Airflow within major river valleys on the south side of the Alps as observed during the MAP Special Observing Period

The Special Observing Period (SOP) of the Mesoscale Alpine Program (MAP) documented the precipitation and airflow structure over a section of the Mediterranean side of the Alpine range and its major river valleys. This was achieved by means of a nested array of operational and research radars. In the Lago Maggiore region, the Swiss Monte Lema and several Italian operational radars, and research radars from the U.S., France, Germany, and Switzerland documented the precipitation systems as they approached and moved over the Alpine barrier. These radars either scanned above the ridges and mountain crests, and thus were not able to see what was happening within the major river valleys, or they pointed only vertically. The Doppler-on-Wheels (DOW), a mobile scanning Doppler radar platform, was thus crucial for the documentation of the precipitation and airflow within individual river valleys. The DOW was operated at different sites within the Toce and Ticino river valleys during the MAP SOP.

One of the most intriguing observations obtained with the DOW was a persistent and sometimes quite strong flow of air down and out of the valleys. This phenomenon was particularly pronounced in IOP 8 (October 20-21, 1999), when highly stable air was pushed towards the Alps and the atmospheric stability apparently prevented the air from crossing over the barrier. The air seemed to be seeking ways to escape by diving under the up-sloping air and pushing back out along the valley bottom. This down-valley flow was likely enhanced (if not initiated) by the evaporation of precipitation, cooling the air within the valley and causing subsidence.

Using the DOW data, we analyze the airflow within the major river valleys. In particular, we seek to determine connections between the flow within the valley (up- or down-valley, strength, and depth in relation to the surrounding mountain crest line) and the environmental flow (strength, direction relative to the valley orientation, and atmospheric stratification) impinging upon the Alps. Does this down-valley flow constitute an important feedback mechanism of orographic precipitation, contributing to the lift of air and subsequent condensation?