Predictability Aspects of Two Severe European Winter Storms

We explore predictability aspects of two severe European winter extratropical cyclones. The first event considered is referred to as ‘Xynthia', crossed Western Europe on 26–28 February 2010 and has been described as the most intense in this region in more than a decade. The violent storm claimed the lives of more than 60 people with many of the deaths in France related to strong winds and a storm surge that caused a rapid rise in water. Hurricane force winds were reported along the Atlantic Coast of France flooding low-lying coastal areas. The second storm we consider has been referred to as ‘Klaus', which made landfall over central and southern France, Spain and parts of Italy in January 2009. The extratropical cyclone was considered the most damaging storm in Europe in nearly a decade and caused widespread damage across France and Spain, especially in northern Spain including twenty-six fatalities.

In this study, the adjoint and tangent linear models for the atmospheric portion of the nonhydrostatic Coupled Atmosphere/Ocean Mesoscale Prediction System (COAMPS) are used to explore the mesoscale sensitivity and predictability characteristics associated with both severe cyclones. Unique aspects of the adjoint modeling system include a full adjoint to the microphysics and a nested grid capability that allows for multi-scale sensitivity calculations. The adjoint is applied using the nesting option with 45 and 15 km meshes. The adjoint diagnostics indicate that the intensity of severe winds in both storms just prior to landfall was sensitive to perturbations in the moisture and temperature fields and to a lesser degree the wind fields. Only a relatively small filament of moisture within an atmospheric river present at the initial time for both storms was critically important for the development of ‘Xynthia' and ‘Klaus'. The results of this study underscore the need for accurate moisture observations and data assimilation systems that can adequately assimilate these observations in order to reduce the forecast uncertainties for these severe extratropical cyclones. However, given the nature of the sensitivities and the potential for rapid perturbation and error growth, the intrinsic predictability of severe cyclones such as Xynthia and Klaus is likely limited.