Simulation of Extreme Updrafts in the Tropical Cyclone Eyewall

Abstract

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.

摘 要

极端上升运动(> 10 m s⁻¹)对热带气旋强度和结构变化有重要作用. 在实际观测中, 虽然能发现热带气旋中存在极端上升运动, 但与之联系的小尺度系统仍不清楚. 本文采用结合大涡模拟技术的WRF模式进行了两组高分辨率的数值试验, 发现极端上升运动的分布特征与观测一致, 主要出现在眼墙强对流一侧的低层(750 m)、 中层 (6.5 km)和高层(13 km), 并且对应着小尺度环流. 极端上升运动在低层主要与龙卷尺度涡旋相联系, 高层与沿着切向风分布的一对反向旋转水平涡旋相关, 而中层的极端上升气流与相对复杂的小尺度系统有关. 研究表明, 当水平分辨率约为 100 m 或更小时在WRF中可以模拟出极端上升运动, 但小尺度系统特征在观测和模拟中都需要进一步验证.