On the performance of the new NWP nowcasting system at the Danish Meteorological Institute during a heavy rain period

Abstract

At the Danish Meteorological Institute, the NWP nowcasting system has been enhanced to include assimilation of 2D precipitation rates derived from weather radar observations. The assimilation is performed using a nudging-based technique. Here the rain rates are used to estimate the changes in the vertical profile of horizontal divergence needed to induce the observed rain rate. Verification of precipitation forecasts for a 17-day period in August 2010 based on the NWP nowcasting system is presented and compared to a reference without assimilation of precipitation data. In Denmark, this period was particularly rainy, with several heavy precipitation events. Three of these events are studied in detail. The verification is mainly based on scatter plots and fractions skill scores, which give scale-dependant indicators of the spatial skill of the forecasts. The study shows that the inclusion of precipitation observations has a positive impact on the spatial skill of the forecasts. This positive impact is the largest in the first hour, and then gradually decreases. On the average, the forecasts with assimilation of precipitation are skilful after 4 h on scales down to a few tens of kilometers. For the events studied, the assimilation improves the forecasted frequencies of heavy and light precipitation relative to the control, while there is some tendency to overpredict intermediate precipitation levels.