Climatic Characteristics Of The Rapid Intensification Of Tropical Cyclones In The Western North Pacific And Analysis Of Influencing Factors

Tropical cyclone(TC)is one of the most destructive weather systems on the earth.However,the intensity prediction of tropical cyclones is still a difficult problem for now,especially the rapid intensification(RI)process is difficult to grasp,because the mechanism affecting the intensity change is complex and diverse.Using the best track data of tropical cyclones from 1999 to 2018,reanalysis data and climate factor information,this thesis conducted a systematic investigation of the influencing factors of rapid intensification(RI)of tropical cyclones in the Western North Pacific at different scales.According to a comparative analysis of the time interval,variables and threshold related to the definition of TC RI that combines the statistical characteristics of mean and standard deviation and the method of definition based on the percentile,this study proposed an objective method for the definition of tropical cyclone RI process.For the TCs in Western North Pacific during 1999-2018,the threshold of RI is defined as a 29.82 h Pa reduction of the minimum sea level pressure in TC center within 24 hours.The significant increase of RI frequency indicates that the change of TC intensity is becoming more and more intense.And the RI process mainly occurred from July to October,concentrated in the area of 10°-20°N and 120°-160°E,and moves mainly to the northwest.Based on the different characteristics of relevant environmental factors between rapid intensification(RI)and non rapid intensification(non-RI)processes,as well as the correlation coefficient between RI and environmental factors,it can be concluded that the most obvious influencing atmospheric environmental factor are the divergence of 200 h Pa in the upper layer and vertical wind shear,followed by relative humidity and the divergence of 850 h Pa,water vapor flux divergence and relative vorticity in the lower layer have relatively smaller influence.The correlation between sea surface temperature and RI shows that the stronger the intensity of tropical cyclone is,the higher the required sea surface temperature is.The physical processes and mechanisms of different scale factors such as climate,circulation and environmental variables affecting RI process are studied by using correlation and partial correlation analysis and dynamic synthesis.It is found that the SST in the central East Pacific(CSST)is highly correlated with the number of RI in the Western North Pacific,and can be traced to two climate modes: Pacific interdecadal oscillation(PDO)and Madden-Julian Oscillation(MJO).Based on the characteristics of PDO and MJO impacts on the main regions of RI,the Northwest Pacific can be objectively divided into five regions: Northern South China Sea(N-SCS),northeastern Philippines(NE-PHI),northern Guam(N-GUM),southern Guam(S-GUM)and Eastern Guam(E-GUM).The correlation between PDO and MJO and regional RI in the Northeast Philippines region(NE-PHI)is opposite to that in the other four regions,that is,the number of RIs increases in PDO and MJO negative phase,which is difficult to observe before regional division.In addition,it is found that the areas affected by PDO and MJO are different,which also explains the significance of regional division.PDO and MJO affect the RI process of TC in different regions through their impacts on the atmospheric circulation in the Western North Pacific.PDO affects the main circulation at different altitudes,including the South Asian high,the subtropical high and the monsoon trough,while MJO mainly affects the monsoon trough.In PDO warm years,the main circulations are significantly strengthened,that is,the South Asian high and the monsoon trough extend eastward,and the subtropical high advances westward,the circulation situation in PDO cold years is opposite;In the positive phase of MJO,the annual monsoon trough extends to the East.The South Asian High extends eastward,which induces positive / negative anomaly of divergence in the upper / lower layers of the N-SCS region,and the ocean heat content is a positive anomaly,which is conducive to the RI,otherwise,it is unfavorable..The strengthened eastward extension of the monsoon trough causes the positive anomaly of low-level vorticity,negative anomaly of water vapor transport flux divergence,positive anomaly of middle-level water vapor,negative anomaly of vertical wind shear and negative anomaly of SST in the main areas of RI.As a result,the main occurrence area of RI moves eastward with the extension of the monsoon trough to the East,and vice versa..The advance and retreat of the subtropical high mainly affects the easterly flow between the subtropical high and the monsoon trough,so as to strengthen or weaken the related influence of the monsoon trough.In summary,the physical mechanism affecting the RI in the Western North Pacific is systematically analyzed.From the sea surface temperature in the central East Pacific to the modulation of climate factors PDO and MJO on the monsoon trough,subtropical high and South Asian high of the general circulation system,to the physical process and mechanism of the general circulation system controlling weather environmental factors such as vertical wind shear,divergence,relative humidity and vorticity,and finally affecting the rapid intensification of tropical cyclones.