Predictability Aspects of Two Severe European Winter Storms
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.