The Use of the ERA5 Reanalysis to Identify Compound Extreme Wind and Precipitation Events That Are Associated with Extratropical Cyclones

Extreme events pose a significant challenge to scientists, as well as to stakeholders and policy makers as such events can have a major impact on society and ecosystems. Since extreme events occur rarely and historical data for such events are rare, it is difficult to understand and predict extreme events and their impact. The occurrence of extremes in two or more meteorological parameters, so called compound events, has received little attention up till now. Such events add an additional level of complexity and they can potentially lead to extreme impacts, especially when considering also the ongoing warming of the Earth’s climate. In this presentation we focus on extratropical cyclones and present a new method and climatology of compound wind and precipitation extremes.

To investigate the cyclones that trigger extreme wind and precipitation two different cyclone detection and tracking algorithms are applied. The two tools use a different method to identify low-pressure systems in the extra-tropics. Additionally, they are both able to estimate the radius of the system in different ways. The radius is used to estimate the area that is considered to be influenced by low-pressure systems. The tracking is applied to the 40 years from 1979—2018 of the most recent reanalysis data set, ERA5. A comparison with ERA-Interim data reveals that the tools are able to track extratropical cyclones also in ERA5. The high temporal and spatial resolution of this data is especially valuable, when analysing extreme events. The tracking of the synoptic systems allows to provide a quasi-Lagrangian perspective of such extreme events, which is in contrast to the Eularian approach that is often used when investigating compound events.

To be considered as an extreme event the seasonal 99^th percentile has to be exceeded at several ‘grid points’ within the impacting area, which is defined by the radius. Only extratropical cyclones that fulfil that criterion both in precipitation and 10-meter wind gust are considered as compound extreme event.

We will show that the peak occurrence of the cyclones that exceed this criterion for wind are slightly displaced from those that exceed the required precipitation threshold. Furthermore, the results show that the precipitation criterion is more often exceeded than the one for wind. Several of the compound events are located over the open water following the storm track, but there are also interesting occurrences over land, e.g., in the lee of the Rocky Mountains, the Mediterranean and in the southern part of mainland Australia and in Tasmania. These extreme events will be further investigated according to their atmospheric conditions.