Sensitivity and Predictability of High-Impact Extratropical Cyclones

We explore initial condition sensitivity and predictability aspects of several extratropical cyclones, Xynthia (2010), Klaus (2009), and the St. Jude's Day Storm (2013), which had a severe impact on Europe. We highlight how higher- and lower-latitude interactions contribute to the development of these cyclones. The adjoint, tangent linear, and nonlinear models for the atmospheric portion of the nonhydrostatic COAMPS are applied with 45 and 15-km resolution nested grids. The adjoint sensitivity results for all three storms underscore the importance of a plume of low-level moisture of sub-tropical origin. The adjoint diagnostics indicate that the intensity of severe winds in these storms just prior to landfall was especially sensitive to perturbations in the moisture and temperature fields and to a lesser degree the wind fields. Only a relatively small region of water vapor within an atmospheric river present at the initial time for all three storms was critically sensitive for the development of all three cyclones, in spite of the large differences between the storms in their structure and developmental evolution. We also place the results from these three storms in the context of a very active waveguide that occurred during December 2013-February 2014, which serves to further highlight the importance of low- and mid-level moisture sensitivity along water vapor plumes. 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 high-impact 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, Klaus, and the St. Jude's Day storm is limited.