Specifying Air-Sea Exchange Coefficients in the High-Wind Regime of a Mature Tropical Cyclone by an Adjoint Data Assimilation Method

Uncertainty in the values of air-sea exchange coefficients has a detrimental effect on tropical cyclone (TC) modeling. Since a TC is one of the most destructive disasters, a method is required to reduce such uncertainty with respect to scientific progress and disaster prevention. In this study, we investigate the feasibility of specifying air-sea exchange coefficients in the high-wind regime of a mature TC by an identical twin experiment using the adjoint data assimilation method. The forward integration is executed by an intermediate cloud-resolving atmosphere-ocean coupled model, while the datasets for the backward integration are sampled as in multiple aircraft missions. Our results show that the air-sea exchange coefficients are successfully improved toward the "True" values. The updated air-sea exchange coefficients yield persistent improvements in the maximum wind speed, the radius of maximum wind, the radius of strong updraft, and in the distribution of water vapor. Without adjustment of the exchange coefficients, the analysis field of the inner-core is contaminated, even if the initial state is modified by the adjoint method.