Sensitivity of High-Impact Extratropical Cyclones to Mesoscale Water Vapor Structure

We explore initial condition sensitivity and mesoscale predictability aspects of the extratropical waveguide and several extratropical cyclones [Desmond, St. Jude’s Day, Nina], which had a severe impact on Europe. In this presentation, we will also address multi-scale predictability aspects that impact waveguide as part of the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX). Results from the adjoint, tangent linear, and nonlinear models for the atmospheric portion of the nonhydrostatic COAMPS model 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 waveguide and cyclones. The adjoint diagnostics indicate that the intensity of severe winds and rainfall in these storms are especially sensitive to perturbations in the moisture and temperature fields and to a lesser degree the wind fields. The forecast sensitivity to diabatic heating is also quantified using the adjoint. Despite large differences in structure and evolution, cyclone development was always critically sensitive to relatively small filaments of water vapor within the larger-scale plumes of enhanced moisture often similar to an atmospheric river near the waveguide. We place the predictability results in the context of a very active period of waveguide activity that occurred during Dec. 2013-Feb. 2014, which serves to further highlight the importance of 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.