Influence of Multi-scale Topographic Factors on Vortex Development during an Eastward-Propagating Rainstorm Event in Southwest China
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摘要: 由观测和数值模拟结果分析发现,2019年8月5~6日中国西南部的东移型致灾暴雨事件中存在三涡(南北双高原涡、西南涡)相继发展并导致暴雨加强和移动的现象。借助数值试验,研究了多尺度地形因子(青藏高原、横断山脉和四川盆地三大地形)各自对涡旋演变的作用。结果表明,横断山脉对西南涡的形成起关键作用,四川盆地影响着西南涡的位置和强度。对于高原涡(南侧高原涡)的移动,四川盆地地形只影响涡旋强度演变,但不会改变高原涡的移动路径。一旦横断山脉被移除,高原涡的东移现象随之消失。进一步分析青藏高原和四川盆地交界处的陡峭地形坡度改变对涡旋发展的影响发现,发现坡度越陡,高原涡移动速度越快,且盆地内二涡合并后的西南涡强度越强。最后借助于倾斜涡度发展理论,解释了不同坡度对涡旋强度演变的影响:随着坡度变陡,倾斜涡度发展系数沿涡旋下滑路径快速减小,对垂直涡度局地倾向的强迫作用,加剧了涡旋的快速加强。Abstract: Using observation and numerical simulation results, we reveal that three vortexes, namely the northern plateau vortexes (TPV1), southern plateau vortex (TPV2), and Southwest vortex (SWV), developed successively during a disaster-causing rainstorm event in Southwest China from August 5 to 6, 2019, which led to the intensification and eastward propagation of the rainstorm. Through numerical experiments, we study the effects of multi-scale topographic factors (Tibetan Plateau [TP], Hengduan Cordillera [HC], and Sichuan Basin [SB]) on vortex evolution. The results show that HC plays a key role in SWV formation, while SB influences the SWV location and intensity. The topography of the SB only affects the intensity of TPV2 but does not change the propagation path. In the absence of HC, the plateau vortex does not propagate. The influence of slope change of the steep terrain at the boundary between TP and SB on vortex development was further analyzed. The steeper the slope, the faster the propagation speed of the plateau vortex, and the stronger the SWV after the merging of TPV2 and SWV. Finally, the impact of the terrain slope on the evolution of vortex intensity was analyzed according to the theory of slantwise vorticity development. As the slope becomes steeper, the development coefficient of inclined vorticity decreases rapidly along the vortex slide path, and the forcing effect on the local tendency of vertical vorticity intensifies the rapid strengthening of vorticity.
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Key words:
- Rainstorm /
- Vortex /
- Topography /
- Numerical simulation
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图 1 2019年8月5日(a1、a2)00:00、(b1、b2)06:00、(c1、c2)12:00、(d1、d2)18:0和(e1、e2)6日00:00(协调世界时,下同)观测(左列)和模拟(右列)的6小时累积降水量(彩色阴影,单位:mm)。灰色阴影表示地形高度,单位:m
Figure 1. Observed (left column) and simulated (right column) 6 h accumulative precipitation (color shaded, units: mm) at (a) 0000 UTC, (b) 0600 UTC, (c) 1200 UTC, (d) 1800 UTC 5, and (e) 0000 UTC 6 August 2019. The gray line represents terrain height (units: m)
图 3 表1中Group1不同数值试验中的地形设置:(a)平滑的实际地形;(b)理想地形的总体特征;(c)模拟区域内IDEAL_ALL试验的青藏高原、横断山脉、四川盆地组成的理想地形;(d)IDEAL_TP+HC试验的青藏高原和横断山脉理想地形;(e)IDEAL_TP试验的青藏高原理想地形。(f)表1中Group2坡度改变试验中各地形坡度的设置
Figure 3. Various terrain configurations for different numerical experiments in Group 1 simulation in Table 1: (a) Smoothed real terrain; (b) overall characteristics of the ideal terrain; (c) ideal terrains consist of the Tibetan Plateau (TP), Hengduan Cordillera (HC), and Sichuan Basin (SB) in IDEAL_ALL run; (d) Ideal terrains of the TP and HC for an IDEAL_TP+HC experiment; (e) ideal terrains of TP in IDEAL_TP run. (f) Different topography slopes for Group2 experiment in Table 1
