The observational characteristics and mechanism analysis of low-level meso-γ-scale vortices during the extreme rainfall event in Shenyang on 16 August 2019
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摘要: 2019年8月16日沈阳市区受伴有低空γ中尺度涡旋的强对流系统影响,发生了1951年有观测记录以来的最强小时降水(102 mm)。为了提高对此类涡旋所致强降水天气的认识和预报能力,综合利用多源观测和ERA5再分析资料,对此次过程中低空γ中尺度涡旋的观测特征、形成原因和对强降水的影响机制开展研究。此次过程期间,500 hPa沈阳位于东北冷涡东南侧,850 hPa以下低空位于“利奇马”台风残涡西侧的偏北气流水汽输送带内,16日午后沈阳市区具有低层大气气温廓线接近干绝热递减、较低抬升凝结高度和逐渐加强的风垂直切变等环境条件特征;风廓线雷达资料显示降水前0—6 km风矢量差最大达17 m/s,有利于较浅薄中尺度涡旋对发生。16时前后,沈阳市区内γ中尺度辐合风场首先触发局地风暴,随后有对流风暴群移入沈阳,在局地风暴具有反气旋式旋转处合并。合并风暴在初期产生强降水后,回波顶高降低、降水强度减小,低空出现了生命史约30 min的γ中尺度涡旋对,更是出现罕见的具有辐合特征的反气旋式涡旋加强的现象,伴有低空涡旋的风暴再次加强并导致后期更强的降水。相比中国中气旋统计特征,本次低空浅薄涡旋生命史较短、尺度小、移速慢、产生了非常强的垂直涡度,但涡旋厚度薄于中气旋。涡旋出现后所有5 min降水量超过10 mm的自动站均出现在中尺度涡旋对之间的区域,更是导致了自从1951年沈阳有观测记录以来的最强小时降水量。涡旋的旋转强度、伸展高度以及两个涡旋之间的距离表征了强降水的强度和范围。本次过程中的合并风暴具有暖云低质心回波特征,前期局地风暴降水使得近地面空气增湿、减少了蒸发作用,使得后期降水具有更高效率,配合低空涡旋在近地面形成上升气流同时促进雨滴增长和碰并过程,进而增强雨强;强烈旋转的涡旋造成近地面强上升气流,有利于风暴再次发展,进而使得降水持续更长时间。低空反气旋式涡旋生成加强原因是:冷涡风暴低空准线状的出流边界形成由北指向南的水平涡管,在降水初期下沉气流向下的扭曲作用下,近地面生成初始涡旋对;由于环境风切变矢量随高度逆时针旋转,更有利于反气旋式涡旋加强,合并后风暴内的强上升气流的拉伸作用也进一步加强了反气旋。最后给出了本次低空γ中尺度涡旋的形成机制和导致暴雨的物理模型。Abstract: The urban area of Shenyang was affected by a rainfall system with low-level meso-γ-scale vortices on 16 August 2019. This system caused record high hourly precipitation (102 mm) since the observations started in 1951. In order to improve the analysis and forecasting ability of heavy rain caused by such vortices, multi-source observations and ERA5 reanalysis data are comprehensively used to analyze the characteristics of the low-level meso-γ-scale vortices in this process, the environment for their generation and their roles in the formation of the torrential rain. The results show that during this process, Shenyang was located in the front of the Northeast Cold Vortex at 500 hPa and in the water vapor conveyor belt on the west side of the residual vortex of typhoon “Likima” in low levels. Moreover, a near-ground near-dry adiabatic lapse rate prevailed in the urban area of Shenyang in the afternoon with lower lifted condensation level and increasing vertical wind shear. At nightfall, the meso-γ-scale convergent wind field in the urban area of Shenyang triggered the local storm, and the cold vortex storm then entered Shenyang. The storm group merged in the area with anticyclonic rotation of the local storm. The merged storm strengthened this extreme precipitation event, and a meso-γ-scale vortex pair with a life history of 30 minutes appeared in lower levels, followed by a rare phenomenon of strengthening of converging anticyclonic vortex. Compared with the statistical characteristics of mesocyclones in China, this low-level shallow vortex demonstrated a short life span with small scale and slow moving speed and strong vertical vorticity. All the automatic weather stations with precipitation exceeding 10mm in 5 minutes after the vortex are located in the area between the vortex pair, and the record high hourly precipitation occurred since observations started in 1951. The strength and extent of the heavy precipitation can be characterized by the strength of the vortex's rotation, the height of its extension, and the distance between the two vortices. The occurrence of extreme precipitation events requires strong rainfall intensity and long duration time. The merged storm in this process has the characteristics of warm clouds and low centroid echo in radar observations. The early local storm precipitation also reduced the difference between ground temperature and dew point, which ensured high precipitation efficiency. Furthermore, the updraft near the ground generated by low-level vortex promoted the growth and collision of raindrops, thereby enhancing the rain intensity. The strong rotation of the vortex caused an updraft near the ground, which was conducive to the re-development of storms and prolonged the precipitation duration time. The reason for the emergence and strengthening of the low-level anticyclonic vortex is likely attributed to the following factors. The low-level quasi-linear outflow boundary of the cold vortex storm formed a horizontal vortex tube from north to south. Under the downward twisting action of the downdraft in the initial precipitation, meso-γ-scale vortex pair appeared near the ground, and since the environmental wind shear vectors rotated counterclockwise with altitude, the clockwise rotation was more conducive to the strengthening of anticyclonic storms. This is consistent with the theoretical research. In addition, this anticyclone was closer to the strong updraft area in the merged storm, and it could also be reinforced under its stretching. Finally, the formation mechanism of this low-level meso-γ-scale vortices and the rainstorm model are summarized, which provide a reference for future weather analysis and forecast research.
