Surface Rainfall Processes during the Genesis Period of Tropical Cyclone Durian (2001)

Abstract

The rainfall processes during the formation of tropical cyclone (TC) Durian (2001) were investigated quantitatively using the three-dimensional (3D) WRF-based precipitation equation. The rain rate (P_S) decreased slightly as the TC approached to formation, and then increased as Durian began to intensify. The rate of moisture-related processes (Q_WV) in the equation contributed around 80% to P_S before TC genesis, and made more contribution during and after TC genesis. The rate of hydrometeor-related processes (Q_CM) contributed about 20% before TC formation, followed by less contribution during and after TC formation. Q_WV were dominated by the 3D moisture flux advection rate (Q_WVA), while the surface evaporation rate (Q_WVE) also played an important role. Just before TC genesis, moisture from Q_WVA and Q_WVE helped the local atmosphere moisten (negative Q_WVL). Q_CM were determined by the 3D hydrometeor advection rates (Q_CLA and Q_CIA) and the local change rates of hydrometeors (Q_CLL and Q_CIL). During TC formation, Q_CM largely decreased and then reactivated as Durian began to intensify, accompanied by the development of TC cloud. Both the height and the strength of the net latent heating center associated with microphysical processes generally lowered before and during TC genesis, resulting mainly from lessening deposition and condensation. The downward shift of the net latent heating center induced a more bottom-heavy upward mass flux profile, suggesting to promote lower-tropospheric convergence in a shallower layer, vorticity amplification and TC spin-up.

摘要

本文利用基于 WRF 模式的三维降水方程, 定量研究了热带气旋(tropical cyclone, 简称 TC)“榴莲”(2001)生成期间的降水过程. 研究表明, 当 TC“榴莲”趋近于生成时, 降水率(P_S)有所减小, 当 TC生成后开始加强时, 降水率逐渐增大. 降水方程中的水汽相关过程贡献了约 80%的 TC生成前降水, 对 TC 生成后的降水贡献则更大; 而降水方程中的云水凝物相关过程贡献了约 20%的 TC 生成前降水, 对 TC 生成后的降水贡献则更小. 其中, 水汽相关过程由三维水汽通量平流项(Q_WVA)主导, 同时, 海表蒸发项(Q_WVE)也起到重要作用. 在 TC 生成前, 由Q_WVA 和 Q_WVE 带来的水汽还起到增湿局地大气的作用. 云相关过程主要由三维云水凝物的平流项(Q_CLA, Q_CIA)和水凝物局地变化率来决定(Q_CLL, Q_CIL). 在 TC 生成期间, 云相关过程对降水的贡献率先显著减小, 接着随着“榴莲”的加强而有所增大, 伴随着 TC 云系的发展. 此外, 与云微物理转化过程相联系的净潜热加热率中心的高度和强度在 TC 生成过程中分别呈现降低和减弱的趋势, 这主要是由云微物理过程中的凝华和凝结过程减少所引起的. 净潜热加热率中心的向下转移引起了向上质量通量廓线中心的下移, 该变化有利于低层大气在近地面的辐合, 涡度的增长和 TC 的自旋发展.