图 4 2019年8月5日(a)00:00、(b)06:00、(c)12:00、(d)18:00、6日(e)00:00和(f)06:00基于GFS资料的500 hPa相对涡度(填色,单位:10−5 s−1)、位势高度(黑色等值线,单位:dagpm)以及温度场(红色等值线,单位:°C)分布
Figure 4. Distributions of relative vorticity (shaded, units: 10−5 s−1), geopotential height (black contours, units: dagpm), temperature (red contours, units: °C) at 500 hPa level based on GFS (Global Forecasting System) data at (a) 0000 UTC August 5, (b) 0600 UTC August 5, (c) 1200 UTC August 5, (d) 1800 UTC August 5, (e) 0000 UTC August 6, and (f) 0600 UTC August 6, 2019
图 5 2019年8月(a–l)5日00:00至6日09:00(间隔3小时)模拟的500 hPa相对涡度(填色,单位:10−5 s−1)、位势高度(蓝色等值线,单位:dagpm)分布(红色等值线代表3000 m地形高度)
Figure 5. Evolution of simulated relative vorticity (color shaded, units: 10−5 s−1), geopotential height (blue contour lines, units: dagpm). The red contour lines represent the 3000-m-height terrain (a–l) from 0000 UTC 5 to 0900 UTC 6 (with an interval of 3 h) August 2019
图 6 沿图5g中直线的相对涡度(填色,单位:10−5 s−1)、垂直速度(等值线,单位:m s−1)、1小时降水量(黑色直方图)剖面:(a–f)2019年8月5日10:00~20:00,时间间隔2小时。灰色垂直线段代表涡旋中心位置。
Figure 6. Cross–sections of relative vorticity (color shaded, units:10−5 s−1), vertical velocity (contours, units: m s−1), and precipitation (black histogram, units: mm) along the line in Fig. 5g: (a–f) From 1000 UTC 5 to 2000 UTC 5 (with an interval of 2 h) August 2019. The gray vertical line denotes the position of the vortex center
图 8 2019年8月5日00:00至6日18:00沿图5c中直线的高原北部涡旋(TPV1,左列)与沿图5g中直线的高原北部涡旋和西南涡(TPV2+SWV,右列)的时间—经度分布:(a、b)涡度(单位:10−4 s−1);(c、d)垂直速度(单位:m s−1);(e、f)1小时降水量(单位:mm)
Figure 8. Time–longitude distribution of the northern Plateau vortex (TPV1, left column) along the line in Fig. 5c and the northern Plateau vortex and Southwest vortex (TPV2+SWV, right column) along the line in Fig. 5g from 0000 UTC to 1800 UTC on August 5, 2019: (a, b) Vorticity (units: 10−4 s−1); (c, d) vertical velocity (units: m s−1); (e, f) 1-hour precipitation (units: mm)
图 9 基于CNTL控制试验利用涡度方程(5)对(a)高原北部涡旋(TPV1)、(b)高原南部涡旋(TPV2)和(c)西南涡(SWV)从8月5日00:00到6日06:00的诊断分析。图中Coriolis、Div、Solenoid、Til、VA、HA和RES分别为科氏力项、拉伸项、力管项、扭转项、垂直输送、水平输送项和剩余项;Vort为方程左侧涡度的局地倾向(单位:10−8 s−2)
Figure 9. Based on CNTL control test, vorticity equation (5) was used to analyze (a) the northern vortex (TPV1), (b) the southern vortex (TPV2) and (c) southwest vortex (SWV) from 0000 UTC on August 5 to 0600 UTC on August 6. In the figure, Coriolis, Div, solenoid, Til, VA, HA and RES are stand for Coriolis force term, stretch term, the solenoid term, the tilting term, the vertical advection term, the horizontal advection term and residual term respectively. Vort represents the local tendency of vorticity on the left-hand-side of the vorticity equation (units: 10−8 s−2)
图 10 Group1试验中2019年8月5日22:00 500 hPa(左列)、700 hPa(中间列)和850 hPa(右列)高度层的涡度(填色,单位:10−5 s−1)、地形(灰色等值线,单位:m)和位势高度(蓝色等值线,单位:dagpm)分布:(a–c)IDEAL_ALL试验;(d–f)IDEAL_TP+HC试验;(g–i)IDEAL_TP试验。图中绘制的地形等值线从右到左分别为600 m、900 m、1200 m、1500 m、2000 m、3000 m、4000 m
