Compound heat wave and heavy rainfall events in Poland

Compound extreme events often cause consequences that are much greater than the sum of the consequences of the component extreme events, when they occur separately, In this study, we focused on events consisting of a heat wave followed by exceptionally heavy precipitation. We investigated whether the presence of heatwave increases the likelihood of extreme rainfall events following it and if so, by how much, and whether the frequency of such compound events changes with warming. For this purpose, daily maximum temperature values and 6-hour precipitation totals from 44 meteorological stations in Poland for the years 1966-2024 were used. It was shown that compound heatwave – extreme rainfall events occurred in Poland with spatially differentiated frequency. They were least frequent on the coast and most frequent in the Southwestern, Southeastern and Northeastern Poland. The extreme rainfall occurred most often between the noon and midnight on the last heatwave day. During these hours the likelihood of extreme rainfall was on average 3.5 time higher compared to climatology. With warming the increase of compound heatwave – extreme rainfall days was at the rate of 1.22 days dec^-1 and compound heatwave – extreme rainfall events (understood as the occurrence of such a phenomenon at a single station) at the rate of 3.75 events dec-1. Although a detailed analysis of mechanisms responsible for such events is planned for further research, the preliminary study revealed that in most cases the approach of a cold front with a mesoscale thundercloud system was responsible for heatwave termination with extreme rainfall.

This research was funded by the National Science Center (NCN), grant number 2023/51/B/ST10/01926)