Application of Adjoint-Derived Ensemble for Tropical Cyclones Rapid Intensification

There remains uncertainty about the key processes necessary for tropical cyclone (TC) rapid intensification (RI), particularly at the onset of RI. Adjoint-derived sensitivities and perturbations can be used to evaluate the importance of these processes, as perturbations to the control forecast are dependent on the definition of the response function (e.g., forecast aspect being evaluated). Utilizing a variety of response functions important for TC development (e.g., sea level pressure, wind speed, moisture), it is feasible to create an ensemble based on adjoint-derived sensitivities. This concept has been successfully developed and applied in the mid-latitudes for a mesoscale convective system (Xu et al., 2001). Since adjoint sensitivities typically contain both structured mesoscale, and synoptic-scale patterns along with smaller-scale, highly noisy patterns it is possible that by using the ensemble approach, the dominant physical processes can be revealed.

The presented work uses the U. S. Navy's Coupled-Ocean Atmosphere Mesoscale Prediction System (COAMPS) and its adjoint to evaluate the application of an adjoint-derived ensemble for TCRI onset. Through this approach, the ensemble is designed to capture the potential spread in the forecast to assess the predictability of the TC. This includes what regions and processes in the forecast are important for multiple metrics related to development. Results can be compared to typical ensemble members to understand the ability to capture the forecasted spread in intensity and track.