Sensitivity of Tropical Cyclone Rapid Intensification Onset Using an Adjoint Model

Tropical cyclones (TCs) undergoing rapid intensification (RI) are an extreme weather hazard. Both the onset of RI and the constraining factors that impact RI are not clearly understood, with multiple hypotheses for the mechanisms involved. To assess the influence mechanisms, adjoint-derived sensitivities to initial conditions can be calculated to help determine the key dynamic and thermodynamic processes that influence TC intensity. Adjoint sensitivity studies evaluate the gradient of a user-selected response function with respect to the model state. The response function is chosen to represent some aspect of the forecast the user is interested in evaluating, such as intensity.

In this presentation, we implement a case study approach using the U. S. Navy's Coupled-Ocean Atmosphere Mesoscale Prediction System (COAMPS) and its adjoint to evaluate the initial state sensitivity and its evolution. The sensitivity and prediction of TCRI onset can utilize adjoint sensitivity to diagnose regions of importance for development through evaluating the influence of small changes to the initial conditions. This technique can elucidate the role of error growth on the storm and its environment. Through identifying and evaluating the evolution of sensitivity-based perturbations, information can be gained about the key processes driving forecast sensitivity. Furthermore, the relative role of moist and dry processes on development and the vertical distribution of perturbations can be quantified.