Climate Change Research Letters
Vol. 08  No. 05 ( 2019 ), Article ID: 32231 , 11 pages
10.12677/CCRL.2019.85069

Numerical Simulation on a Squall Line Process in Jiangsu Province and Analyses of Its Formation Mechanism

Yongqiang Jiang1, Mingbo Jiang2, Yufeng Zhou2, Rui Han3, Huawen Wang4, Chaohui Chen1

1College of Meteorology and Oceanography, National University of Defense Technology, Nanjing Jiangsu

2Army 61540 of PLA, Beijing

3Army 93117 of PLA, Nanjing Jiangsu

4Army 61741 of PLA, Beijing

Received: Sep. 1st, 2019; accepted: Sep. 12th, 2019; published: Sep. 19th, 2019

ABSTRACT

A squall line process in northern Jiangsu Province on May 16, 2012 was simulated by using a mesoscale numerical model WRF. The maximum horizontal resolution of the model was 5 km with triple nesting. Based on the high resolution model output data, the squall line structures in the surface and middle levels and the squall line development mechanism are analyzed. The results show that there exists a divergence ring and three convergence lines in the simulated surface field. A divergence line formed along the thunderstorm high zone, a convergence line formed in the front and another back of the squall line. There are positive and negative vorticity centers to the south and north sides of the vertical velocity center at 500 hPa, respectively. The airflow in squall line is different from the typical model which contains an upshear updraft and a downshear downdraft. The rear inflow at middle level separates into an updraft and a downdraft which are basically in the same column. The liquid or ice particles produced in the middle and upper levels fall into the downdrafts not dragging the updraft. When they evaporate in the dry downdrafts, they cool the air and then trigger the formation of surface thunderstorm high and cold pool. Based on the vorticity equation, the formation mechanism of mid-level vorticity couple is analyzed. The interaction between vertical wind shear and strong gradient of updraft induces the negative vorticity to the north side of the vertical velocity center and positive vorticity to the south side. The interaction between cold pool in thunderstorm high and vertical wind shear induces thunderstorm regeneration at the front of cold pool, weakening at the back of cold pool, and squall line self-excitation development.

Keywords:Squall Line, Divergence Ring, Vorticity Couple, Cold Pool, Numerical Simulation

1国防科技大学气象海洋学院，江苏 南京

261540部队，北京

393117部队，江苏 南京

461741部队，北京

1. 引言

2. 模式和资料

WRF模式(Weather Research and Forecasting Model)是美国多所科研机构的科学家们共同研发了业务与研究共用的新一代高分辨率中尺度预报模式，是一种完全可压非静力模式。本文采用WRF 3.3版本，模式水平方向采用Arakawa C网格，垂直方向采用地形跟随坐标。采用三重嵌套方案，中心为(32˚N，120˚E)，从外到内区域水平分辨率分别为45、15、5 km，格点数分别为181 × 175、133 × 127、100 × 94。垂直分层35层，模式顶气压为50 hPa。对于对流降水的计算，不选择对流参数化方案，采用云微物理过程Lin等的方案 [17] ，边界层潜热、感热、动量的垂直通量采用ACM2方案 [18] [19] ，长波辐射采用RRTM方案 [20] ，短波辐射采用Dudhia方案 [21] 。初始场采用2012年5月16日0000UTC NCEP 1˚ × 1˚全球再分析资料，积分24 h，本文的分析采用5 km分辨率资料。

3. 飑线概况及大尺度形势分析

2012年5月16日，在东北冷涡背景下发展起来的飑线所带来的冰雹、大风、强降雨袭击苏北地区。该飑线发生发展直至消亡历时约7 h。自江苏徐州生成，途经宿迁、淮安、连云港、盐城，而后入海消亡。初生位置位于徐州附近的柳泉站(114.2˚E，39.5˚N)。0630UTC，柳泉开始出现几个孤立的对流单体，随后逐渐发展强盛，0830UTC对流单体发展强盛，回波最大强度达到65 dBZ。1100UTC，飑线移至盐城附近，强度减弱，其后入海消亡。过程中苏北大片地区出现大风、冰雹和强降水天气，宿迁、睢宁、连云港、灌云、灌南等地出现冰雹，部分地区出现的最大风力达到了24 m/s以上。

Figure 1. Synoptic pattern at 0000 UTC on 16 May 2012 ((a) 500 hPa; (b) 700 hPa; (c) 850 hPa, the black solid line denotes geopotential height (gpm), the red dashed line denotes temperature (˚C), the green arrowdenotes wind (m/s), the brown thick line denotes trough, and the shading denotes the wind speed greater than 12 m/s)

4. 模拟飑线的中尺度结构及发展机制分析

4.1. 雷暴高压、尾流低压和中尺度辐合线

Figure 2. Simulated surface pattern((a) t = 7 h, (b) t = 8 h, (c) t = 9 h, (d) t = 10 h. The black solid line denotes sea level pressure (hPa), the red solid line denotes temperature (˚C), the blue arrow denotes wind (m/s), and the shading denotes 1 h accumulated precipitation (mm))

Figure 3. Simulated surface streamlines and isotaches (m/s) ((a) t = 7 h, (b) t = 8 h, (c) t = 9 h, (d) t = 10 h. The green arrow denotes stream line, the shading denotes isotach (m/s), the red and blue dashed lines in (c) denote convergence and divergence lines, respectively)

Figure 4. Simulated surface divergence and wind field ((a) t = 9 h, (b) t = 10 h. The black arrow denotes wind (m/s), and the shading denotes divergence (10−3/s))

