The Effects Of Convective Activities On Intensity And Size Changes Of Tropical Cyclones

The forecasting of tropical cyclone(TC)intensity showed limited improvement in the past few decades.The lack of understanding of the physical mechanism of TC intensity changes,including how the convective activities affect the intensity and structure change,limits the improvement of TC intensity forecasting.This dissertation focused on the study of the role of the convective activity on TC intensity changes in the following three aspects:(1)The role of convective activity on TC intenfication need to be quantified.(2)The role of convective activity on the contraction of the radius of maximum tangential wind(RMW)of TCs.(3)The role of convective activity on the intensification of TCs and the contraction of the RMWs in the early stage of storms.At first,the effects of different convective activities on intensity and intensity change are studied.The convective clouds were grouped as very deep convective clouds(infrared(IR)brightness temperatures(BTs)≤208 K)and high clouds(208 K<IR BTs≤240 K)based on different characteristics of diurnal vatiations.Effects of two types of convective clouds and precipitation on the intensity and intensity change of TCs in the western North Pacific between 2000-2017 are studied.Results show that TC intensity increases as the amount of precipitation and the high clouds increases.However,this relationship does not mean that changes in precipitatoin and high clouds lead to the changes of TC intensity.The changes of the very deep convective clouds are closely associated with TC intensity change.TCs will intensify rapidly when the areas of very deep convective clouds are sufficiently large,which suggests to be a good predictor of TC intensity change.The areas of very deep convective clouds are larger for rapid intensifying TCs than non-rapid intensifying TCs at beginning and following intensification hours.Secondly,the contraction between convective activities and contraction of the RMW are studied.The composite analysis shows that,the distribution of very deep convective clouds is the trigger of rapid contraction of RMW,the very deep convective clouds are mainly distributed within RMW and closer to the center of TCs when RMW contracts rapidly(contract more than 20 nmi within 24 h).A smaller initial RMW(usually less than 30 nmi)is favorable for TC rapid intensification.When the initial RMW is larger(ususlly larger than 30 nmi),more very deep convective clouds are required for rapid intensification.The statistical results of these observations support previous theoretical studies that convective heating induced inflow increases tangential wind below and outside the convective heating region through advection of angular momentum.RMW contraction is associated with spin-up of tangential wind being much faster inside the RMW than at the RMW itself.At last,the relationship between intensity and size of TCs is studied.The negative correlation between intensity and size during ’mature stage’ of mature TCs increases the nonlinear relationship between intensity and size of TCs.In addition,the angular momentum analysis show that,relative angular momentum contributes more during intensifying stage,and earth angular momentum contributes more during ’mature stage’.Furthermore,the decrease of relative angular momentum is smaller than the increase of earth angular momentum at the end of ’mature stage’ in North Atlantic,which prolongs the ’mature stage’ by 12 h roughly.