An approach for the classification of European windstorms

Abstract

Winter windstorms are among the most severe natural hazards in Europe and are frequently the cause of enormous economic losses. Therefore, it is not surprising that studies on windstorms often revolve around their potential insured losses, return values, or overall severity. Each of these measures is an important indicator for decision-makers to understand the potential impacts of current and future windstorm events. Especially now, as evidence of advancing climate change becomes more apparent. A much less researched but equally informative topic in this context is the characteristics of windstorms. Duration, size, and intensity are only a few examples of windstorm characteristics that are not only statistical properties of the event but also measures of severity themselves. In this dissertation, we study multiple European winter windstorm characteristics with the goal of identifying and understanding key parameters determining these characteristics, quantifying their impact, and investigating potential trends. Using the new ERA5 reanalysis product of the European Centre for Medium-Range Weather Forecasts, windstorms are tracked and matched to a parent cyclone. A set of windstorm characteristics is designed based on commonly evaluated characteristics from the windstorm and natural hazard community. Based on these characteristics, a set of windstorms is partitioned for the purpose of constructing objective windstorm classes, not only for further evaluation but also in an effort to create universally applicable windstorm classes for a wide range of end-users. In the process, we introduce our newly developed quasi-supervised k-means (QSKM), a semi-supervised clustering technique for grouping windstorm events with respect to a catalog of historically severe windstorm events. QSKM constructs three different windstorm classes, one of which closely resembles the windstorm catalog in its inherent characteristics. In a comprehensive evaluation of the constructed classes, we show that large events with high wind speeds and an exceptionally long lifetime often originate in the West Atlantic near the US east coast, intensify over the mid and eastern parts of the Atlantic, and usually hit Central or North Europe. Their occurrence can be associated with a strong jet stream and a deep parent cyclone. Affected areas experience strong wind gusts between 15-20 hours, with peak wind speeds in the early 10 hours of occurrence. Comparable weaker and smaller events are found in the Mediterranean region. Those events often develop in the East Atlantic or Mediterranean Sea where they also dissolve due to their short lifetime. However, due to their slow-moving character, they often affect local areas for up to 20 hours and more regardless of their short lifetime. Similar small and short events, but with high wind speeds are typical for northern Europe. These types of events usually originate in the western parts of the North Atlantic and further intensify as they travel across the open water. Although only exhibiting a third of the size of the first class of events, the core pressure of their parent cyclone can also drop below 970 hPa.