Nowcasting of orographic rainfall by using Doppler weather radar

A novel radar-based heuristic tool for nowcasting orographic precipitation is presented. The system benefits from the strong relation, due to the orographic forcing, between mesoscale flows, air-mass stability and rainfall patterns. The system is based on an analogue approach: past situations with mesoscale flows, air mass stability and rainfall patterns most similar to those observed at the current instant are identified by searching in a large historical data set. Deterministic and probabilistic forecasts are then generated every five minutes as new observations are available, based on the rainfall observed by radar after the analogous situations. This approach constitutes a natural way to incorporate evolution of precipitation into the nowcasting system and to express forecast uncertainty by means of ensembles. A total of 127 days of long-lasting orographic precipitation constitutes the historical archive in which the analogous situations are searched. The system is originally developed for the Lago Maggiore region in the southern part of the European Alps, but it can be extended to other mountainous regions given the availability of radar data and the presence of a strong orographic forcing. An evaluation of the skill of the system shows that the heuristic tool performs better than both Eulerian persistence and the COSMO2 numerical model.