Recent trends and variabilities of convective parameters relevant for hail events in Germany and Europe

Abstract

This study investigates whether any evidence can be found to support the occurrence of alterations in the hailstorm frequency in the last few decades (1978–2009) over Germany and Europe. Due to their local-scale extent and a lack of appropriate monitoring systems, hailstorms are not captured reliably and comprehensively over long periods by current observation systems. To overcome this constraint, we consider various convective indices and parameters (CPs) derived from soundings at 1200 UTC, and we evaluate which of them are appropriate for predicting hail damage days according to loss data from a building insurance company.Most of the CPs that rely on moisture at the near-surface layers exhibit a significant positive trend towards a higher convective potential. This finding applies to the 90% (10%) percentiles of the annual distribution and to the number of days that exceed (or fall below) a specific threshold, which is suitable for hailstorm prediction. Negative and, at most stations, insignificant trends of CPs that rely on moisture at higher levels or if the initial parameters of the lifting curve were mixed over the lowest layers can be attributed to inconsistency in the time series of the dew point. This in homogeneity is caused by changes in the instrumentation around the year 1990. The investigations show considerable spatial differences in the convective potential both in the mean and the trends, with a distinct north-to-south gradient and a less marked west-to-east gradient over Europe and Germany. The spatial distribution of the trends, however, is fairly consistent among the CPs that are based on the same principles.