Severe wind gust thresholds for Meteoalarm derived from uniform return periods in ECA&D

This study presents guidelines for wind gust warning thresholds in Meteoalarm, the severe weather warning website for Europe (meteoalarm.eu). To that end we calculated the return value of the annual maximum wind gust associated with various return periods using validated wind gust data from the European Climate Assessment and Dataset (ECA&D, eca.knmi.nl). Return pe-riods indicate how extreme an event is under local climate conditions and can be used as a meas-ure of the possible danger of the event and its impact on society. Furthermore, warning thresholds based on return periods that are the same for the whole of Europe do not change abruptly at coun-try borders the way the current thresholds do, which solves the problem of unrealistic differences in frequency and levels of warning between neighbouring countries. This makes for a more uni-form pan-European warning system.
The chosen return periods reflect current Meteoalarm warning levels. Codes orange and red are based on the average return periods for median sized countries in the current guideline: re-spectively 2 and 5 years. These return periods are realistic and also represent the exten-sive experience of the participating National Hydrological and Meteorological Services with the damage caused by earlier extreme events. For some large and many smaller countries however, the return periods become unrealistic when they are adjusted for the size of the country as pre-scribed by the current guideline. Although the average return period for code yellow warnings in a median sized country is 8 times a year, we linked warning level yellow to a return period of 1 year for pragmatic reasons.
Analysis of return values shows that regional warning thresholds are required for wind gust warnings and that stations should be grouped into 3 "climate regions": coastal, inland and moun-tainous regions. Whether warning thresholds should depend on the season, can be established with data and tools provided in ECA&D and statistics can be used to account for population den-sity differences. We based the choice of the new warning thresholds for wind gusts on the median of the return values.
At the time of the study ECA&D contained gust data from 8 of the 10 countries providing wind measurements. Mainly due to the standard completeness requirements and the fact that some countries provided too few stations, representative medians of the return values of annual maximum gust could be calculated for 5 countries. New thresholds for Norway, Ireland, the Netherlands, Germany and Spain are given, where the need to distinguish between coastal, inland and mountainous regions is demonstrated. The thresholds put forward in this study differ signifi-cantly from current Meteoalarm threshold for coastal and mountainous regions, for code orange and red alarm levels.