Impact of the Monsoonal Surge on Extreme Rainfall of Landfalling Tropical Cyclones

Abstract

A comparative analysis and quantitative diagnosis has been conducted of extreme rainfall associated with landfalling tropical cyclones (ERLTC) and non-extreme rainfall (NERLTC) using the dynamic composite analysis method. Reanalysis data and the tropical cyclone precipitation dataset derived from the objective synoptic analysis technique were used. Results show that the vertically integrated water vapor transport (Q_vt) during the ERLTC is significantly higher than that during the NERLTC. The Q_vt reaches a peak 1–2 days before the occurrence of the ERLTC and then decreases rapidly. There is a stronger convergence for both the Q_vt and the horizontal wind field during the ERLTC. The Q_vt convergence and the wind field convergence are mainly confined to the lower troposphere. The water vapor budget on the four boundaries of the tropical cyclone indicates that water vapor is input through all four boundaries before the occurrence of the ERLTC, whereas water vapor is output continuously from the northern boundary before the occurrence of the NERLTC. The water vapor inflow on both the western and southern boundaries of the ERLTC exceeds that during the NERLTC, mainly as a result of the different intensities of the southwest monsoonal surge in the surrounding environmental field. Within the background of the East Asian summer monsoon, the low-level jet accompanying the southwest monsoonal surge can increase the inflow of water vapor at both the western and southern boundaries during the ERLTC and therefore could enhance the convergence of the horizontal wind field and the water vapor flux, thereby resulting in the ERLTC. On the other hand, the southwest monsoonal surge decreases the zonal mean steering flow, which leads to a slower translation speed for the tropical cyclone associated with the ERLTC. Furthermore, a dynamic monsoon surge index (DMSI) defined here can be simply linked with the ERLTC and could be used as a new predictor for future operational forecasting of ERLTC.

摘要

利用NCEP/NCAR 再分析资料和客观天气图分析法 (OSAT) 热带气旋降水数据集, 采用动态合成分析方法, 对登陆后引发极端降水类台风 (ERLTC) 和非极端降水类台风 (NERLTC) 做了合成对比分析和定量诊断. 结果表明:ERLTC整层水汽通量明显高于NERLTC, ERLTC在极端降水发生前1-2天达到峰值, 极端降水开始后迅速减小. 相比NERLTC, ERLTC整层水汽通量辐合更强, 水平风场辐合更强, 但水汽平流要弱。两组个例的整层水汽通量 (辐合)、 风场辐合均以对流层低层贡献为主. 边界水汽收支表明:ERLTC发生前四个边界均有水汽输入, 而NERLTC的北边界在暴雨发生前水汽持续输出, 其余边界均为水汽输入. ERLTC西边界和南边界的水汽输入均比NERLTC大, 东边界差异不大, 这主要是由TC环境场中的西南季风涌强弱不同造成. 在东亚夏季风背景下, 西南季风涌伴随的低空急流可加强ERLTC的西边界和南边界的水汽输入, 增强其水平风场辐合和水汽通量辐合, 削弱台风的引导气流而使其移速显著减慢, 从而对ERLTC产生影响。动态季风涌指数DMSI对ERLTC有较好指示意义, 有潜力成为一个业务预报新指标.