Nowcasting of hailstorms simulated by the NWP model COSMO for the area of the Czech Republic

Abstract

This study deals with the evaluation of the possibility of using the current numerical models for nowcasting of large hail (diameter≥2.5cm). Evaluation is focused on deterministic forecast with the lead times up to 2h. As in our previous work (Sokol et al., 2014) the COSMO NWP model with a two moment cloud microphysics explicitly considering hail was applied. The model is run at a horizontal resolution of 1.1km and with the assimilation of radar reflectivity. The model forecasts are evaluated for five events with large hail determined by direct observations and derived using a recognition algorithm based on radar and sounding data. The accuracy of model forecasts was evaluated subjectively by eye, by Fractions Skill Score and by bias. In addition distances between locations of forecasted and observed hail were analyzed and evaluated. Basic results of this study are that (i) forecasts longer than 90min are problematic because of large errors and significant underestimation of forecasted hail, (ii) when the model forecasts large hail then there is a 50% chance that large hail will be observed within approximately 30km.