If Lagrange fails, Doppler steps in: A solution for nowcasting intense orographic rainfall?

Amplitude and location of intense orographic rainfall are largely driven by meso-scale wind flow and airmass stability. Patterns of heavy rainfall often persist as long as direction and intensity of lower- and upper-level flow do not change. This is the result of a multi-sensor superposed epoch analysis combining radar precipitation maps, Doppler radar wind fields, radiosounding profiles, and measurements of mesonet stations during 106 days with significant widespread rainfall over the Southern European Alps.

The paper discusses the result of the superposed epoch analysis and explores its usage for nowcasting intense orographic rainfall. Whereas techniques based on Lagrangian persistence of radar maps fail to produce useful precipitation forecasts over orography, wind flow from Doppler radar and airmass stability may step in as short-term predictors: in saturated air similar constellations of lower- and upper-level flow and airmass stability are supposed to result in similar amplitude and location of heavy orographic rainfall. First results of a prototype nowcast system seem to confirm the above hypothesis. The output of the system is probabilistic.

The overall goal is to integrate these findings into the orographic precipitation radar ensemble nowcast system currently being developed at MeteoSwiss as part of the EU-funded research project IMPRINTS. A test chain coupling the radar ensemble REAL with the runoff model PREVAH is already running in real-time for a steep Alpine river catchment since May 2007.