The Relationship Between Convective Bursts And Tropical Cyclone Rapid Intensification

The rapid intensification of tropical cyclone(TC)is a very complicated problem,especially for the offshore TC before landing,which will seriously endanger the safety of life and property in coastal areas.Therefore,the relevant researches on rapid intensification have attracted many scholars' attention.Among these studies,the relationship between convection burst(CB)and rapid intensification,that is,whether the emergence of CB is a byproduct or the cause of the rapid intensification,has aroused a lot of controversy.So,this paper carried out corresponding observational and numerical simulation studies on the relationship between CB and rapid intensification,and proved that CB is more likely to be the cause of rapid intensification than the result.Based on the analysis of observation data,we found that CB activity and rapid intensification in TC correspond to different large-scale environments.The environment related to CB activity is mainly dominated by thermodynamic factors.The contributions of thermodynamic factors such as temperature difference between upper and lower atmosphere,ocean heat capacity and relative humidity are greater than vertical wind shear and other kinematic factors.While in the large-scale environmental conditions corresponding to the rapid intensification of TC,the relative weights of kinematic factors such as vertical wind shear and upper-level divergence are significantly larger than those of thermodynamic factors such as relative humidity and sea surface temperature.This indicated that the CBs are not a byproduct in the process of rapid intensification,otherwise the corresponding large-scale environment of them should be consistent.In addition,the results showed that CB activity at the current moment is more closely related to the subsequent TC intensity change compared with the previous TC intensity change.More CB activities occurred in the stage when the TC was about to strengthen,while less CB activities occurred in the stage when the TC was about to weaken.This meant that CB activity may be followed by rapid intensification,which indirectly proved that CB is not the result of rapid intensification.Based on numerical simulation,we analyzed the distribution characteristics of CBs and the process of CB leading to the rapid intensification in typhoon "Saomai"with the high resolution three-dimensional data.The results indicated that a large number of CBs occur before the onset of rapid intensification,and most of them locate in the inner part of the radius of maximum wind(RMW)(circle area between the 0.25 times and 0.75 times RMW)and near RMW(circle area between the 0.75 times and 1.25 times RMW).However,CBs began to decrease rapidly with the occurrence and maintain of the rapid intensification.There were hardly any CBs left after the end of the rapid intensification.This also indirectly proved that CBs are not a result of rapid intensification and can play an important role in the occurrence of rapid intensification.Further results showed that the latent heat release and the subsidence of upper-level air related to CBs caused a significant warming in the middle and upper troposphere in the inner part of the RMW and near the RMW.The warming in the inner part of the RMW leaded to the formation of the warm-core structure in the center of"Saomai",which caused the rapid drop of central pressure and the rapid increase of tangentwind speed through the hydrostatic adjustment and the gradient wind balance,and thereby resulted in the rapid intensification of "Saomai".On the other hand,the warming near the RMW leaded to the positive buoyancy of the middle and upper troposphere in the corresponding region,and casused the increase of the secondary circulation,incluing stronger and deeper radial inflow and stronger updraft and the high-level radial outflow,and triggered further the increase of primary circulation and tangential wind speed through the momentum of the horizontal and vertical transportation.Eventually,the intensity of system strengthed.This indicated that the formation of warm-core structure and the increase of secondary circulation associated with the warming caused by CBs played an important role in the occurrence of rapid intensification.