Studies On Characteristics Of Convection And Precipitation Of Tropical Cyclones In The Northwest Pacific

Precipitation in tropical cyclones(TCs)can lead to catastrophic damage and significant loss of life through destructive winds,heavy rainfall,and large storm surges.Compared with non-TCs weather systems over the ocean,precipitation and convection(especially in the inner core region)in TCs are unique,and many studies have showed that their development and evolution are close related to TC intensity change and rapid intensification.However,concerning the convective and precipitation characteristics of TCs,there is still a lack of complete and systematic understanding,and the necessary and sufficient conditions for the rapid increase of TCs are controversial.This thesis statistically examines the convective and rainfall characteristics in different TC regions and intensity stages in the Northwest Pacific basin from 11 years of TRMM data.The properties of inner-core deep convection with different depths in different TC intensity and intensity change categories,and before/during/after rapid intensification were also investigated.The main conclusions are as follows:(1)The strongest convection generally presents in the IC region,less in the IB region and least in the OB region based on radar and passive microwave.The convective intensity in the IC and IB regions increases as TCs strengthen.When TC intensity increases,the frequency of deep convection increases,but the intensity decreases except for CAT35 stage.The lightning density of OB PFs is 1.2(2.7)times higher than that of IC(IB)PFs when considering all features,however,the lightning density of IC PFs is 1.4(4.8)times than that of OB(IB)PFs only for the features with area larger than 1000 km~2.IC(OB)PFs have the highest(lowest)threshold to have a lightning flash.The occurrences of lightning are mainly determined by the strength of convection,and the area of convection has little effect.(2)TCs are dominated by stratiform precipitation,which accounts for more than78%of the total raining area.The highest fraction of the stratiform raining area exists in the IB region and increases as the TC intensity increases(from 80%to 93%).Stratiform precipitation in the IC region generally has comparable or even stronger ice scattering signatures and higher 20-dBZ radar echo heights than the convective precipitation in the IB and OB regions.Weak convection decreases significantly as the TC intensity increases,which leads to increased convective intensity.Stratiform(convective)precipitation accounts for 61%(39%)of the total TC volumetric rain and25%(75%)of the total TC lightning flash,respectively.Moreover,stratiform precipitation's contribution to the total TC volumetric rain and lightning flash increases as the TC intensity increases,which indicates that stronger TCs are favorable for maintaining more stratiform precipitation.The stratiform and convective precipitation properties in different TC regions and intensities cooperatively change with the enhanced ascending branch in the IC region and the radial outflow at the upper levels of the secondary circulation.(3)The probability of deep convection(defined as the 20-dBZ radar echo height>10,11,12,13,14 km)generally increases as TC intensity increases and rapidly intensifying(RI)TCs have a higher probability than other intensity change categories.However,category-3–5(CAT35)hurricanes have the strongest convective intensity with the highest maximum 30-dBZ radar echo height while RI storms have significantly lower 30-dBZ radar echo height for 13(14)km deep convection at the90%(95%)confidence level compared to other intensity change categories.The deep convection in stronger TCs tends to be larger and more mature(small fraction of convective precipitation).Significant larger size and more mature deep convection are also found in RI category.(4)The convective height of the deep convective system is relatively high(high20-dBZ radar echo height)before RI.The convective intensity(30-dBZ radar echo height)of the deep convection system is strong before/after RI,while it is weak during RI.Deep convection systems in the innercore region matured over time before RI and it is basically stable during RI.The deep convective system was mature at 6 h after RI,but this stable state was broken with time.Before RI,the latent heat of the deep convective system changes little,and it rapidly increases during RI and decreases After RI.A large amount of latent heat release during RI may increase the buoyancy of the TC eyewall and enhance the generation of kineticenergy,thereby rapidly strengthening the system.