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Observational Characteristics of A Hybrid Severe Convective Event in the Sichuan-Tibet Region
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摘要: 利用中国气象局地面自动气象站、探空、天气雷达等观测资料和ERA-Interim再分析资料，分析2016年9月8日川藏高原一次强对流天气过程。结果表明：该过程多站出现8级雷暴大风、10 mm以上小时强降水且伴有最大直径为18 mm的冰雹，是川藏高原一次混合型强对流过程。对流系统发生在500 hPa弱冷平流和低层切变线影响下，中低层深厚湿层、环境中等强度对流有效位能和垂直风切变为超级单体的形成和维持提供有利条件。初始北侧多单体和南侧弱对流在地面辐合线上生成，向东南移入适宜环境后，北侧多单体发展成线状对流系统，与南侧单体合并且促使其迅速发展成超级单体。成熟超级单体低层具有清晰的前侧入流缺口、钩状回波和中气旋特征。强回波区随高度前倾，呈显著的上冲云顶突起、回波悬垂和有界弱回波区。风暴内中层径向辐合、上升气流减弱和反射率因子核心快速下降预示下击暴流的产生。中层干空气的夹卷和水凝物快速下落的拖曳作用加强下沉气流，结合峡谷地形的狭管效应，引起地面大风。Abstract: The Sichuan-Tibet Region is a key area for the development of western China, where severe convective weather such as thunderstorm gales occur frequently. However, due to the complex terrains, synoptic systems, and the lack of meteorological observations, it is especially challenging to make accurate prediction. To better understand the mechanism of severe convective weather over the plateau, a rare severe convective event in the Sichuan-Tibet Region on 8 Sep 2016 is analyzed with weather reports, hourly and minutely surface observations, sounding data and Doppler weather radar data from China Meteorological Administration and ERA-Interim 0.5°×0.5° reanalysis data from European Centre for Medium-Range Weather Forecasts (ECMWF). The result shows that hourly rainfall of over 10 mm and hails of over 18 mm are observed at several weather stations, indicating a hybrid moist convective event. The meso-scale convective system (MCS) occurs near a shear line at low level with weak cold advection at 500 hPa. Large environmental convective available potential energy (CAPE), vertical wind shear, and the thick moist atmospheric layer are conductive to the formation of supercell. The initial convection is generated along a surface convergence line, with multiple γ meso-scale cells embedded in stratiform cloud in the north and cluster cells in the south. They move to the southeast, enter the favorable environment and merge with each other, enabling the cell on the south side to quickly develop into a supercell. When the supercell grows matured, the characteristic of front inflow gap, hook echoes and mesoscale cyclone at low levels are clear. The strong echo region tilts forward with height. There is significant overshooting top with the echo top height up to 15 km above ground in the upper troposphere, and obvious echo overhang capping bounded weak-echo region (BWER) in the middle layer. Mid-altitude radial convergence, weakening of updrafts and rapid drop of the reflectivity core indicate the occurrence of downbursts inside the storm. The cooling effect due to the entrainment of midlevel dry air is favorable to the growing of big hails and raindrops, and the formation of downdrafts. Moreover, the drag effect related to the rapid drop of heavy raindrops and hails, and the narrow tube effect of the canyon terrain, contribute to the formation of thunderstorm gales near the ground.
