A linear upslope-time-delay model for orographic precipitation

A simple advection model is proposed for the prediction of precipitation patterns in complex terrain with arbitrary wind direction. The model includes the time delays associated with the conversion of cloud water to hydrometeors and the gravitational fallout of hydrometeors. These equations are solved using a Fast Fourier Transform. The model is tested against raingauge data from the Alps and radar data from the Lago Maggiore area of northwestern Italy, for the heavy rainfall event on September 20, 1999. Problems with these precipitation data sets are discussed. In the limit of short time delays, the model reduces to the well-known upslope method. Compared with the upslope method, the upslope-time-delay model has a significantly improved predictive skill. In the future, the method may be extended to include airflow dynamics and downslope evaporation.