Meteorological thunderstorm types in Europe

Lightning in western Europe follows mostly a distinct annual cycle with the majority of lightning occurring in summer or fall reflecting the importance of CAPE thunderstorms (thunderstorms accompanied by high CAPE and moisture values) or, in the Mediterranean, the influence of high sea surface temperatures. But lightning occurs also without the typical favorable conditions so other meteorological processes become relevant. These rare thunderstorm types can be very destructive as the high hit rate to elevated infrastructure in winter shows: even in regions with a pronounced annual lightning cycle, tower striking lightning has no seasonality. Neglecting rare thunderstorm types can lead to insufficient lightning risk assessments, especially concerning elevated infrastructure such as radio towers and wind turbines. A proper understanding of the various thunderstorm types as presented in this study helps to find good places for wind farms and to construct more resilient wind turbines.

We use lightning observations from the EUCLID network along dozens of ERA5 variables with a resolution of 0.25 degree longitude/latitude and one hour. Each found thunderstorm type is described by its driving ERA5 variables, which are in turn categorized based on their meteorological characteristics: Mass/temperature field variables, wind field variables, cloud physics variables, moisture variables, and surface exchange variables. As some thunderstorm types may only occur in specific regions, we exert our analysis on several rather homogeneous subdomains in Europe representing the sea, as well as flat and mountainous domains. Further, we assure that lightning from each season is equally represented in our data set to better capture rare lightning conditions in unfavorable seasons.

This comprehensive data-driven description of various thunderstorm types in Europe is based on unsupervised learning methods. Cluster analysis groups observations with and without lightning based on their accompanying meteorological values working out various thunderstorm groups and clearly distinct from them groups without lightning. The thunderstorm groups are then meteorologically described using their cluster means, principal component analysis, and their prevalent weather patterns. 

First results show that in addition to the well-known CAPE thunderstorm type there are at least cloud-physics thunderstorms and wind-field thunderstorms. Cloud-physics thunderstorms are associated with large amounts of cloud particles, strong updrafts, thick clouds, and high precipitation amounts. Wind-field thunderstorms occur together with large wind speeds and high wind shear and in the absence of CAPE.

The presented description of various thunderstorm types in Europe improves the conceptual understanding of thunderstorms and provides a basis to better evaluate lightning risk in Europe.