Dynamical sensitivity analysis of tropical cyclone steering and genesis using an adjoint model

The adjoint of a numerical weather prediction (NWP) model is a powerful tool for efficiently calculating the "sensitivity" of some function of the model forecast state with respect to small but otherwise arbitrary perturbations to the model state at earlier times. Physical interpretation of these sensitivity gradients for functions describing some phenomenon of dynamical interest allows the user to approach a variety of dynamical problems in atmospheric science from the perspective of the potential impact of small perturbations on the future development of that phenomenon; the integration of adjoint-derived sensitivity gradients as a dynamical tool for approaching these problems can be called dynamical sensitivity analysis.;A methodology for dynamical sensitivity analysis is developed and applied to problems related to the steering and genesis of modeled tropical cyclones. Functions defining the steering and genesis of tropical cyclones are developed and tested, and sensitivity gradients of those functions with respect to model initial conditions are interpreted physically. Results indicate that regions of strong sensitivity tend to localize where small vorticity perturbations have the capacity to grow quickly and impact the future state of the model, such as regions of strong ascent and subsidence surrounding midlatitude troughs, or near zonal jets where upshear-tilted perturbations can grow barotropically. Consequences for dynamics and predictability of these events are discussed.