Figure 10. Distributions of vorticity (color shaded, units: 10−5 s−1), terrain (gray contours, units: m), and geopotential height (blue contours, units: dagpm) for (a–c) IDEAL_ALL, (d–f) IDEAL_TP+HC, and (g–i) IDEAL_TP experiments in Group1 at 500 hPa (left column), 700 hPa (middle column) and 850 hPa (right column) at 2200 UTC August 5, 2019. The terrains with heights of 600, 900, 1200, 1500, 2000, 3000, and 4000 m are depicted from right to left, respectively
图 11 图11 Group1中(a–f)IDEAL_ALL、(g–l)IDEAL_TP+HC和(m–r)IDEAL_TP试验的涡度方程主要源、汇项(填色,单位:10−8 s−2)的垂直剖面:第一、三、五行分别为沿高原涡的涡旋主体移动通道29°~33°N、29°~32°N、29°~34°N的纬向平均;第二、四、六行分别为沿西南涡移动通道29°~33°N、29°~32°N、29°~34°N的纬向平均;(a、g、m)是高原上空涡旋(即原TPV2)的汇(均为Til+HA);(d、j、p)为盆地内涡旋(即原SWV)的汇(均为Til+Div);(b、h、n)为高原上空涡旋的源项(均为Div+VA);(e、k、q)为盆地内涡旋的源项(均为HA+VA);(c、i、o)为高原上空涡旋的源汇项(阴影)和相对涡度(黑色等值线,单位:10−5 s−1);(f、l、r)为盆地内涡旋的源汇项和相对涡度;红色方框表示涡旋主体位置
Figure 11. The cross-section diagrams of main source and sink terms (shaded, units:10−8 s−2) of vertical vorticity development calculated by vorticity equation for (a–f) IDEAL_TP, (g–l) IDEAL_TP+HC and (m–r) IDEAL_ALL experiment in Group1. Where (a–c), (g–i), (m–o) are the zonal average of 29°–33°N, 29°–32°N, 29°–34°N along the main moving channel of TPV above TP; and (d–f), (j–l), (p–r) are the zonal average of 29°–33°N, 29°–32°N, 29°–34°N along the main moving channel of SWV within SB. (a, g, m) Sinks of TPV2 (Til+HA) above TP; (d, j, p) sinks of SWV (Til+Div) within SB; (b, h, n) sources of TPV2 (Div+VA) above TP; (e, k, q) sources of SWV (HA+VA) within SB; (c, i, o) sources + sinks of TPV2 above TP, superposed is relative vorticity (black contours, units: 10−5 s−1) ; (f, l, r) sources + sinks of SWV within SB, and relative vorticity
图 12 Group2不同试验中29°~31°N纬向平均的500 hPa逐时相对涡度(左列,阴影,单位:10−5 s−1)、垂直速度(右列,阴影,单位:10 m s−1)时间—经度分布:(a、b)IDEAL_0.3Rs试验;(c、d)IDEAL_0.8Rs试验;(e、f)IDEAL_2.0Rs试验;(g、h)IDEAL_4.0Rs试验;(i、j)IDEAL_6.0Rs试验
Figure 12. Time–longitude diagrams of the hourly, zonally averaged (between 29°–31°N) relative vorticity (left panels, shaded, units: 10−5 s−1) and vertical velocity (right panels, shaded, units: 10 m s−1) at 500 hPa for (a, b) IDEAL_0.3Rs, (c, d) IDEAL_0.8Rs, (e, f) IDEAL_2.0Rs, (g, h) IDEAL_4.0Rs, and (i, j) IDEAL_6.0Rs experiments in Group2
图 13 (a)高原涡(TPV2)和西南涡(SWV)的涡度峰值强度(直方图,单位:10−5 s−1)和最大涡度中心伸展高度(折线,单位:hPa);(b)高原上空和盆地涡度发展系数CD(单位:10−10 s−1)的演变;(c)不同坡度地形数值试验中高原涡(TPV2)沿青藏高原和四川盆地交界处陡峭地形的下滑速度
Figure 13. (a) The peak vorticity intensity of TPV2 and SWV (histogram, units: 10−5 s−1) and the upwards-stretching heights of the maximal vorticity center (curve, units: hPa); (b) the evolution of the development coefficient of inclined vorticity (CD) (units: 10−10 s−1) over TP and within SB; (c) the downward-sliding velocity of TPV2 along steep terrain slopes between TP and SB for various terrain numerical experiments
表 1 试验设计与描述
Table 1. Experiment design and description
试验名称 试验描述 Group1 研究三大地形(TP、HC和SB)各自对涡旋演变的作用 CNTL 真实地形 IDEAL_ALL 理想地形(TP+HC+SB) IDEAL_TP+HC 青藏高原和横断山脉的理想地形(TP+HC) IDEAL_TP 只有青藏高原的理想地形(TP) Group2 研究坡度改变对TPV2移动的影响 IDEAL_0.3Rs 坡度为0.3Rs的理想地形 IDEAL_0.8Rs 坡度为0.8Rs的理想地形 IDEAL_2.0Rs 坡度为2.0Rs的理想地形 IDEAL_4.0Rs 坡度为4.0Rs的理想地形 IDEAL_6.0Rs 坡度为6.0Rs的理想地形 注:TP、HC、SB分别代表青藏高原、横断山脉、四川盆地;Rs为地形坡度系数。 -
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