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图 1 2019年8月16日沈阳降水量 (单位:mm) 分布 (a. 17—20时沈阳市区累积降水量 (彩色圆点) 分布 (五角星为沈阳多普勒雷达位置,三角为GNSS和风廓线雷达位置);b. 累积降水量前三名站点的5 min降水量、所有自动站最大5 min降水量 (实线) 和5 min降水量大于10 mm站次 (虚线,单位:站次) 的时间序列)
Figure 1. Distribution of precipitation (unit:mm) in Shenyang on 16 August 2019 (a. distribution of accumulated precipitation (shaded areas) from 17:00 to 20:00 BT (the pentagram star shows the location of the Shenyang Doppler radar station,and the triangle is the location of the GNSS and wind profiler radar station); b. time series of 5-min precipitation at the top three stations of accumulated precipitation,the max 5-min precipitation (solid line) and the number of stations (dash line) which 5-min precipitation larger than 10 mm)
图 2 2019年8月16日16时 (a) 500 hPa位势高度 (等值线,单位:dagpm)、风场 (风向杆) 和相对湿度 (色阶,单位:%) 以及 (b) 850 hPa位势高度 (等值线,单位:dagpm)、风场 (风向杆) 和大气可降水量 (色阶,单位:mm)(红色方框为沈阳市区位置)
Figure 2. Geopotential height (purple solid lines,interval:4 dagpm),relative humidity (shaded,unit:%) and wind (wind barbs,unit:m/s) at 500 hPa (a) and geopotential height (white solid lines,interval:4 dagpm),precipitable water vapor (shaded,unit:mm) and wind (wind barbs,unit:m/s) at 850 hPa at 16:00 BT 16 August 2019 (the red box denotes the location of Shenyang)
图 3 2019年8月16日沈阳强降水风暴环境条件 (a. 16时沈阳T-lgp图,b. 14—20时大气可降水量的时间演变 (单位:mm),c. 16时925 hPa风场和水汽通量 (色阶,单位:g/(cm·hPa·s),d. 14时—18时40分沈阳风廓线演变 (风向杆,水平风场,单位:m/s;色阶,垂直速度,单位:m/s;红色三角号为降水开始时间,e. 13—17时风矢量随时间的演变,f. 沈阳风廓线雷达的速矢端轨迹和风切变矢量随高度的变化)
Figure 3. Environmental condition for the Shenyang intense rainfall storm on 16 August 2019 (a. skew T-logP diagram in Shenyang at 17:00 BT 16 August 2019, b. variation of precipitable water vapor from 12:00 BT to 20:00 BT 16 August 2019,c. 925 hPa winds and vapor flux (shaded,unit:g/(cm·hPa·s)) at 17:00 BT 16 August 2019, d. wind profile evolution in Shenyang from 14:00—18:40 16 August 2019 (wind barbs,unit:m/s;shaded:m/s),e. wind shear evolution in Shenyang from 13:00—17:00 16 August 2019 (unit:m/s), f. hodograph based on the Shenyang wind profile radar at 16:30 BT 16 August 2019)
图 4 2019年8月16日 (a) 16时06分对流风暴沈阳雷达4.3°仰角反射率因子 (单位:dBz) 及 (b) 15时30分地面风场 (风向杆,单位:m/s) 和散度场 (绿色等值线,单位:10−5 s−1)(黑色线为16时06分风暴A内超过10 dBz回波区域)
Figure 4. Radar reflectivity (unit:dBz) at 4.3° elevation angle of convective storm at 16:06 BT (a) ,and surface wind (barbs,unit:m/s) and divergence field (contours,unit:10−5 s−1) at 15:30 BT (the black line shows area with radar reflectivity larger than 10 dBz at 16:06 BT) (b) 16 August 2019
图 5 2019年8月16日 (a1—c1) 4.3°仰角沈阳雷达反射率因子和 (a2—c2) 沿着对应时刻雷达图直线的反射率垂直剖面 (a1、a2. 17时33分,b1、 b2. 17时39分,c1、c2. 17时50分;白色圆圈表征风暴合并期间强降水区域)
Figure 5. Radar reflectivity at 4.3° elevation angle (a1—c1)and vertical cross section of reflectivity along the write line (a2—c2)on 16 August 2019 (a1,a2. 17:33,b1 ,b2. 17:39,c1,c2. 17:50;white circles represent areas of heavy precipitation during storm merge)
