Studys On The Influence Of Environmental Factors On The Rapid Intensification Of Tropical Cyclone In Northwestern Pacific

The best track datasets of JTWC and typhoons yearbooks for the past decades （2002-2011）in Northwestern Pacific are used to study the statistical characteristics of the rapid intensification（RI）of tropical cyclones（TCs） with different tracks.The different effects of the environmental factors between the RITCs and slow deepening tropical cyclones（Non-RITCs） are analyzed by using the dynamic composite analysis technique with the NCEP FNL data. It is showed that the probability of RI reaches the maximum when the TC track is turning（especially turning with the angle between 120°-125°）.Then the northeastward and northwestward tracks follow in second and third position. There are differences of value and phase distribution in monthly and daily variations of RI among different tracks. And the east of Philippines and Taiwan Island is the region where TC rapid intensification is most likely to occur. If TC moves westward, some RI processes can also appear in the northern South China Sea. The statistics also indicate that once a TC takes a turning track, it often intensifies rapidly at its turning point. The environment fields of RITC and Non-RITC are dramatically different. In contrast with Non-RITC, the South Asia high, which influences the RITC, is much weaker. The strength of subtropical high is stronger.The RITCs show the larger lower-level relative humidity in front of the moving direction. Differences of environmental effects are found among the RITCs with different tracks.The connection between the moisture and TC’s ability to subsequently undergo rapid intensification （RI） are investigated with the NCEP FNL data and the best track datasets of JTWC, which is organized into 24-h intervals of either RI and slow intensification/constant intensity （non-RI） periods. The environmental and internal distributions of moisture and its transport at each interval are statistically analyzed to reveal their significant influence on the intensity change in subsequent 24 hours. The result shows that relative humidity area-averaged from the radius of 3 to 10 latitude at 900hPa（RH₃-10） can obviously distinguish RI from Non-RI,which means the sensitive vertical level of moisture in Northwestern Pacific is lower than that in the Atlantic. In addition, larger RH₃-10 values and smaller horizontal moisture flux（F_all） mark the beginning of RI. With the consuming of the convection development in RI, the environmental humidity decreases and the moisture flux is enhanced. The obvious differences of the moisture distribution and transfer between TC inner-core and outer-rainband region are also studied, showing that the net convergence region of moisture flux in RI is larger than that in Non-RI.The correlative analysis also indicates that intensity forecasts and RI predictions in particular may be aided by the use of the moisture and its flux both in environmental and internal region.Finally, the changes of environmental relative humidity, net water vapor flux and warm core structure are examined using a WRF simulation of Typhoon Chanchu with a high resolution. The results of statistical analysis are validated preliminarily.