Variation and Uncertainty in European Windstorm Footprints

David stephenson and laura dawkins

Windstorms, or extra-tropical cyclones with intense surface wind speeds, are a major source of risk for European people. Windstorms can cause aggregate insured losses comparable to that of US landfall hurricanes: for example, the cluster of three storms Anatol, Lothar and Martin in December 1999 led to insured losses of US$11bn. Windstorms cause economic damage of US$2.5 billion per year, and insurance losses of US$1.8 billion per year (1990-1998). They rank as the second highest cause of global natural catastrophe insurance loss after U.S. hurricanes.

The loss due to European windstorms depends not only on the frequency and intensity but the spatial extent of the resulting footprint. The footprint of a storm is defined as the maximum 3 second gust at each grid point in the model domain over a 72 hour window centred on the time at which the maximum wind speed over land along the track is achieved during the storm.

This project aims at building and testing methodologies that can be used to estimate the risk of intense windstorm events by understanding the variation and uncertainty in windstorm footprints and the mechanisms which cause these variations and uncertainties (e.g. Large scale atmospheric circulation systems and model error).

A common assumption in catastrophe models is that the spatial extent of a windstorm footprint is constant. However, it is observed that windstorms can have very different spatial extents. This is evident from the image below which displays the footprints and exceedance footprints (threshold = 25m/s over land and 30 m/s over sea, chosen to ensure relevance to wind damage risk) for 3 major insurance loss windstorm events.

The model used to generate the windstorm footprints is based on the Met Office North Atlantic-European operational Numerical Weather Prediction (NWP) model (UM vn 7.4, parallel suite PS24). The model is run at a resolution of 25km.

Currently a historical eXtreme Wind Storms (XWS) catalogue is being developed by the Met Office, the University of Exeter and the University of Reading due for release in August 2013. The catalogue will be freely available for commercial and academic use and will contain information on the storm tracks and footprints of 50 extreme windstorms. The catalogue aims to facilitate research into storm characteristics and the influence of atmospheric variability and climate change on European windstorms and will be a wonderful resource for windstorm work within both CREDIBLE and RACER.

My role in developing the catalogue has been to derive an objective method for selecting severe and high insurance loss storms to be included within the catalogue. To achieve this the key indicators of high insurance loss storms were investigated and found to be the maximum wind speed over land and the exceedance area over land (threshold=25m/s). This result is illustrated in the plot below where 23 famous high insurance loss storms are shown in red. This finding reiterates the importance of varying the spatial extent of windstorms within catastrophe models.

Later in the CREDIBLE project, work will be carried out at the UK Met Office using ensemble prediction methods to allow for the quantification of forecast uncertainty of windstorm footprints on different time and length scales. The aim is to exploit this information within the larger and more complete framework for uncertainty estimation being developed within the CREDIBLE consortium project. A further aim is to use the windstorm footprint uncertainty in a risk assessment tool as part of the Met Office’s Hazard Impact Model.