Simultaneous Optimization of Air̶Sea Exchange Coefficients and Initial Conditions near a Tropical Cyclone Using JNoVA

The impact ofoptimizing the air̶sea exchange coefficients and the initial state is examined for a tropical cyclone (TC) with the aim ofimproving an operational mesoscale four-dimensional variational data assimilation system. In our optimization approach for TC Chaba that approached the Ryukyu Islands in October 2010, the drag coefficient values are adjusted so that they saturate at extreme conditions, while the first guess increases with increasing wind speed. In addition, the latent heat exchange coefficient values are adjusted so that they significantly increase under extreme conditions relative to the first guess. Consequently, the term of the cost function, which represents the discrepancy between the model results and the observational data, decreases by 4.1̶22.4% relative to the existing system after some spin-up cycles. The intensity and location ofthe TC are brought close to those ofthe corresponding best track produced at the Japan Meteorological Agency. Our optimization approach has the potential to improve the forecast skill