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Key words:
- the Sichuan-Tibet Region;
- thunderstorm gales;
- supercell
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图 1 2016年9月8日05:00—09:00南侧和北侧强对流中心移动路径
(时间间隔为30 min，黑色和红色线段分别为雷达最大组合反射率因子在35~60 dBZ和不小于60 dBZ)(a)及2010—2017年青藏高原地区冰雹直径的概率分布(b)
Fig. 1 Moving paths of strong convection centers(the time interval is 30 min, the black and red lines denote the reflectivity factor ranging from 35-60 dBZ and more than 60 dBZ) from 0500 UTC to 0900 UTC on 8 Sep 2016(a) and probability distribution of hail diameter in the Qinghai-Tibet Region during 2010-2017(b)
图 2 2016年9月8日00:00高度场(等值线，单位：dagpm)和风场
(红色方框为发生对流区域)
(a)500 hPa(填色为温度，棕色实线为槽线)，(b)600 hPa(填色为相对湿度，棕色实线为切变线，灰色为地形)，(c)200 hPa(风羽指示大于等于30 m·s-1高空急流区，填色为散度)，(d)地面(蓝色等值线为海平面气压，单位：hPa;填色为温度)Fig. 2 Geopotential height(the contour, unit:dagpm) and wind at 0000 UTC on 8 Sep 2016
(the red rectangle denotes convection area)
(a)500 hPa(the shaded denotes temperature, the brown curve denotes trough), (b)600 hPa(the shaded denotes relative humidity, the brown curve denotes shear line, the grey denotes terrain), (c)200 hPa(the barb denotes upper level jet stream with wind velocity no less than 30 m·s-1, the shaded denotes divergence), (d)surface(the contour denotes sea-level pressure, unit:hPa;the shaded denotes temperature)
图 3 2016年9月8日环境对流有效位能(填色)和500~700 hPa垂直风切变(蓝色等值线，单位：m·s-1)
(红色方框为超级单体成熟的区域)
Fig. 3 Environmental convective available potential energy(the shaded) and 500-700 hPa vertical wind shear(the blue contour, unit:m·s-1) on 8 Sep 2016
(the red rectangle indicates mature supercell area)
图 4 2016年9月8日甘孜站雷达观测和地面观测
(圆点为气温，数字为相对湿度(单位:%)，棕色实线为地面辐合线，红色椭圆分别指示南、北侧对流位置)
Fig. 4 Radar and surface observations at Ganzi on 8 Sep 2016
(the dot denotes surface temperature, the number denotes relative humidity(unit:%), the brown curve denotes surface convergence lines, the red ellipse denotes convection positions)
图 5 2016年9月8日07:45甘孜雷达0.5°仰角观测
(a)反射率因子(白色圆指示勾状回波，白色箭头指示前侧(B侧)入流缺口)，(b)沿图 5a中AB的垂直剖面(白色椭圆指示回波悬垂区域)，(c)径向速度(白色椭圆指示中气旋区域)，(d)沿图 5c中AB的垂直剖面(白色椭圆指示中层径向辐合区域，白色箭头指示风暴内气流的方向)
Fig. 5 Observation at 0.5°elevation angle by Ganzi radar on 8 Sep 2016
(a)reflectivity factors(the white ellipse denotes hook echo, the white arrow indicates inflow gap), (b)vertical cross-section along line AB in Fig. 5a(the white ellipse denotes echo overhang), (c)reflectivity factors(the white ellipse denotes mesocyclone), (d)vertical cross-section along line AB in Fig. 5c(the white arrow denotes storm inflow direction)
图 6 2016年9月8日甘孜雷达0.5°仰角观测
(白色圆圈指示中气旋位置，白色箭头指示超级单体低层入流方向)
Fig. 6 Observation at 0.5°elevation angle by Ganzi radar on 8 Sep 2016
(the white circle denotes mesocyclone, the white arrow denotes supercell inflow direction)
图 8 2016年9月8日07:45—08:25地面气象站要素时间演变
(a)雅江站分钟降水量(柱状)和逐5 min气温(曲线)，(b)雅江站逐5 min气压(曲线)和风(风羽)，(c)838181自动气象站分钟降水量，(d)838181自动气象站极大风矢量
Fig. 8 Surface meteorological elements evolution from 0745 UTC to 0825 UTC on 8 Sep 2016
(a)precipitation(the column) and temperature(the curve) at Yajiang station, (b)pressure(the curve) and wind(the barb)at Yajiang Station, (c)precipitation(the column) at auto weather station 838181, (d)hourly extreme wind(the barb) at auto weather station 838181
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