Two cases of heavy Rain on the Mediterranean Side of the Alps in MAP

During September 1999, the Lago Maggiore region, which lies in a broad concave indentation on the Mediterranean side of the European Alps, experienced heavy rainfall and minor (but deadly) flooding in three storms during the Mesoscale Alpine Programme (MAP). These rainstorms occurred during Intensive Observation Periods (IOPs) 2B, 3, and 5, on September 20, September 25, and October 3, respectively. Each storm occurred ahead of an upper-level baroclinic trough. The IOP 2B trough extended far south with a southerly 500 mb jet over the Lago Maggiore region. It resembled the famous Piedmont flood storm of November 1994. The 500 mb flow ahead of the IOP 3 and IOP 5 troughs had a less favorable southwesterly fetch, which led to less overall precipitation in the Lago Maggiore region.

In all three cases, the lower tropospheric flow (below ~850 mb) was conditionally unstable and flowing up and over the terrain in a manner favorable for orographic precipitation. At 850 mb, the wind direction was generally southerly over the Lago Maggiore region in IOP 2B and IOP 3. It was southeasterly in IOP 5. Dual-Doppler winds at the 2 km level showed that the flow at the 2 km generally turned to a more down-the-pressure-gradient direction as it approached the Alpine barrier. Superimposed on this turning, the curving walls of the concavity in the barrier deflected the low-level wind inward toward the center of the concave Lago Maggiore valley, thus enhancing and focussing mesoscale convergence over the foothills near the center of the concavity. On a finer scale, individual river valleys and intervening ridges strongly controlled the precipitation pattern in this convergent region. In all three storms, polarimeteric radar data confirmed by rain gauges showed local rainfall maxima in the hundreds of millimeters within the general region of orographic convergence and lifting. In IOP5, heavy local rains produced damage, which at one location led to a fatality when the excessive rain caused a stone wall to collapse.

The mobile X-band Doppler on Wheels radar located in the Ticino River valley in all three storms showed return flow at low levels, down the river valley, as well as strong upslope flow above the 1.5-2 km level. Vertical cross sections of radar data over the heaviest rain areas showed that small convective cells, generally with echo tops below 7 km, formed repeatedly over local ridges for many hours as low level air continued to rise up the slopes.

Vertically pointing radar data indicated that the precipitation seen on scanning radars contained extremely fine substructure with individual maxima moving over the radar in short times of ~1 min. These elements took the form of generating cells in the melting layer with fallstreaks below. Polarimetric radar measurements indicated that the coalescence below the melting layer (the "warm rain" process) was an important, if not dominant microphysical process. Ice processes were also evident, as measurements suggested that graupel, and perhaps occasionally small hail, occurred in the individual convective cells where the heaviest rain occurred.