Compound precipitation and wind extremes over Europe and their relationship to extratropical cyclones

Extratropical cyclones and their associated extreme precipitation and winds can have a severe impact on society and the co-occurrence between the two extremes is important when assessing risk. In this study the extremal dependency measure, χ, is used to quantify the co-occurrence of extreme precipitation and wind gusts, and is investigated at individual grid points and spatially over Europe. Results using three observational datasets and a higher spatial and temporal resolution version of ERA5 than previously used confirm previous studies. Over Europe high co-occurrence is found over western coasts and low co-occurrence is found over eastern coasts. All datasets have qualitatively similar spatial patterns over most regions of Europe excluding some regions of high topography where ERA5 χ values are much larger. ERA5 represents the timings of daily extreme co-occurring events well, compared to observations. The differences in precipitation accumulation timescales are also accounted for by considering hourly, 6, 24 and 48 hourly co-occurrence. In a few regions co-occurrence changes with longer accumulations, indicating the different speeds and sizes of weather systems affecting these regions. χ in most regions has little increase by allowing a 24 h lag and lead between the precipitation and wind, with a few exceptions where χ is increased by up to 24%. Regions with the larger of these increases are on or around elevated topography. Using an objective feature tracking method, insight into the spatial pattern of extreme precipitation and wind within cyclones over Europe is given. As well as suggesting how many hours apart the extremes occur from one another in a particular location. Extreme co-occurring events are associated with cyclones far more of the time than non extreme events. Given an extreme co-occurring event the chance of a cyclone being within 1110 km is more than 70% for much of Europe. Regions with low co-occurrence have extremes caused by different weather systems and regions with large co-occurrence have both extremes caused by the same weather system. Cyclones linked to extreme events, particularly co-occurring and extreme wind, have larger intensity than those not and for most of Europe these cyclones also have faster mean speed.