Reconstructing Long-Term (1950-2021) Trends in Convective Hazards using Additive Logistic Regression Models

Additive Logistic Regression Models for different convective hazards were developed across Europe and a portion of North America using lightning observations, severe weather reports and convective parameters from the ERA5 reanalysis. To model convective hazards, convective initiation was taken explicitly into account by computing, for instance, P_hail (probability of hail) as the product of P­_storm (probability of convective initiation) and P_hailstorm (conditional probability of hail given a storm). We will report on the development of models that are skilful across Europe and North America, and on the regionally dependent skill of convective parameters used as model predictors. To reconstruct the probability of lightning, large hail, and very large hail from 1950 to 2021, the models were applied to the ERA5 reanalysis, at one hourly intervals across both regions. The modelled hazard climatologies are in strong agreement with observed patterns and can accurately resolve local-scale features thanks to the (0.25 x 0.25° degree) spatial resolution of the ERA5 reanalysis. We analysed long-term trends over the 71 year period and detected important differences between the regions. Across North America, 1950–2021 hail trends were found to be weak and mostly non-significant, but a period of enhanced lightning activity (+30% to 1950–2021 average) was detected between 1980 and 1990 across the Central Plains. In Europe, trends are mostly positive and significant, with the highest trend modelled across Northern Italy. Here, the convective activity has seen an abrupt increase with very large hail 3 times as likely in recent years (2012–2021) than in the 1950s. Apart from a sharp increase in frequency, the year-to-year variability has also increased with yearly differences in occurrence exceeding 100% for large and 200% for very large hail compared to the long-term average. In addition to (very) large hail, preliminary results on the development of models for severe wind gusts and tornadoes models along with the corresponding long-term trends will be presented.