Dependence of lightning occurrence in Europe on large-scale flow patterns

There is ample evidence that the occurrence of deep convection changes as a result of global warming and that, across Europe, increases in convective instability as measured by CAPE are an important driver in many regions. This study is a first step in disentangling the role that climate change induced changes in flow pattern occurrence plays on the evolution of the frequency of thunderstorms. Here we evaluate the association between large-scale flow patterns with the (temporal and spatial) distribution of lightning in Europe as detected by the Met Office Arrival Time Difference Network (ATDnet). The seasonal cycle shows that the largest number of lightning days occurs in the summer from May to August, the period we, therefore, focus on. The large-scale flow pattern is expressed using the daily mean 500 hPa geopotential extracted from ERA5 reanalysis data. A hierarchical clustering algorithm (Ward's method) is applied to the daily mean geopotential heights in the selected four-month period between 2007 and 2019. The algorithm produces 9 patterns (Fig. 1), with cluster 1 being the most frequent, occurring around 20% of the time and pattern 3 being the least frequent, occurring around 4% of the time. The distributions of lightning associated with the clusters show that lightning often occurs in synoptically quiescent conditions or even underneath a ridge. Furthermore, lightning occurrence over western Europe seems to be more dependent on the synoptic situation, where it is strongly associated with clusters that have a southerly flow at 500 hPa, compared to lightning over the Alpine range or south-eastern Europe.

 

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Fig. 1: Large-scale flow patterns are shown in nine clusters, geopotential heights of ERA5 at 500 hPa are plotted in the foreground with 50hPa intervals, and the mean number of lightning strikes per day is shown as filled contours.