NUMERICAL SIMULATION OF THE FORMATION OF THE MONSOON GYRE IN AUGUST 1991
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摘要: 季风涡旋对台风活动有重要的影响, 因此研究季风涡旋的形成机制有利于提高台风预报的准确性。此研究利用中尺度非静力数值模式WRF-ARW模拟1991年8月季风涡旋的生成过程, 并对其生成机制进行分析。模式结果表明, 此次季风涡旋个例是由一个中纬度气旋性低压发展而来。初期中纬度高层正位势涡度的强迫作用有利于对流层低层气旋性低压的发展和维持, 随后高层动力强迫作用减弱, 但中纬度气旋性低压在南移过程中其东南侧对流带逐渐与低纬地区的对流带合并, 使得对流潜热释放增强, 进而使低压在Gill响应的作用下不断加强并最终形成季风涡旋。同时, 涡旋的对流结构表现出明显的非对称性, 因而使其得以维持较大尺度。敏感性试验的结果表明对流层高层强迫对于初始低层扰动的发展至关重要, 而后期热带地区的潜热释放有利于季风涡旋的增强。Abstract: The analysis of the genesis mechanism is beneficial to improve the accuracy of typhoon forecast, as the monsoon gyre has an important influence on typhoon activities. The mechanism responsible for the formation of the monsoon gyre in August 1991 is investigated based on a series of numerical simulations using the non-hydrostatic Advanced Weather Research and Forecasting model (WRF-ARW). The results suggest that the monsoon gyre was evolved from a mid-latitudinal low. Upper-tropospheric forcing associated with the anomalous potential vorticity at mid-latitude was favorable for the development of the nascent low. While the subtropical low moved equatorward, the area of active convection to the southeast of the low gradually merged with the convective region in tropics, strengthening the latent heat release to the southeast of the low. Through the Gill-type response to the enhanced heat source, the low further intensified and became the monsoon gyre eventually. Meanwhile, the gyre's asymmetrical convective structure helped maintain the gyre size. Sensitive experiments show that upper-tropospheric forcing plays an important role in the development of the mid-latitudinal disturbance and the latent heat release in tropics is conducive to the intensification of monsoon gyre.
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图 2 a.8月5-7日平均的200 hPa风场(单位: m/s; 箭矢)与风速场(单位: m/s; 阴影); b.8月11日5~25 °N经向平均的大气视热源(Q1;单位: K/day)的纬向-高度剖面图。
图 4 CTL试验中5日00时(a)、5日12时(b).6日00时(c).6日12时(d)、7日00时(e)、7日12时(f)位势涡度(单位:PVU, 1 PVU=10-6 m2. K/(s·kg))垂直经向剖面
图 5 CTL试验中850~200 hPa垂直积分的水汽通量散度(单位: 10-5 g/(s·m2), 阴影)和水汽通量(单位: 106 m2/s, 箭矢)
a.8月9日; b.8月10日; c.8月11日。
图 9 8月5-7日CTL与EXP1试验中Q矢量计算得到的准地转垂直速度(单位:Pa/s)随高度的分布
红色实线代表CTL, 蓝色虚线代表EXP1, 平均区域选取的是以低压外围垂直上升速度最大值为中心的900 km × 900 km区域。
图 10 8月6-8日CTL试验与EXP1试验的850 hPa风场(单位:m/s, 箭矢)和相对湿度(单位:%, 阴影)偏差
a.6日; b.7日; c.8日。黑色三角形代表CTL试验中850 hPa祸旋中心。
图 11 两组敏感性试验8月12日累计降水量(单位: mm, 阴影)和850 hPa风场(单位: m/s, 箭矢)
黑色实心三角表示850 hPa的低压中心。a. EXP1;b. EXP2。
表 1 试验设计情况
试验名称 模式设置 CTL 初始场来自ECMWF再分析资料 EXP1 对88°E~140 °W，35~54 °N, 500~50 hPa区域的再分析资料进行10天低通滤波后构建初始场与边界条件 EXP2 在150~168 °E, 7~15 °N范围内关闭微物理方案及积云对流方案 
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