Classification of North Atlantic and European extratropical cyclones using multiple measures of intensity

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Abstract. The question of how to quantify the intensity of extratropical cyclones (ETCs) does not have a simple answer. To offer some perspective on this issue, we analyse multiple measures of intensity for North Atlantic and European ETCs for the extended winter season between 1979 and 2022 using ERA5 reanalysis data. The most relevant intensity measures are identified by investigating relationships between them and by performing a sparse principal component analysis on the set of measures. We show that dynamical intensity measures correlate strongly with each other, while correlations are weaker for impact-relevant measures. Based on the correlations and the sparse principal component analysis, we find that five intensity measures, namely 850 hPa relative vorticity, 850 hPa wind speed, wind footprint, precipitation, and a storm severity index, describe ETC intensity comprehensively and non-redundantly. Using these five measures as input, we objectively classify the ETCs with a cluster analysis based on a Gaussian mixture model. The cluster analysis is able to produce four clusters between which ETCs differ in terms of their intensity, life cycle characteristics such as deepening rate and lifetime, and geographical location. A fourth of all ETCs belong to the weakest cluster and occur mostly over Europe and in the Mediterranean area. Nearly half of all ETCs belong to the average-intensity cluster and occur mostly at the northeastern parts of the main North Atlantic storm track. A fifth of all ETCs belong to the second most intense cluster and occur mostly at the start of the North Atlantic storm track. Finally, less than a 10th of all ETCs belong to the most intense cluster and occur almost equally everywhere. This last cluster includes a clear majority of a set of investigated impactful storms (17 out of 21), which demonstrates the ability of the method to identify potentially damaging ETCs.