Figure 5. (a) 500 hPa reflectivity (shading, dBz) and sea level pressure (black solid line, hPa) at t = 9 h, (b) vertical cross section of circulation (ching line), reflectivity (shading, dBz), and specific humidity (purple dashed line, 10−3 g/kg) along the purple line in (a). The red and blue ellipses denote the positions of wake low and thunderstorm high, respectively. The yellow arrow denotes rear inflow, and the brown and blue arrow denoteupdraft and downdraft, respectively)

4.2. 对流层中层涡度偶

Figure 6. (a) Simulated 500 hPa vorticity (shading, 10−3 s−1), vertical velocity (black line, m/s), and stream line (green arrow) at t = 9 h, (b) The magnification in the red box in (a)

$\frac{fD}{SL}\ll 1$ (D、L分别为上升运动的垂直和水平尺度，S为垂直风切变的强度尺度)时，由科氏力作用引起的垂直涡度变化和倾斜项引起的垂直涡度变化相比是比较小的。因为在强对流环境里， $\frac{fD}{SL}\text{~}0.01$

$\begin{array}{l}\frac{\partial \zeta }{\partial t}=-V\cdot \nabla \zeta +\omega \cdot \nabla w\\ =-u\frac{\partial \zeta }{\partial x}-v\frac{\partial \zeta }{\partial y}-w\frac{\partial \zeta }{\partial z}+\xi \frac{\partial w}{\partial x}+\eta \frac{\partial w}{\partial y}+\zeta \frac{\partial w}{\partial z}\\ =-u\frac{\partial \zeta }{\partial x}-v\frac{\partial \zeta }{\partial y}-w\frac{\partial \zeta }{\partial z}+\left(\frac{\partial w}{\partial y}-\frac{\partial v}{\partial z}\right)\frac{\partial w}{\partial x}+\left(\frac{\partial u}{\partial z}-\frac{\partial w}{\partial x}\right)\frac{\partial w}{\partial y}+\left(\frac{\partial v}{\partial x}-\frac{\partial u}{\partial y}\right)\frac{\partial w}{\partial z}\end{array}$ (1)

$\frac{\partial {\zeta }^{\prime }}{\partial t}=-\overline{u}\frac{\partial {\zeta }^{\prime }}{\partial x}-\overline{v}\frac{\partial {\zeta }^{\prime }}{\partial y}+\frac{\partial \overline{u}}{\partial z}\frac{\partial {w}^{\prime }}{\partial y}-\frac{\partial \overline{v}}{\partial z}\frac{\partial {w}^{\prime }}{\partial x}=-\overline{V}\cdot \nabla {\zeta }^{\prime }+S×\nabla {w}^{\prime }\cdot k$ (2)

Figure 7. Schematic representation of the inclination of the vortex tube (The green thin arrow denotes the ambient wind V, the red arrow denotes the updraft, and the thin arrow on the green vortex tube denotes the rotation direction of the vortex tube)

4.3. 冷池和飑线自激发展机制

Figure 8. Vertical cross section of circulation and θ (K) along the purple line in Figure 5(a) at t = 9 h

700 hPa以下低层，冷池中下沉气流向前后流出，因下沉出流风速大，冲击前方和后方的空气，辐合产生上升运动，前方和后方的上升运动在120.1˚E、119.6˚E附近，这两处形成新的水平涡度，如图9所示。前方(后方)的垂直环流分别形成不同方向的水平涡度。冷池后部由于冷空气造成的环流与环境风场的环流方向相反，导致上升运动减弱，飑线后部雷暴消亡；而冷池前部由于环境水风速随高度增大产生的环流和冷池产生的垂直环流方向一致，两环流迭加造成低层新的上升运动，飑线向前自激发展。该理论即为Rotunno等提出的冷池和垂直风切变相互作用导致飑线移动方向前方新生单体的理论(即RKW理论) [23] 。

Figure 9. Schematic representation of interaction between cold pool and vertical wind shear (The green arrow denotes the environmental wind, the red circle denotes the circulation generated by cold pool, and the blue circle denotes the circulation generated by vertical environmental wind shear)

5. 结论

1) 东北冷涡南部低槽是飑线过程的大尺度背景形势，飑线发生在低层暖平流、中层冷平流的形成强位势不稳定层结大气中。

2) 云微物理过程Lin等的方案能够成功模拟此次飑线过程，模拟的雷暴高压、尾流低压及降水等天气过程与实况接近，模拟地面最大风速达到27 m/s。模拟的地面散度场存在一个“散度环”，雷暴高压带形成一条辐散线，其前部和后部各存在一条辐合线。

3) 飑线的气流与经典模式的顺切变和逆切变气流不同，上升气流和下沉气流基本处在同一垂直气柱中。由于上升气流源于中空，其在中高空产生的降水物质掉入下半部分的下沉气流中，不会对上升气流形成拖曳作用，同时，下降的降水物质在下沉气流中蒸发降温，在地面形成雷暴高压。

4) 利用线性化涡度方程，分析了飑线中层涡度偶形成的机制，垂直风切变和强垂直运动梯度相互作用在上升运动中心北侧形成负涡度，南侧形成正涡度。模拟的垂直环流和冷池分析验证了RKW理论，即雷暴高压中的冷池和垂直风切变相互作用的结果使得雷暴在冷池前缘新生，而在冷池后部消亡，飑线自激发展。

Numerical Simulation on a Squall Line Process in Jiangsu Province and Analyses of Its Formation Mechanism[J]. 气候变化研究快报, 2019, 08(05): 625-635. https://doi.org/10.12677/CCRL.2019.85069

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