Statistical Analysis Of Relationship Between Tropical Cyclone Intensity And Size Change Over The Western North Pacific

Tropical cyclone(TC)intensity and size are important indicators to measure its destructiveness.By using the best track data from JTWC(Joint Typhoon Warning Center)from July to November 2004 to 2020 over the western North Pacific(WNP),this study is carried out on climatological characteristics of TC intensity and size change;the impact of initial vortex size on later size development;and the lifetime covariation characteristics of the intensity and size change rate under the combined effects of size and intensity.Based on the understanding of the impact of TC’s own factors,combined with the SHIPS(Statistical Hurricane Intensity Prediction Scheme)reanalysis dataset,the impact characteristics of various environmental factors on size change rate under different given intensity parameter distributions are analyzed.Finally,a size forecast model is build using neural networks and Bayesian methods so as to provide support for the establishment of a TC size prediction system.Statistical analysis of the spatiotemporal and covariation characteristics of TC intensity and size(selected as 26 m /s wind speed radius,R26a)reveals that the intensity and size of tropical cyclones reach their peak in October,mainly showing a higher proportion of strong and large-sized TCs with longer lifetime at sea than other months.by a higher proportion of large and strong tropical cyclones with a longer lifetime at sea than in other months.the TC size expands with the increase in intensity and contracts as the TC weakens.TCs reach the lifetime maximum size(LMS)later than the lifetime maximum intensity(LMI),with a mean lag time of 40 hours.T Compared to the TC rapid intensification(RI)and LMI,the mean meridional positions of TC rapid expansion(RE)and LMS are closer to the coastal continent.Initial vortex size of TC affects the size development,especially LMS.Specifically,58% of small initial vortices maintain the size in the small to medium category.While71% of vortices with large initial size develop to large vortices in later periods,with59% intensify to strong TCs(≥59m/s)at LMI stage.The size of the latter stage has a high correlation(≥0.45)with the initial R26 a for 66 h,indicating that the initial size of TCs can be a key predictor.The peak of size change rate(DR26)is located at moderate intensity(25～50 m/s)and the peak intensity change rate(DVMAX)is located at medium and small size(50～100 km).The TC size is also influenced by environmental factors.Through statistical analysis,under a given relative intensity,the TC size change rate is related to six environmental factors,namely,vertical shear of ambient wind,middle layer relative humidity,200 h Pa divergence,850 h Pa relative vorticity,ocean heat content,and dry air prediction factors.It is found that ocean heat content and environmental humidity have a significant impact on the TC size change.High ocean heat content and environmental humidity are conducive to the expansion of the TC size.Especially under conditions of high divergence,high relative humidity,high ocean heat content,moderate and weak vertical shear of ambient winds,and weak dry air intrusion,the size is more likely to occur outward expansion,even rapid expansion.At the same time,in the cases of strong TCs(intensity close to MPI),TC size is more sensitive to the changes of the surrounding environment.An attempt was made to establish a TC size forecast model for WNP using a combination of neural networks and Bayesian methods.The results shows that the 24-h and 48-h forecast errors(MAE)of R17 are better than those of the CLIPER forecast method by about 20% and 24%,and the 24～72 hours forecast MAE of R26 a reached32km～34km,respectively,showing a good application prospect.