A physically based statistical model with the parameterized topographic effect for predicting the weakening of tropical cyclones after landfall over China

Accurate prediction of tropical cyclone (TC) intensity is important but challenging. Previous exponential decay models of TC weakening after landfall assume the TC weakening rate after landfall being proportional to TC intensity at the time of landfall. In this study, a physically based algebraic decay model with topographic effect for predicting TC weakening after landfall over mainland China is constructed based on the best-track TC data during 1980–2020. In this model, the TC weakening rate is proportional to the square of the TC maximum near-surface wind speed with the “effective” surface drag coefficient being an internal parameter. The topographic enhancement of surface drag coefficient is introduced to include the topographically enhanced TC weakening over the southeastern coastal region of mainland China. This is achieved by parameterizing the drag coefficient as a linear function of the normalized topographic height in the study domain, which is calibrated by minimizing the forecast errors for all landfalling TCs over mainland China during 1980–2020. Different from most decay models discussed in previous studies, the algebraic decay model that we have developed in this study is a time-dependent decay parameter along the TC track due to the topographic variation. Results show that the previous decay models underestimated the weakening rate enhanced by topographic effect over the southeastern China while overestimated the weakening rate of TCs after landfall over the eastern China where plains are dominated. The algebraic decay model with the introduced parameterized topographic effect shows considerably better performance. Thus, the algebraic decay model with topographic effect considered performs better than the commonly used exponential decay model for TCs after landfall over mainland China, especially when TCs move further inland.