Dynamical Sensitivity Analysis of Tropical Cyclone Steering Using an Adjoint Model

Abstract

hrough the use of an adjoint model, the sensitivity of the steering of a simulated tropical cyclone (TC) to various aspects of a model forecast trajectory can be calculated. This calculation, providing a priori information about how small perturbations to the model state will impact the steering of the TC at some future time, provides a wealth of dynamical information about the importance of synoptic-scale features and associated processes to the steering of a modeled TC that is difficult or impossible to obtain by other means. Regions of strong sensitivity to cyclone steering are regions where, if errors in the model state exist, those errors would have the largest effect on TC steering at a specified time in the future. However, without a dynamical understanding of why the steering of a simulated TC is sensitive to changes in these regions, errors in the methodology of implementing an adjoint model for calculating these sensitivities may result in sensitivity gradients that do not represent sensitivity of TC steering at all, and without a strong dynamical interpretation of these sensitivities, these errors may escape notice.An adjoint model is employed for several cases of simulated TCs in the west Pacific to determine the dynamical significance of regions for which sensitivity to TC steering is found to be particularly strong. It is found that the region of subsidence upstream of a passing midlatitude trough can play a crucial role in the development of perturbations that strongly impact a recurving TC. A dynamical interpretation of this relationship is described and tested.