图 6 2019年8月16日17时30分 (a) 和17时50分 (b) 地面风场 (风向杆,单位:m/s)、气温 (数字,单位:℃)、1小时降温幅度分布 (不同颜色色块,单位:℃) 和地面散度场 (等值线,单位:10−5 s−1)(图a中圆圈的站点17时00—30分的降水量≥10 mm,图b中圆圈的站点17—18时的降水量≥20 mm;红色和蓝色椭圆区分别为基于低仰角雷达径向速度识别的反气旋式涡旋和气旋式涡旋的区域)
Figure 6. Surface wind (wind barbs,unit:m/s),temperatures (numbers,unit:℃),1-hour cooling intensity (colored rectangles,unit:℃) and divergence field (contours,unit:10−5 s−1) at 17:30 (a) and 17:50 (b) 16 August 2019 (the circle stations in Fig. 6a indicate precipitation more than 10mm during 17:00—17:30 BT ,the circle stations in Fig. b indicate precipitation more than 20 mm during 17:00—18:00 BT; the red and blue ellipses are the regions of the anticyclonic and cyclonic vortexes identified based on radial velocity of the low-elevation radar data,respectively)
图 7 2019年8月16日17时45分 (a)、17时50分沈阳雷达4.3° (b)、0.5° (c) 径向速度及沿着图7 (c) 所示白色直线的径向速度剖面 (d)(单位:m/s;图a—c中粗黑色实线为相应时刻和仰角上反射率因子为35 dBz等值线,椭圆形白色虚线为风暴合并的位置,图c中蓝色虚线为风暴B冷池出流与暖空气形成的边界层辐合线,图d中紫色椭圆圈表示风暴合并后加强的中层的反气旋式涡旋,低空紫色区域为风暴B在地面形成的冷池区域)
Figure 7. Relative velocity at 4.3° elevation angle at 17:45 BT (a),relative velocity at 4.3° (b) and 0.5°(c) elevation angles at 17:50 BT 16 August 2019,and the vertical cross section of velocity (d) along the write line in Fig.7c (unit:m/s; the black line is the contour with a reflectivity factor of 35dBz,the ellipse dotted line is the location of the storm merge,and the blue dotted line is the boundary layer convergence line)
图 8 2019年8月16日沈阳雷达1.5°仰角反射率因子和相应时刻沿着雷达PPI图所示直线的反射率因子垂直剖面 (单位:dBz;a1、a2. 17时55分, b1、 b2. 18时01分,c1、c2. 18时07分,d1、 d2. 18时12分;a2—d2三角号为反气旋式涡旋所在位置)
Figure 8. Radar reflectivity at 1.5° elevation angle and the vertical cross section of reflectivity along the write line on 16 August 2019 (unit:dBz;a1,a2. 17:55,b1 b2. 18:01,c1,c2. 18:07 ,d1,d2. 18:12;the triangle number a2—d2 shows the location of the anticyclonic vortex)
图 10 2019年8月16日 (a—b) 0.5°、(c) 1.5°沈阳雷达观测的径向速度及沿着相应时次图形中所示白色直线垂直剖面 (单位:m/s;a1、 a2. 18时01分, b1、b2. 18时07分,c1,c2. 18时12分;粗黑色实线为反射率因子为35 dBz等值线,a1中黄色圈为原始涡旋,a2—c2实线圈为反气旋式涡旋,虚线圈为原始涡旋),以及 (d) 18时01分—24分反气旋和气旋中心移动路径、18时00—30分降水量≥40 mm的站点分布 (黑色数字,单位:mm)
Figure 10. Relative velocity at (a—b)0.5° and(c)1.5°elevation angles and vertical cross section of relative velocity along the write line on 16 August 2019 (unit:m/s;a1,a2. 18:01 BT,b1 b2. 18:07 BT,c1,c2. 18:12 BT;the black line is the contour with the reflectivity factor of 35 dBz,the yellow circle in a1 is the original vortex,the solid circle in a2—c2 is the anticyclonic vortex,and the dotted circle is the original vortex),and (d) the tracks of anticyclone and cyclone centers from 18:01 to 18:24 and precipitation distribution at stations with precipitation ≥40 mm from 18:00 to 18:30 BT (black numbers,unit:mm)
