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

• COSMO NWP model is used with a resolution of 1.1 km a two moment microphysics.
• COSMO NWP model is applied to nowcasting of hail.
• Deterministic forecast of hail is useful for lead times up to 90 min.

This study deals with the evaluation of the possibility of using the current numerical models for nowcasting of large hail (diameter ≥ 2.5 cm). Evaluation is focused on deterministic forecast with the lead times up to 2 h. 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 1.1 km 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 90 min 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 30 km.

Sample forecast of occurrence of large hailstones with a diameter of at least 2.5 cm. The left figure shows the truth, i.e., the place of occurrence of hail on 22nd August 2012 between 1400 and 1500 UTC derived from radar, sounding measurements and observations. The right figure shows the forecast by the COSMO NWP model. Forecast length is 60 min.Figure optionsDownload as PowerPoint slide