Study On The Impact Of Environmental Field Of The Northbound And Westbound Landfalling Tropical Cyclone

Tropical cyclones (TC) landfalling is one of the most serious natural disasters. Statistics show that Zhejiang and Fujian province in China's eastern coastal areas have the most active TC landfall regions each year, where the TC has strong intensity and a large impaction after landing, thus study samples selection in this two provinces have very important significance. Typhoon disasters which closely related to environmental field are often caused by the landing and after; therefore the research of the environmental field after TC landfalling can help the forecasting of TC landing path and intensity.Using the NCEP/NCAR reanalysis atmosphere data, we make use of the dynamic composite analysis and band-pass filter method for studying the influence of environmental field on the north and westbound TC categories landfalling, the result has show:1. By contrast of the large-scale environmental field character of north and westbound TC categories, we found that large-scale environmental field of northbound TC is conductive to its maintenance, while the westbound TC is not conducive to its maintenance. More details as follows: (1) After landfalling, northbound TC category move toward mid-latitude baroclinic front zone in the front of westerly trough, and then move toward into the trough. While westbound TC category have no long wave trough as former category, and far from the mid-latitude baroclinic front zone; (2) Northbound TC category have one water vapor conveyer belt which provided mid-latitude low jet stream, and have another water vapor conveyer belt when landing TC move towards north. While westbound TC category have not such another vapor conveyer belt; (3) After landing, northbound TC category toward near high jet stream which increased its upper level divergence and gain westerly momentum, it do good for TC sustaining. While westbound TC category don't move toward jet stream, high layer's divergences is weak, and have no westerly momentum stir down. (4) After landfalling, northbound TC category gain baroclinic energy from mid-latitude, its vertical wind shear and relative vorticity would be increased in this process, divergence in the centre of the TC have experienced weaker and then stronger on the high layer, they would be experienced a process of extratropical transition. But westbound TC category gain no extra energy and decay after landfall.2. On aspect of the sub-synoptic scale environmental field of the two TC categories, we found some main result as follows: (1) Two TC categories exhibited a kind of wave structure which can influence its intensity around TC circulation, it was found that the interaction between TC and mid-latitude systems could stir strong disturbances on the periphery of TC. Northbound TC category wave structure could sustain a long time after landfalling, but westbound TC categories wave structure sustain a short time. (2) After landing, northbound TC category gain positive relative vorticity from sub-synoptic scale environmental field to enhance its relative vorticity, as to westbound TC category, the positive relative vorticity of sub-synoptic scale environmental field is very weak, it hard to enhance its relative vorticity; (3) Northbound TC category gain kinetic energy from sub-synoptic systems. Butwestbound TC category gains less kinetic energy as northbound TC category.