Sensitivity and Predictability of the Waveguide and High-Impact Extratropical Cyclones

We explore initial condition sensitivity and mesoscale predictability aspects of the extratropical waveguide and several extratropical cyclones [Xynthia (2010), St. Jude’s Day Storm (2013), Nina (2015)], which had a severe impact on Europe.  In this presentation, we will also address opportunities to further investigate multi-scale predictability aspects that impact waveguide as part of the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX).  The adjoint, tangent linear, and nonlinear models for the atmospheric portion of the nonhydrostatic Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) are applied with 45 and 15-km resolution nested grids.  The adjoint sensitivity results for the cyclones underscore the importance of the low- and mid-level moisture distribution and multi-scale interactions with the waveguide.  We utilize the adjoint to understand how small perturbations of moisture, winds and temperature evolve into downstream disturbances that impact the wave guide and cyclones.  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.  Despite large differences in structure and evolution, cyclone development was always critically sensitive to relatively small regions of water vapor within the larger-scale  plumes of enhanced moisture often similar to an atmospheric river near the waveguide. .  We also place the results from several of the 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 these severe cyclones may be limited.