Evaluation of the hailstone size of certain hail events over Germany in 2024, comparison with ICON-RUC forecasts and estimation of the maximum hail size.

Large hail can cause considerable damage (e.g. crop losses in agriculture or damage to house roofs) and so the predictioon of its maximum size is of particluar interest to the population. Our aim is to provide an estimation of the size of large hailstones at the surface from the forecasts of the Rapid Update Cycle ICON-RUC. The ICON-RUC produces hourly updated forecasts for Germany with a two-moment microphysics scheme, which particularly improves the prediction of severe convective storms. The prognostic variables of the two-moment scheme are the mass and the number concentrations of each hydrometeor, including hail. This allows a direct calculation of the mass-weighted mean diameter for hail, which can serve as an indication of the larger particles in the distribution.

Unfortunately, there are no area-wide hail observations over Germany that could be taken as the real truth.  However, the DWD's weather warning app (WarnWetter) offers the option of submitting reports on the current weather situation. This also includes hail. Although the accuracy of the individual reports cannot be determined, the data as a whole can still be a good help to evaluate the predicted hail size in the model. In addition, there are estimates of hail size from radar reflectivities, which can also be used to determine the adjustment for estimating the maximum hailstone size on the surface.

Days from the 2024 convective season are evaluated. These are matched with the hailsizes from the radar database for quality control and then compared to  model forecasts from the ICON-RUC. This provides information on how accurate the hail forecast is (location and time errors) and whether the estimate of the (average) hail size is realistic. In addition, we investigate how to correct the mean averaged hail size in order to obtain a realistic maximum possible diameter on the ground. In the end, this should help to improve the overall hazard assessment of a thunderstorm and thus minimize the hazard risk.