A method for assessing model extensions: application to modelling winter precipitation with a microscale obstacle-resolving meteorological model (MITRAS v3.3)

The microscale, obstacle-resolving meteorological transport and stream model MITRAS has been extended with a snow cover and precipitation scheme. The performance of the model extension is assessed by comparing the results of different model versions using a method based on hit rates originally developed for assessing wind performance. For temperature, radiation and precipitation, estimates for the threshold values were derived based on computational accuracy; these are used in the hit rate calculation for these variables. The threshold values for the deviations are 0.02 m s −1 (5 %) for the wind components, 0.05 K (0.02 %) for temperature, 0.5 W m −2 (0.5 %) and 0.5 W m −2 (0.2 %) for the net long and short wave radiation, and 0.001 mm (1 %) for precipitation on ground. The model extensions produce plausible results and better represent winter precipitation. This opens the opportunity to study with higher accuracy the influence of obstacles on precipitation heterogeneities.