Adjoint-Based Forecast Sensitivity to Quasigeostrophic States during the Rapid Intensification of Typhoon Surigae (2021)”
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Abstract: A novel approach to deriving an analytical expression for the sensitivity to quasi-geostrophic vorticity (QGPV) from sensitivities to the horizontal wind field and potential temperature is developed. The sensitivity to QGPV along with sensitivity gradients with respect to geopotential, three-dimensional balanced geostrophic streamfunction, sensitivities to geostrophic winds, and potential temperature under hydrostatic approximation are demonstrated with the adjoint sensitivity case study of Typhoon Surigae (2021) during its rapid intensification using the Weather Research and Forecasting (WRF) model. Further, it is shown that using geostrophically balanced sensitivities as the adjoint forcing can effectively reduce the higher-frequency wave structures in the sensitivities field when integrating backward while maintaining the leading order balance. A comparison between two forward model runs with the adjoint-informed optimal initial perturbations (Errico 1997) and the similar initial perturbation but informed by sensitivities to the balanced geostrophic state variables is also shown in this study. This further allows us to investigate how the geostrophically balanced and unbalanced perturbations affect a change on the response function.