Simulation of Extreme Updrafts in the Tropical Cyclone Eyewall

Strong vertical motion(>10 m s-1)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 an analysis of the extreme eyewall updrafts in two numerical experiments conducted with the Advanced Research version of the Weather Research and Forecasting(WRF)model,in which the large-eddy simulation(LES)technique was used with the finest grid spacings of 37 and 111 m,for the first time this study demonstrates that the simulated extreme updrafts that occur mainly in the enhanced eyewall convection on the down-shear left side are comparable to available observations.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.The study suggests that extreme updrafts can be simulated when the grid spacing is about 100 m or less in the WRF-LES framework,although the simulated small-scale features need further verification in both observation and simulation.

Simulation of Extreme Updrafts in the Tropical Cyclone Eyewall

Strong vertical motion(10 m s~(-1)) 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 an analysis of the extreme eyewall updrafts in two numerical experiments conducted with the Advanced Research version of the Weather Research and Forecasting(WRF) model, in which the large-eddy simulation(LES) technique was used with the finest grid spacings of 37 and 111 m, for the first time this study demonstrates that the simulated extreme updrafts that occur mainly in the enhanced eyewall convection on the down-shear left side are comparable to available observations. 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. The study suggests that extreme updrafts can be simulated when the grid spacing is about 100 m or less in the WRF-LES framework, although the simulated small-scale features need further verification in both observation and simulation.