STUDY ON THE MECHANISM OF LOW-LEVEL JET IN A HEAVY RAINFALL IN THE YANGTZE RIVER BASIN
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摘要: 利用WRF模式对2016年6月30日—7月6日长江流域的一次大暴雨天气过程中的低空急流进行数值模拟，在成功模拟低空急流基础上，分析此次急流过程中可能的影响机制；同时对地形高度进行敏感性试验，分析地形因素对此次低空急流可能的影响。(1)此次低空急流发生时，东侧为西太平洋副热带高压，西侧则为西南涡。这种“东高西低”的高低压配置为低空急流的形成与发展提供了有利的背景场。(2)高空急流和低空急流的耦合作用是低空急流发展的一个重要背景条件。(3)垂直方向高空动量不断下传为低空系统的发展提供了动力支持，是低空急流发生的一个重要条件。(4)逆温和垂直风切变之间的正反馈机制是低空急流形成与加强的因素之一。(5)山体在急流生成及发展过程中对气流有摩擦和阻挡作用，这种阻挡作用随着山体地形高度的增加而有所加强，同时山脉的走向会改变原始的风向，使得急流前端超前或滞后。青藏高原有强背风波效应，它的绕流和挤压作用会使得低空气流表现为狭长的带状，使动量更加聚集从而风速增加形成低空急流。Abstract: The Weather Research and Forecast (WRF) model is used to simulate the low-level jet (LLJ) during a heavy rainfall process that occurred in Yangtze River valley from 30th June to 6th July 2016. Based on the successful simulation of LLJ, the possible mechanism of LLJ was analyzed. Meanwhile, a sensitivity test of the terrain height was performed to analyze the effect of topographic factors on the LLJ. The results are showed as follows: (1) This LLJ occurred with the western Pacific subtropical high in the east and the southwest vortex in the west. The situation of "east high and west low in the pressure field" provides a favorable background for the formation and development of LLJs. (2) The coupling of high-level jets and LLJs is an important background condition for the development of LLJ. (3) Constant vertical momentum downward from high-altitude provides momentum for LLJ, which is also an important condition for the occurrence of LLJ. (4) Positive feedback between temperature inversion and vertical wind shear is one of the factors that strengthen LLJ. (5) The mountain body has friction and blocking effect during the formation and development of LLJ which increases with the mountain height. Meanwhile, the direction of the mountains will change the original wind direction, making LLJ leading or lagging. Tibetan Plateau has strong lee wave effect, its flow and squeezing effects will make the air flow appear as a narrow band, which will accumulate the momentum and increase the air speed to form a low-level jet stream.
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Key words:
- rainstorm /
- low-level jet(LLJ) /
- numerical simulation /
- mechanism analysis /
- terrain /
- sensitivity test
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图 4 观测的2016年6月30日02时(a、c)和08时(b、d)500 hPa(a、b)、700 hPa(c、d)位势高度(粉色实线，单位：gpm)和温度(蓝色线，单位：℃)
a、b中，棕色线为槽线，红色线为副热带高压线。
图 5 模拟的7月1日02时(a)、08时(b)200 hPa高空急流(阴影，单位：m/s)、风矢量(箭头，单位：m/s)及散度(紫线，单位：10-5 s-1)；模拟的7月1日02时(c)、08时(d)850 hPa低空急流(阴影，单位：m/s)、风矢量(箭头，单位：m/s)及散度(紫线，单位：10-5 s-1)；c、d中红线、黄线为两个剖面线；e.高低空急流耦合示意图
图 7 模拟的2016年7月1日02时(a)、08时(b) 950~900 hPa高度垂直风切变(阴影，单位：m/s)及逆温分布图(黑线，单位：℃)；c. 6月29日14时—7月2日02时850 hPa高度全风速随时间变化(曲线，单位：m/s)；d.垂直风切变和逆温相互作用示意图
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