Analysis Of Convective Structure Characteristics Of Tropical Cyclone Main Rainbands In Large Eddy Simulation Experiments

Forecast the intensity and precipitation of tropical cyclone(TC)is still an international challenge,one of the major reasons being the poor understanding of the fine-structure of TC rainband and their evolution.An in-depth study of the finestructure of TC rainband convection is important for improving the forecast of TC intensity,structure and heavy precipitation as well as disaster prevention and mitigation.Based on three sets of high-resolution numerical experiments,this study analyses the differences in TC intensity and principal rainband convective-scale structure in different numerical experiments,the azimuthal variations of the principal rainband convective-scale structure in the LES-111 m numerical experiment,and the azimuthal variations of inner-edge downdraft and the mechanism of its generation and development.The results are shown as follow:(1)Three experiments are capable to simulate the convective-scale structure of the TC principal rainband,including overturning updraft,secondary horizontal wind maximum(SHWM),mid-level radial outflow and low-level radial inflow.There are great differences in simulated downward motion among the three experiments,only LES-111 m experiment well simulates the low-level downdraft(LLD)on the outer side and inner-edge downdraft(IED)from upper level.Combined with the large eddy technique,the numerical experiment with horizontal resolution at the scale of 100 m is helpful to improve the simulation of convective scale of principal rainband.(2)In the LES-111 m experiment,the azimuthal variations of the convective scale structure are examined by dividing the simulated principal rainband into the upwind,middle and downwind portions and some new feature are found.The inner-edge downdraft is combined with a branch of inflow from the upper levels in middle and downwind portions,carrying upper-level dry air to the region between the overturning updrafts and eyewall,and the intrusion of the upper-level dry air further limits the altitude of the overturning updrafts in the middle and downwind portions of the principal rainband.From the middle to downwind portions,the strength of the SHWM is gradually replaced by low-level wind maximum(LLWM)collocated with LLD.Small-scale rolls are embedded in the overturning updrafts,which further mix the water vapour,heat and momentum in the rainband.(3)During the transition from upwind to downwind portion,the IED is combined to the upper-level dry air inflow,suppressing the altitude of overturning updraft and leading dry air intrusion into the eyewall.The combined effect of buoyancy and the perturbation pressure forces leads to the intensification of IED.The main contribution to IED intensification is made by buoyancy,while thermal buoyancy in buoyancy makes the largest contribution,followed by the water loading.In the upwind portion,the upper-level dry air inflow convergence may result in the appearance of the downward motion.In the middle portion,the IED is enhanced by the combined effects of negative perturbation pressure forces,negative thermal buoyancy,strong water loading and negative buoyancy radial gradient.In the downwind portion,positive lowlevel buoyancy weakens the low-level downward motion,and upper-level inflow results in the accumulation of hydrometeors,which further enhances upper-level water loading.Therefore,vertical tilt of IED decreases.