Orographic forcing and Doppler winds, the key for nowcasting heavy precipitation in the mountains

Due to the presence of the orographic forcing, in mountainous regions precipitation is often observed to be persistent over several hours exhibiting distinct spatial patterns. Both the persistence and the spatial patterns can also be found in the mesoscale wind field. Thus by careful analysis of a large set of past events systematic patterns in the mesoscale wind field can be detected and their changes used as short-term predictors for nowcasting the location, the intensity and the duration of heavy orographic precipitation.

A tool for nowcasting heavy orographic precipitation in the Lago Maggiore region, which is located in the European Alps between southern Switzerland and northern Italy, is currently being developed at MeteoSwiss. A radar climatology of 49 orographic precipitation events that have occurred in this region from January 2004 through May 2008 is here presented. This analysis constitutes the basic framework necessary to develop such a nowcasting system, which benefits from the presence of the orographic forcing and makes use of high-quality precipitation and Doppler wind radar estimates and other observational data.

A strong relation between the upstream flow, air mass stability and heavy precipitation in the mountains is found: the velocity of the upstream wind is highly related to the intensity of the rain, whereas its direction determines over which geographical areas the largest accumulations are measured. Moreover, unstable lower atmospheric conditions lead to a larger orographic enhancement of the rain.