Development and application of an objective storm severity measure for the Northeast Atlantic region

An objective index for the estimation of storm severity has been developed, validated and applied to both reanalysis data and an AOGCM simulation for the Northeast Atlantic region. The index is based on the exceedance of local thresholds of the daily maximum wind speed. Positive trends for both the severity of storms during the historic ERA40 period (1960-2000), and under anthropogenic climate change (ACC) conditions (SRES A1B and A2) are identified. Additionally an increase in the spatial extent of storms is diagnosed, amounting up to about 10 % between present day and the scenario climate. Two types of the index are introduced: One area integrated measure for the investigation domain of 50 degrees W to 20 degrees E, 45 degrees N to 70 degrees N, called ASSI (Area Storm Severity Index), per day; and one event integrated index, called ESSI (Event Storm Severity Index), per storm event. ASSI and ESSI clearly identify extreme "cyclone bombs" under ACC conditions, which exceed the range found in observed (ERA40) and simulated data for the recent climate. ASSI and ESSI identify an increase in the severity of storms for the Northeast Atlantic and western Central Europe under ACC conditions. The index can easily be calculated for large data sets, and is thus well applicable to multi-model ensemble simulations in order to objectively estimate climate change signals and related measures of uncertainty. Reasons for the increase in severity could be seen mainly in the occurrence of higher wind speeds, and in larger areas affected by storms. These larger areas result from longer tracks combined with a common broadening along the path.