LAUSR

The impact of mid- and upper-level dry air, represented by low relative humidity (RH) values, on the genesis of tropical cyclone (TC) Durian (2001) in the South China Sea was investigated by a series of numerical experiments using the Weather Research and Forecasting model. The mid-level RH was lowered in different regions relative to TC Durian (2001)’s genesis location. Results suggest that the location of dry air was important to Durian (2001)’s genesis and intensification. The rapid development of the TC was accompanied by sustained near-saturated mid- and upper-level air, whereas low humidity decelerated its development. Water vapor budget analysis showed that moisture at mid and upper levels was mainly supplied by the vertical convergence of moisture flux and the divergence terms, and consumed by the condensation process. The horizontal convergence of moisture flux term supplied moisture in the air moistening process but consumed moisture in the air drying process. With a dryer mid- and upper-level environment, convective and stratiform precipitation were both inhibited. The upward mass fluxes and the diabatic heating rates associated with these two precipitation types were also suppressed. Generally, convection played the dominant role, since the impact of the stratiform process on vertical mass transportation and diabatic heating was much weaker. The vorticity budget showed that the negative vorticity convergence term, which was closely related to the inhibited convection, caused the vorticity to decrease above the lower troposphere in a dryer environment. The negative vorticity tendency is suggested to slow down the vertical coherence and the development rate of TCs.摘要本文利用 WRF 模式开展了一系列敏感性实验, 通过改变南海热带气旋“榴莲”生成地点周围不同区域的中高层相对湿度, 研究了中高层干空气对热带气旋生成过程的影响. 结果表明, 干空气的分布位置对“榴莲”的生成和加强十分重要, 热带气旋的快速发展往往伴随着持续的, 近饱和的中高层空气, 而湿度较小的空气会减缓热带气旋的发展. 水汽收支分析表明, 中高层水汽主要由水汽通量的垂直辐合项以及散度项供应, 同时被凝结项消耗; 其中, 水汽通量的水平辐合项在大气的增湿过程中提供水汽来源, 而在空气的变干过程中则消耗水汽. 当中高层空气较干时, 对流降水和层云降水, 以及由二者引起的向上质量通量和非绝热加热率均被抑制. 一般来说, 层云降水对应的质量通量和非绝热加热率均远远小于对流降水, 因此, 对流降水的作用占据主导地位. 涡度收支表明, 在较干的环境中, 由被抑制的对流引起的负的涡度辐合项使低层的涡度减小, 负的涡度变化项进而减慢热带气旋的垂直耦合和发展速率.