A Comparative Study On The Upper-level Trough's Effect On The Intensity Changes Of Two Types Of Tropical Cyclones Landing In China During Extratropical Transition

Based on the China Meteorological Administration(CMA)tropical cyclone(TC)database and reanalysis data from European Centre for Medium-Range Weather Forecasts(ECMWF),the post-ET intensifying TCs(ITC)and weakening TCs(WTC)making landfall in China and undergoing extratropical transition(ET)in the presence of upper-level trough are examined in this paper in terms of structure characteristics,environment fields and dynamic diagnosis by use of dynamic composite analysis.In the meantime,complete form of vertical vorticity tendency equation is also conducted for vertical vorticity budget analysis.Results show that affected by the South Asia High and the Subtropical High,the upstream trough of the ITC extends from northwest to southeast in the process of ET while the trough of the WTC extends from northeast to southwest.In terms of ITC,cold air carried by upper-level trough intrudes into TC distinctly and mostly encircles the warm core vortex,making the ITC acquire characteristics of a warm seclusion;For WTC,the cold air only wanders on the northwest side of TC without further intrusion.The upper-level divergence is also in favour of ITC by the influence of pumping.According to the diagnostic analysis of moist potential vorticity,moist baroclinity can cause changes in vertical vorticity to some extent.The vertical vorticity budget analysis further indicates that there is stronger and wider positive vorticity advection in upper troposphere and more enhanced vertical vorticity transport in low-middle layer of the troposphere near TC center for ITC.And the contribution of enhanced baroclinity to the growth of vertical vorticity in ITC is more significant than WTC.In order to further verify the preliminary conclusions from composite analysis,typhoon Winnie and Wipha are selected from ITC and WTC for numerical simulation.Results show that the WRF model reproduces the track,intensity and basic structure well.We can find from the results of case study that the ET process of Winnie is less affected by the South Asia High and then the direction of trough in the northwest presents northwest-southeast trend.In the early stage of re-development of Winnie,mainly the front and baroclinity play a key role.In the later stage,Winnie continues to deepen under the joint influence of baroclinity,warm-seclusion,upper-level divergence and vorticity advection.However,strong South Asia High and Subtropical High heavily affect the ET process of Wipha,making the trough hardly move and then extend from northeast to southwest,the weak cold air intrusion therefore limit the strength and area of baroclinic zone.In addition,the unfavorable upper-level divergence and vorticity advection fail to provide a favorable environment for reintensification of Wipha.The case study confirms the conclusions from composite analysis well.