Analysis Of Factors Affecting The Post-landfall Weakening Of Landfalling Tropical Cyclones

The disaster potential caused by strong winds,torrential rainfall,and storm surge is largely related to the tropical cyclones(TC)intensity.Due to the involvement of complex,multiscale dynamic and thermodynamic processes,accurate numerical forecasting models of landfalling TC intensity is still challenging with relatively limited skills.Therefore,improving the understanding of factors and processes that affecting the weakening of landfalling TCs(LTCs)is important both scientifically and practically for disaster prevention.Using the TC best-track datasets from the Shanghai Typhoon Institute(STI)of China Meteorological Administration(CMA),6-hourly interim reanalysis data from the European Centre for Medium-Range Weather Forecasts(ECMWF),and the highresolution topographic observation data from the National Oceanic and Atmospheric Administration(NOAA)of the United States,we attempted to identify factors that affect the weakening of LTCs over China Mainland during 1979-2018.We quantitatively explored the spatial and temporal distribution and relative contributions of several key factors,including the pre-landfall intensity change characteristics,dynamic and thermodynamic large-scale environment.We further introduced the vertical wind shear parameter an a major environmental factor into the existing LTC dynamical-statistical intensity prediction model to improve the prediction skill of the model.The main conclusions are as follows:The results show that the pre-landfall intensity change is significantly related to the post-landfall weakening rate.Most of TCs making landfall in East China experienced weakening prior to landfall and tended to decay faster after landfall.The weakening of TCs after landfall in South China was relatively slow.Almost all TCs intensified prior to landfall in South China and experienced relatively small weakening rate after landfall.Namely,the intensifying TCs prior to landfall were easier to maintain their original intensity after landfall than the weakening TCs prior to landfall.The we further analyzed and discussed the large-scale environmental factors and their relative importance that affect LTC post-landfall weakening rate.In general,the lower and decreasing sea surface temperature(SST),and thus the greater SST gradient reduces the supply of energy from the underlying ocean.At the same time,the increasing deep-layer vertical wind shear,especially the larger westerly vertical wind shear,LTCs would exhibit a larger weakening rate.In addition,the weakening of upperlayer divergence and the increasing of lower-layer water vapor flux divergence block the vertical transport of airflow and water vapor supply,respectively,contributing to the rapid weakening of TCs after landfall.Environmental change is more drastic in East China,and even more crucial to LTC weakening,than in South China.Among them,the contributions of SST,SST gradient,and zonal vertical wind shear are the most significant.By contrast,in South China,the sensitivity of LTC weakening to environmental factors is much weaker,with only a slightly greater relative contribution by SST.This also explains why the LTCs in East China has a greater poet-landfall weakening rate.In order to consider the impact of the most important environmental factor,we introduced the standardized vertical wind shear as a input parameter into the algebraic decay model of LTCs(M-ALG＿VWS).This was done by using the relationship that the TC weakening rate is proportional to the square of the TC maximum near-surface wind speed to fit the TC weakening over time.The new model was compared with the non-physical classical exponential decay model(M-EXP),the physically based algebraic decay model(M-ALG),and the algebraic decay mode with parameterized topographic effect(M-ALG＿H).The results reveal that the new model including the environmental vertical wind shear M-ALG＿VWS performed better with smaller rootmean square error(RMSE)and mean absolute error(MAE)than the abovementioned previous three models,especially during 0–3 hours after landfall.Furthermore,because environmental vertical wind shear is larger over East China than that over South China,and previous models did not include the impact of environmental vertical wind shear,M-ALG＿VWS model reduces negative biases over South China and positive biases over East China appearing in earlier models.This demonstrates that the M-ALG＿VWS with the environmental vertical wind shear effect can effectively improve the postlandfall intensity prediction skill over China Mainland.