图 9 2019年8月16日18时08分 营口雷达0.5° (a1—c1) 和1.5° (a2—c2) 仰角差分反射率因子 (a)、相关系数 (b)、差分传播相移率(c)(黑色方框为反气旋式涡旋所在位置,蓝色框为气旋式涡旋所在位置,黑色虚线区域为强降水区,灰色线为45 dBz强回波区域,此区域位于营口雷达东北方向150 km,0.5°仰角探测高度约2.6 km,1.5°仰角探测高度约5.2 km)
Figure 9. ZDR(a),CC(b) and KDP(c) at 0.5°(a1—c1) and 1.5°(a2—c2)elevation angles at 18:08 BT 16 August 2019 (unit:dBz;the black box is the location of the anticyclonic vortex,the blue box is the location of the cyclonic vortex,the black dotted line area is the heavy precipitation area,the gray line shows area with radar reflectivity larger than 45 dBz and this area is located 150 km to the northeast of Yingkou radar,0.5° elevation angle detection height is about 2.6 km,1.5° elevation angle detection height is about 5.2 km)
图 11 (a) 对称上升气流风暴内低层γ中尺度涡旋形成机制 (引自Trapp,et al,2003) 和 (b) 本次过程γ中尺度涡旋形成机制示意 (绿色带刺的线为阵风锋,箭头代表垂直平面内的空气运动,蓝色阴影区代表强降水区,黑色粗实线代表垂直平面内的涡线,上面的小箭头为涡度的方向,红色和紫色分别代表垂直平面内正和负的垂直涡度区域,D表示扰动低压,H表示扰动高压,右侧蓝色箭头代表台风利奇马的水汽)
Figure 11. (a)Schematic diagram showing the proposed effect of low-level meso-vortices on QLCS structure and their roles in the generation of damaging surface winds (Trapp,et al,2003), and (b) schematic diagram of the formation mechanism of γ mesoscale vortex in this process (the green barbed line indicates the gust front and red circles denote low-level mesovortices,the red area in the vertical plane shows vertical extent and tilt of positive vertical vorticity and corresponding mesovortex,the implication is an associated downward-directed vertical pressure-gradient force (bold blue arrow) that acts to locally eliminate or “fracture” the updraft above the mesovortex location,black stippling on the south-southwest flank of this mesovortex shows the area of instantaneous damaging “straight line” winds driven by the vortex circulation,a smaller area,or a narrow strip of such winds,is indicated well southeast of the vortex,at the apex of the primary bowing segment,these winds are due to a rear-inflow jet that descends to the ground,represented by the black streamlines in the other vertical plane,the blue arrow on the right represents the “Lekima” water vapor)
表 1 反气旋式低空γ中尺度涡旋和降水的特征
Table 1. Characteristics of low-level meso-γ-scale vortex and precipitation
物理量 18时01分 18时07分 18时12分 18时18分 18时24分 反气旋顶高度(km) 2.1 2.9 2.9 2.5 2.3 旋转中心高度(km) 0.3 0.3 0.7 0.6 0.6 正负速度中心直径(km) 1.58 1.20 1.01 1.51 1.84 最大正、负(括号内)速度(m/s) 27.5(−6) 29.5(−26) 29.5(−8) 24(−16.5) 8(−25) 涡旋中心垂直涡度(10−2×s−1) −2.12 −4.63 −3.71 −2.73 −1.79 涡旋倾斜程度 倾斜 垂直 垂直 倾斜 倾斜 涡旋位置强度达到45dBz的回波顶高(km) 6.2 7.3 7.3 6.6 4.7 距离涡旋最近观测站的5分钟降水量(mm) 5.2 9.8 12.2 13.1 10.7 5 min最大雨量(mm)以及≥10 mm的站次(括号内) 15.5
(3)13.9
(6)18.3
(8)12.8
(5)11.2
(3)最低仰角反射率和组合反射率(括号内)(dBz) 50
(51)50
(54.5)50.5
(56)49.5
(56.5)52
(52) -
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