Analysis of an Ensemble of High-Resolution WRF Simulations for the Rapid Intensification of Super Typhoon Rammasun (2014)

Abstract

Diagnostics are presented from an ensemble of high-resolution forecasts that differed markedly in their predictions of the rapid intensification (RI) of Typhoon Rammasun. We show that the basic difference stems from subtle differences in initializations of (a) 500–850-hPa environmental winds, and (b) midlevel moisture and ventilation. We then describe how these differences impact on the evolving convective organization, storm structure, and the timing of RI. As expected, ascent, diabatic heating and the secondary circulation near the inner-core are much stronger in the member that best forecasts the RI. The evolution of vortex cloudiness from this member is similar to the actual imagery, with the development of an inner cloud band wrapping inwards to form the eyewall. We present evidence that this structure, and hence the enhanced diabatic heating, is related to the tilt and associated dynamics of the developing inner-core in shear. For the most accurate ensemble member: (a) inhibition of ascent and a reduction in convection over the up-shear sector allow moistening of the boundary-layer air, which is transported to the down-shear sector to feed a developing convective asymmetry; (b) with minimal ventilation, undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud; and (c) this process seems related to a critical down-shear tilt of the vortex from midlevels, and the vertical phase-locking of the circulation over up-shear quadrants. For the member that forecasts a much-delayed RI, these processes are inhibited by stronger vertical wind shear, initially resulting in poor vertical coherence of the circulation, lesser moisture and larger ventilation. Our analysis suggests that ensemble prediction is needed to account for the sensitivity of forecasts to a relatively narrow range of environmental wind shear, moisture and vortex inner-structure.

摘 要

针对超强台风 “威马逊” 的南海迅速加强 (RI) 过程进行了集合数值模拟, 模拟结果具有明显的集合预报离散度. 分析了集合成员的环境风场和中层湿度条件差异对风暴结构、 对流组织和RI发生时点的影响, 结果表明: (1) 风垂直切变数值大小是影响 RI 发生时点的重要因素; (2) 风垂直切变较弱时, 通风干冷下沉气流对边界层影响较小, 边界层湿空气得以输送到下风一侧, 不对称对流发展活跃, 切变下风左侧的对流云团明显地朝着眼心螺旋式发展, 使得上风方向的高层与低层环流垂直耦合, 削弱了涡旋倾斜. 发生 RI 时点较迟的集合成员, 其环境风垂直切变较强, 通风干冷下沉气流较强, 高低层环流垂直耦合度较低. 基于上述分析, 设计了简单的诊断工具--同步指数, 用于判断 RI 是否发生.