Extreme hazard cyclones – concept and ERA5 climatology

Abstract. Extratropical cyclones shape much of the weather variability outside the tropics and can be associated with extreme precipitation and damaging winds with potentially substantial socioeconomic impacts. Consequently, identifying and understanding the cyclone characteristics associated with widespread weather extremes along their tracks is essential. This study uses 75 years of ERA5 reanalyses to characterize “cyclone extremeness” by quantifying the occurrence of extreme precipitation and surface winds along cyclone tracks. We do this by linking precipitation and 10 m wind speed extremes (values exceeding the local climatological 99.9^th percentile) to individual cyclones and quantify the number of extremes associated with each cyclone along its track on a global scale. Extremes are considered at two time scales, 1 hour and 3 days, reflecting that the impact of an extreme surface weather event may vary with its duration. An “extreme hazard cyclone” is then defined as a cyclone that accumulates a large number of precipitation and/or wind extremes along its track. Compared to all cyclones on average, extreme hazard cyclones form at lower latitudes and are deeper, larger in cyclone area, longer-lived, and more rapidly intensifying. Most of the top 100 extreme hazard cyclones globally form in the tropics and undergo extratropical transition. When restricting the ranking to cyclones producing extremes in the North Atlantic storm track, the top 100 extreme hazard cyclones predominantly occur in summer and autumn for precipitation extremes (including extratropical transition events) and mainly in winter for wind extremes. Extreme wind hazard cyclones typically form in strongly baroclinic environments at the left exit of intense upper-level jets. In contrast, extreme precipitation hazard cyclones are slightly less deep and form in weaker baroclinic environments, but they develop a prominent vertically coherent potential vorticity tower indicative of strong diabatic processes. At the time a precipitation or wind extreme occurs, extreme hazard cyclones are generally classified as ‘symmetric’ in the classical cyclone phase space and those linked to many wind extremes exhibit a ‘shallow warm core’. Long-duration extremes are associated either with more stationary cyclones or with cyclone clustering, both of which keep the affected region under cyclonic influence for an extended period, whereas short-duration extremes are linked to more intense and rapidly moving cyclones. The concept of extreme hazard cyclones, as introduced in this study, reveals key characteristics of cyclones associated with many precipitation or wind extremes along their track, and it provides a framework to investigate how climate change influences cyclone-driven surface weather extremes. The concept can easily be applied to specific regions of interest and extended to study cyclone impacts by including particular fields of exposure.