 i n close agreement with each other. The radar data are consistent with previ ous rain-gauge studies in showing that the rain was heaviest over the lower windward slopes, and decreased toward higher terrain. The three-dimensiona l reflectivity fields show that precipitation growth occurred mainly at low altitudes. The composite radar data show that the precipitation was most i ntense when the mean flow around the 2 km level was southerly or south-east erly, 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 o f the Lago Maggiore region. When the Froude number was high, the flow proce eded directly up and over the terrain of the lower Alpine slopes. Under the se unblocked conditions, the low-level flow (including the layer from the s urface 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. Unde r unblocked conditions, the precipitation enhancement only extended a short distance (a few tens of kilometres) upstream of the Alps. When the upstrea m Froude number computed from the Milan sounding was low (blocked condition s), the Doppler radial velocities indicated that the low-level flow (in the layer below 2 km above mean sea level) turned cyclonically as it approache d the Alpine barrier, instead of rising over the terrain. The composite rad ar reflectivity data showed less precipitation enhancement directly over th e windward slope, but in contrast to the unblocked case showed that the pre cipitation 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 co nditions. The 1998 and 1999 autumn data sample further indicates the relative roles o f wind speed and stability, which are combined in the Froude number. When t he wind speed upstream was strong (>8 m s(-1)), significant precipitation e nhancement occurred on the windward slope of the Alps in the Lago Maggiore region, regardless of the static stability. However, the enhancement was fa r 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 P o 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.