Effects Of Rainwater Evaporation In Tropical Cyclone Inner Core On Its Intensity And Structure

Tropical cyclone(TC)can cause strong wind and precipitation.It is of great importance to study the factors that affect the intensity and structure of TC for the prediction of strength and the resulting disasters.TC will cause precipitation and disasters in the affected areas,and it can be affected by the evaporative cooling of rainwater,including future strength and development of rainbands.In addition,the effects of rain evaporation are different at different radii of TC.Therefore,the following three scientific issues will be addressed in the thesis:(1)the influence of rain evaporation on TC intensity and inner-core structure;(2)the structure features and influencing factors of different types of inner rainbands,and to what degree the influence of rain evaporation on them;(3)the relationship between the low-level cold pool and evaporation of rainwater in the inner core,and the role of eyewall playing.Intensity of TC,inner-core wind field,inner-core precipitation and characteristics of cloud are analyzed by sensitivity experiments.Evaporation of rainwater includes two functions: the evaporative cooling and the evaporative wetting.The variation of water vapor mixing ratio mainly depends on the vertical advection and microphysical process.In the middle and lower layers outside the eyewall,the evaporation of rainwater will increase the moisture content of this area.The control group and sensitivity experiments show that the evaporative cooling in the inner core will weaken strength,while the evaporative wetting is the enhancement effect,and the influence of both on strength almost offset.But the inner-core horizontal circulationand secondary circulation of TC are weak in the experiment without evaporation.There are two reasons: no evaporative cooling effect,the subsidence movement outside the eyewall is weak;no evaporative wetting,the water vapor mixing ratio in the circulation of TC is reduced,resulting in the reduction of latent heat release of condensation in the eyewall,and weakening the rise and tangential wind.At the same time,the water vapor flux on the underlying surface increases in the experiment without evaporation.This shows that evaporative wetting will supplement the water vapor in TC and reduce the dependence on the underlying surface.What's more,when there is no inner-core evaporation,the cloud water and ice mixing ratio in the inner core also decreases,and the overall distribution of the inner rainbands also changes,that is,the inner-core evaporation will affect the convection activity in this area.The rainbands within stagnation radius or three times the maximum wind speed radius are called inner rainbands.There are still differences in structure and influencing factors between the inner rainbands.Structurally,some of the inner rainbands incline inward,while some incline outward;moreover,some of them have cold pools in the low levels.As for the factors affecting the inner rainbands,some of them are related to the vortex Rossby wave,others are related to the advection of the low-level rapidly rotating wind field.In this study,it is also found that some of them are related to the low-level cold pool.Sensitivity experiment shows that some of the inner rainbands are indeed affected by the cold pool.When there is no evaporation of rainwater,and then there is no cold pool in the low levels,they weaken or even disappear.As for the cold pool in the lower layer of the inner core,this paper analyzes the causes of its formation.Microphysical processes are the main reasons for the formation and maintenance of the cold pool,and the evaporation of rainwater accounts for a large proportion.In addition,not only the evaporation of rainwater in the boundary layer plays an important role in the formation and maintenance of the cold pool,but also the evaporation of rainwater above the boundary layer promotes the subsidence through the cooling effect,bringing the dry air from the middle layer into the boundary layer,which is conducive to the further evaporation of rainwater inthe boundary layer.Similarly,sensitivity experiment also confirms the above results.In addition to the inner rainband,eyewall is in the inner core.The low-level cold pool is located under the eyewall,and there is a certain relationship between the two.The positive anomaly of eyewall cloud is located in the downwind side of the cold pool,and the positive anomaly phase of precipitation particles is slightly backward,which is located directly above the cold pool.The vortex Rossby wave represents the activity of eyewall convection,and it within height of 2-4 km has a certain corresponding relationship with the cold pool below.The results show that the activity of eyewall has a certain effect on the formation and maintenance of low-level cold pool,to achieve which the evaporation of rainwater is a critical process.