Verification of precipitation from the NWP nowcasting system at the Danish Meteorolgical Institute during a heavy rain period

At the Danish Meteorological Institute (DMI) the NWP nowcasting system utilizes a newly developed nudging based technique to assimilate radar-derived precipitation observations in the NWP model. In this method the observed rain rates are used to estimate a vertical profile of horizontal divergence changes corresponding to the rain deficit in the model.

Verification of precipitation forecasts by the nowcasting system using this technique is presented for 17 days of august 2010. This period was particularly wet with several heavy precipitation events with observed precipitation above 36 mm of rain during one hour. Three of these events are studied in detail.

The verification is based on the fractions skill score (FSS), which give scale selective scores of forecast skill relative to observations (radar-derived precipitation fields). The study shows that the nudging technique have a strong impact on the spatial skill oftheforecast in the first 1-2 hours of the free forecast, and a smaller but still clear impact 3-4 hours in to the free forecast.