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A Heavy Precipitation Process over the Tibetan Plateau Under the Joint Effects of a Tropical Cyclone and Vortex
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摘要: 利用联合台风预警中心（Joint Typhoon Warning Center，JTWC）最佳路径资料、逐小时降水资料和ERA5再分析资料，研究2017年5月26—31日孟加拉湾风暴与高原低涡共同影响下青藏高原一次强降水过程，结果表明：风暴和南支槽共同作用下建立的孟加拉湾至青藏高原的水汽输送带为高原低涡-切变线区域的降水提供水汽。南支槽后冷气流在青藏高原南部陡坡下沉形成冷垫，孟加拉湾偏南暖湿气流首先沿冷垫向北抬升，爬上青藏高原后向北在高原切变线附近再次抬升，增加降水区地表至对流层高层大气中的可降水量。风暴偏南风暖湿气流与青藏高原北部干冷空气交汇产生锋生，大气湿斜压性显著增长，湿等熵线密集陡立导致垂直涡度剧烈发展，有利于高原低涡加强。风暴北上过程中其高层反气旋式出流加强青藏高原槽前西南风高空急流，辐散增强有利于低层切变线发展和高原低涡东移，产生大范围强降水。高原低涡切变线与风暴水汽输送的正反馈作用，为降水区提供持续视热源和视水汽汇，有利于青藏高原降水系统的维持和发展。Abstract: The plateau vortex and the tropical cyclone over the Bay of Bengal have common active periods but there are few studies on the influence of their interaction on the plateau precipitation. Therefore, a large scale heavy precipitation process over the Tibetan Plateau under the joint effects of a tropical cyclone over the Bay of Bengal and plateau vortex is analyzed which occurs during 26-31 May 2017, based on the Joint Typhoon Warning Center (JTWC) best-track data, hourly precipitation observation data and combined with hybrid single-particle lagrangian integrated trajectory (HYSPLIT) model. The results show that, with the cooperation of tropical cyclone over the Bay of Bengal and the India-Burma though, the water vapor transport jet from the Bay of Bengal to the southeast of the Tibetan Plateau is established, providing water vapor for the low vortex and shear line of the plateau. The cold air behind the India-Burma though forms a cold cushion on the steep slope of the southern Tibetan Plateau, and the warm water vapor from the Bay of Bengal first rises northward along the cold cushion, then sinks after ascending to the plateau, and rises northward again near the low vortex and shear line of the plateau, which increases the precipitable water between the plateau surface and the upper troposphere atmosphere. Meanwhile, frontogenesis is generated by the confluence of tropical cyclone southerly warm water vapor with the cold and dry air in the northern part of the plateau. In the process of frontogenesis, the atmospheric wet baroclinicity increases significantly and the wet isentropic line rises sharply, which promotes the sharp development of vertical vorticity and the enhancement of plateau low vortex. During the northward movement, the anticyclonic outflow from the upper layer of the tropical cyclone strengthens the southwest jet in front of the upper trough of the Tibetan Plateau, and the enhancement of divergence favors the development of the plateau shear line and the eastward movement of the plateau low vortex, resulting in a large-scale heavy precipitation. On the other hand, the positive feedback of the water vapor transportation of the tropical cyclone over the Bay of Bengal and the shear line of the plateau vortex provides continuous apparent heat source and apparent moisture sink for the precipitation area, which is favorable for the maintenance and development of the precipitation system on the plateau.
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图 1 2017年5月26日20:00—31日20:00高原低涡、未编号低涡间隔12 h和孟加拉湾风暴间隔6 h的移动路径及累积降水量分布(散点)
(橙色框表示高原降水关键区)
Fig. 1 Path of plateau vortex, unnumbered low vortex at 12 h intervals and tropical cyclone over the Bay of Bengal at 6 h intervals and distribution of accumulated precipitation from 2000 BT 26 May to 2000 BT 31 May in 2017 (the dot)
(the orange box denotes the main area of plateau precipitation)
图 2 500 hPa位势高度(紫线，单位：dagpm)、风(矢量)、水汽通量(填色)
Fig. 2 Geopotential height (the purple line, unit:dagpm), wind (the vector), water vapor flux (the shaded) at 500 hPa
图 3 2017年5月28日08:00—30日08:00质点前向和后向追踪60 h轨迹聚类分型
(括号内数字为水汽贡献占比)
Fig. 3 Trajectory clustering of particles tracked forward and backward for 60 hours from 0800 BT 28 May to 0800 BT 30 May in 2017
(numbers in brackets denote the contribution of water vapor)
图 4 2017年5月28日08:00—30日20:00洛隆站、八宿站、波密站、林芝站和米林站以南380 km处各等压面的湿弗劳德数
Fig. 4 Moist Froude number values at different pressure levels about 380 km to the south of Luolong, Basu, Bomi, Linzhi and Milin stations from 0800 BT 28 May to 2000 BT 30 May in 2017
图 5 500 hPa相当位温(红线，单位：K)，锋生函数(填色)，风场(矢量)
Fig. 5 Equivalent potential temperature (the red line, unit:K), frontogenesis function (the shaded), wind vector (the vector) at 500 hPa
图 6 沿95°~96°E平均经向剖面的相当位温(黑线，单位：K) 和垂直环流(矢量，由v与w×200倍合成)
(灰色阴影为地形, 五角星表示风暴所在纬度)
Fig. 6 Meridional sections of the equivalent potential temperature (the black line, unit:K) and vertical circulation averaged along 95°-96°E (the vector, derived from the combination of v and w×200)
(the gray shaded denotes topography,the pentacle denotes the latitude of tropical cyclone)
图 7 高原低涡中心(五角星) 南、北两侧各1个纬度区域平均的散度(填色)、涡度(黑线，单位：10-5 s-1) 纬向剖面
Fig. 7 Zonal sections of regional averaged divergence (the shaded) and vorticity (the black line, unit:10-5 s-1) along the latitude belt covering one latitude south/north to plateau vortex center (the pentacle)
图 8 沿高原低涡中心(五角星) 的相对湿度(填色)，相当位温(黑线，单位：K), MPV2 (红色虚线，单位：PVU，仅显示小于-0.2的值)(矢量由u与w×100合成) 经向剖面
Fig. 8 Meridional sections of relative humidity (the shaded), equivalent potential temperature (the black line, unit:K), MPV2 (the red dashed line, unit:PVU, only the values less than -0.2 are shown) along plateau vortex center (the pentacle)(the vector derived from the combination of u and w×100)
图 9 高原降水关键区区域平均的视热源Q1和视水汽汇Q2及其贡献项的垂直廓线
(绿线为Q1和Q2，红线为垂直输送项，黄线为平流项，蓝线为局地变化项)
Fig. 9 Vertical profiles of apparent heat source Q1 and apparent moist sink Q2 and their contribution terms in the main area of plateau precipitation
(the green line denotes Q1 and Q2, the red line denotes vertical movement term, the yellow line denotes advection term, the blue line denotes local term)
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