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

There is great potential in combining real-time radar wind and precipitation fields with the knowledge of orographic forcing for nowcasting the location, onset, and let-up of stationary heavy precipitation over the European Alps. The paper presents the concept of a novel heuristic system for nowcasting development and evolution of heavy orographic precipitation using real-time measurements from the MeteoSwiss radar network.

In a mountainous region, such as the European Alps, simple extrapolation of radar precipitation maps is not successful, because the mountains strongly influence the motion and development of precipitation. Instead of using extrapolation we suggest to make use of the orographic forcing. As a result of this forcing heavy orographic precipitation is often persistent over several hours and exhibits distinct spatial patterns. Both the persistence and the spatial patterns can also be found in the meso-scale wind field. A typical configuration for heavy precipitation in the Southern European Alps is a moist low-level inflow from south to south-east and an upper-level flow from south-west to south. A significant change in the meso-scale wind field, such as a change in the strength and/or direction of the low-level inflow, is a short-term precursor of a change in the location and intensity of the heavy precipitation over the orography, which is crucial information for issueing warnings.

We are currently developing a nowcasting system that exploits the strong orographic forcing and the relation between the meso-scale wind field and the patterns in heavy rainfall. The goal is to provide forecast guidance for short-term decision making in an operational context.