Adjoint Sensitivity Analysis of Rapid Intensification of Tropical Cyclone Nammadol (2022)

Adjoint sensitivity analysis of rapid intensification (RI) of Tropical Cyclone (TC) Nammadol (2022) is performed using the Weather Research and Forecasting (WRF) adjoint model. The adjoint sensitivity analysis enables one to obtain sensitivities of a response function to the initial model state. As a response function, lower-tropospheric kinetic energy within 200-km radius from the TC center is employed in this study. The adjoint sensitivity analysis shows that the sensitivity to water vapor mixing ratio is the highest among sensitivities to zonal wind, meridional wind, vertical wind, potential temperature, and water vapor mixing ratio when Nammadol rapidly intensifies. The high sensitivity to water vapor mixing ratio is observed slightly inside the radius of maximum wind (RMW), and in the lower troposphere. This is consistent with previous studies that showed that RI is favored when convection occurs in the higher-inertial-stability environment inside the RMW. In the azimuthal direction, the sensitivity to water vapor mixing ratio is high in the northwest quadrant of Nammadol, where water vapor mixing ratio is relatively large. To the best of our knowledge, this has not been pointed out by previous studies. These results mean that the sensitivity of kinetic energy near the TC center to water vapor mixing ratio is high in the region favorable for convections that lead to intensification of the TC. To further confirm this result, we plan to run the nonlinear WRF model and to add perturbations of water vapor mixing ratio in the highly sensitive region.