A Synchronous Variation Process of Tibetan Plateau Vortex and Southwest Vortex
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摘要: 青藏高原低涡(简称高原低涡)和西南涡是影响我国降水的重要天气系统,两者同步变化是引发我国西南和东部地区强降水的重要方式,而两低涡同步变化的物理过程和机理目前尚不清晰。为探究高原低涡和西南涡同步变化的物理机制,选取2020年超强超长梅雨期间一次高原低涡与西南涡同步变化过程,利用ERA5逐小时再分析资料及降水观测资料,分析两涡共存时特殊时间节点所对应的强度、结构等演变特征及位涡收支。结果显示:水平位置不重叠的高原低涡和西南涡也可发生同步变化,即强度变化特征大致相似。两低涡在同步变化之前各自的演变机理不同,但同步变化时两者的演变机理基本一致。具体地,未发生同步变化时,高原低涡主要依靠加热场作用维持东移,西南涡则依靠水平位涡通量散度作用得以维持;两涡同步变化时,两者强度变化相似,演变机理一致,两涡维持主要依靠水平位涡通量散度作用,加热场作用次之。Abstract: The synchronous variation of Tibetan Plateau vortex (TPV) and southwest vortex (SWV) is an important way to trigger heavy precipitation in southwest and eastern China. However, the physical process and mechanism of the coordinated change of two vortices are still unclear. A synchronous variation process of the TPV and SWV during the super-strong and super-long Meiyu period in 2020 (during 1-4 June of 2020) is selected to analyze the evolution characteristics of the strength and structure corresponding to the special time node (T1-T5) when two vortices coexist, as well as the potential vorticity budget with the data including the fifth-generation European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5) hourly data and the precipitation observations. It's found that during the period T1-T2, TPV moves eastward on the Tibetan Plateau. When the SWV is generated (T1), it is far away from the TPV, and it is considered that two vortices have no interaction at that time. At the time of T3, when the TPV moves to the lower slope terrain on the eastern side of the Tibetan Plateau, its intensity is significantly weakened. Also at that monment, the TPV begins to change in coordination with the SWV. During T4-T5, two vortices strengthen and continue to change synchronously, and then merge into cyclonic circulation on their east side. Combined with the intensity changes of two vortices, the TPV and SWV with non-overlapping horizontal positions can also undergo synchronous variations, when their characteristics of intensity changes are roughly similar. From the analysis results of the potential vorticity diagnostic equation, two vortices have different evolution mechanisms before the synchronous variation, but their evolution mechanisms are basically the same when the synchronous variation occurs. It can be concluded that, when there is no synchronous variation (T1-T2), the TPV mainly relies on the heating field to maintain the eastward movement, and the SWV is maintained by the horizontal potential vorticity flux divergence. When the two vortices change synchronously (T3-T5), their intensity changes are similar, and the evolution mechanisms of them are consistent. The maintenance of two vortices mainly depends on the horizontal potential vorticity flux divergence, followed by the heating field.
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Key words:
- Tibetan Plateau vortex;
- southwest vortex;
- synchronous variation
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图 1 2020年6月1日23:00—4日09:00高原低涡和西南涡移动轨迹(圆点代表高原低涡,三角代表西南涡,绿色表示高原低涡与西南涡共存时各自的轨迹,红色三角为西南涡生成位置, 橙色线表示青藏高原,下同) (a)及强度随时间变化(b)
Fig. 1 Trajectory (dots denote the TPV, triangles denote the SWV, green lines denote trajectories of the TPV and the SWV, the red triangle denotes the genesis location of the SWV, the orange line denotes the Tibetan Plateau, similarily hereinafer) (a) and intensity(b) of the TPV and the SWV from 2300 UTC 1 Jun to 0900 UTC 4 Jun in 2020
图 2 T1—T5时刻200 hPa位势高度(等值线,单位:gpm) 和全风速(阴影)、500 hPa位势高度(等值线,单位:gpm) 和风场(矢量)、整层水汽通量(矢量) 及水汽通量散度(阴影)
(红线分别代表南亚高压(200 hPa)和副热带高压(500 hPa)北界位置)
Fig. 2 200 hPa potential height (isolines, unit: gpm) and wind speed (the shaded), 500 hPa potential height (isolines, unit: gpm) and wind (the vector), vertically integrated water vapor flux (the vector) and the water vapor flux divergence (the shaded) at T1-T5
(red isolines denote the northern boundary of the South Asia high(200 hPa) and the subtropical high(500 hPa))
图 3 高原低涡涡度(暖色等值线;单位:10-5 s-1)、散度(填色) 垂直速度(蓝色等值线,单位:Pa·s-1) 的经向、纬向垂直剖面图及500 hPa平面图
Fig. 3 Meridional and zonal vertical profiles of vorticity (warm isolines, unit: 10-5 s-1), divergence (the shaded) and vertical velocity (blue isolines, unit: Pa·s-1) with horizontal distribution at 500 hPa for the TPV
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