Extreme Vertical Motion And Fine Structure Analysis Of Tropical Cyclone Eyewalls For Large Eddy Simulations

Strong vertical motion has profound implications for tropical cyclone(TC)structure changes and intensity.While extreme updrafts in the TC are occasionally observed in real TCs,the associated small-scale features remain unclear.Based on the analysis of three numerical experiments with different horizontal resolutions(333 m,111 m and 37 m)conducted with the Advanced Research version of the Weather Research and Forecasting(WRF)model,in which the large-eddy simulation(LES)technique was used in the boundary layer,the spatial and temporal distribution of the extreme updrafts of the eyewall is studied,and its associated small-scale fine structure is classified and traced.Main conclusions are as follows:The results show that the characteristics of the vertical motion distribution in three numerical experiments are consistent with the observations,and all almost satisfy the log-normal distribution,about 70%of vertical velocities range from-2?2 m s^-1,but there are also extreme vertical velocities.Both the drafts(>0.5 m s^-1)and the cores(>1 m s^-1)show the statistical characteristics of a large number and a small radius in the lower level,but the upper level is the opposite.When the horizontal grid spacing is reduced,the intensity of extreme vertical motion is enhanced,the number of drafts increases and the size of drafts decreases.In the two numerical experiments with higher resolution,the simulated extreme updraft exhibits relatively high frequencies in the lower(750 m),middle(6.5 km)and upper(13 km)troposphere,which are associated with different types of small-scale structures.While the lower-level extreme updraft is mainly related to the tornado-scale vortex,the extreme updraft at upper levels is closely associated with a pair of counter-rotating horizontal rolls oriented generally along the TC tangential flow,which are closely associated with the enhanced eyewall convection.The extreme updraft at middle levels is related to relatively complicated small-scale structures,it can be divided into three categories:a ring-like roll,two counter-rotating rolls and a single roll.In the numerical experiment with a horizontal resolution of 37 m,the small-scale systems in different regions were classified:The inner side of the strong convective eyewall is dominated by the combined effect of tornado-scale vortexes,horizontal vortices like rolls and smaller-scale vortices to produce 70 m s^-1 local gusts,and the vertical motion amplitude up to10 m s^-1.In the outer side of the strong convective eyewall,the local strong wind is 40?50 m s^-1,and the vertical motion amplitude is only 4 m s^-1,which is mainly caused by the rolls with a wavelength of about 3?4 km and a length of 10 km.Rolls on the vertical motion field in the weak convection eyewall area,along the tangential direction and inclined outward,with a wavelength of about 2?3 km and a length of about 5?6 km,which does not produce strong surface winds.Because tornado-scale vortexes are accompanied by extreme upward motion and local surface gusts,we then track the evolution of tornado-scale vortexes in the experiment with 37m horizontal resolution,both undergo a process of merging and splitting from smaller-scale vortices to reach TSV intensity,but it can also be grouped into three categories:The first is a gradual merger,the second type is a rapid enhancement,and the third type has been evolving to maintain a multi-vorticity center.