Correlation of wind and precipitation annual aggregate severity of European cyclones

The risk from compound natural hazards (such as extratropical cyclones) can be large, but the aggregate loss over yearly timescales is significantly greater. The total insured losses from three European cyclones in February 2022 was over €3.5 billion.

This study has investigated the correlation between wind and precipitation annual aggregate severity caused by extra-tropical cyclones over the Europe-Atlantic region (from 30°N 100°W to 75°N 40°E ) in the 41 years of ERA5 reanalyses from 1980-2020.

Simple aggregate severity indices were constructed by summing exceedances above chosen thresholds of wind gust speed maxima and precipitation totals for all storms in a year that pass within 5° radius of each grid point location.

At low thresholds, there is a strong positive correlation between wind and precipitation aggregate severity most likely induced by the common dependence on the total number of storms. However, at higher thresholds, where the aggregate indices are expected to be better predictors of wind and flood losses, negative correlations start to appear especially over western Europe e.g. a correlation of -0.22 for severity indices aggregated over France at thresholds of 20ms^-1 and 20mm.

This suggests that accumulated wind and flood losses in Europe should not be assumed to be either independent or positively correlated, and that there is a potential for risk diversification.