Mechanism of the Influence of Topography on the Initial Upscaling of the South China Squall Line: A Numerical Simulation Study
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摘要: 本文利用NCEP/NCAR提供的1°×1°的再分析资料,应用WRF4.0中尺度数值模式对2016年4月13日华南地区的一次飑线升尺度过程进行模拟,并设计一系列的敏感性试验,详细研究了南岭对飑线升尺度增长的影响以及可能的机制。结果表明:WRF模式较好的模拟了本次飑线过山前后的变化以及其降水的分布。强对流在过山后比过山前发展要强烈,水平的尺度增长快。但不同高度的地形敏感性试验表明,适宜的地形高度对于风暴的发展更有利。地形影响了飑线的尺度和组织,地形过高会使得广东北部的对流分散。地形可以通过改变水平流场、水汽场、垂直运动以及低层的垂直风切变等来间接影响飑线中的对流单体的分布和对流单体的强度。无地形阻挡时,有利于急流的北进,水汽输送更为有利。但是,一定的地形高度对低层的垂直运动是有利的。地形较高,则会利于高层的垂直运动,低层更多的可能以绕流为主。当地形超过一定高度时,低层的辐合场也相应的减弱。Abstract: In this study, the National Center for Environmental Prediction and the National Center for Atmospheric Research 1°×1° reanalysis data and the Weather Research and Forecasting (WRF) 4.0 mesoscale numerical model were used to simulate the upscale process of a squall line in South China on April 13, 2016. To examine the role of topography in the upscaling process of the squall line, a number of sensitivity tests was designed to comprehensively examine the influence of Nanling on the upscale growth of the squall line and associated mechanism. The WRF model effectively simulated the changes before and after the squall line crossed the mountain and the precipitation distribution. The convection after the mountain is stronger than that before the mountain, and the horizontal scale grows faster; however, the terrain sensitivity tests at different heights demonstrated that a suitable terrain height is extremely beneficial to storm development. The topography affects the scale and organization of the squall line, and the high topography disperses the convection in the north of Guangdong. Terrain can indirectly affect the distribution of convective cells and the strength of convective cells in the squall line by changing the horizontal flow field, water vapor field, vertical movement, and vertical wind shear at the lower level. The absence of terrain obstruction is beneficial to the jet stream and transporting water vapor toward the northward direction is more convenient. A certain terrain height is beneficial to the vertical movement of the low level; however, the terrain is extremely high to facilitate the vertical movement of the upper level; furthermore, the low level is more likely to be primarily detoured. When the terrain exceeds a certain height, the low-level convergence field is correspondingly weakened.
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Key words:
- Squall line /
- Sensitivity tests /
- Topography /
- Upscale growth /
- South China
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图 6 (a, b)ZE试验、(a1, b1)ZFE试验、(a2, b2)RE试验和(a3, b3)OFE试验模拟的2016年4月12日17:00(第一行)和21:00(第二行)的雷达回波(阴影,单位:dBZ)
Figure 6. Simulated radar reflectivity (shaded, units: dBZ) respectively by (a, b) ZE test, (a1, b1) ZFE test, (a2, b2) RE test, and (a3, b3) OFE test at 1700 UTC (top line) and 2100 UTC (bottom line) 12 April 2016
图 7 2016年4月12日18:00(a, e)ZE试验、(b, f)ZFE试验、(c, g)RE试验和(d, h)OFE试验模拟的850 hPa风场(第一行,单位:m s−1)和散度场(第二行,单位:10−5 s−1)。(a–d)中阴影区为风速大于12 m s−1的急流区
Figure 7. Simulated 850 hPa wind field (top line, units: m s−1) and divergence field (bottom line, units: 10−5 s−1) by (a, e) ZE test, (b, f) ZFE test, (c, g) RE test, and (d, h) OFE test at 1800 UTC 12 April 2016. Shadows in (a–d) for the jet with wind speed more than 12 m s−1
图 8 2016年4月12日18:00模拟的敏感性试验与对照试验在850 hPa高度上散度场的差值(阴影,单位:10−5 s−1):(a)ZE减去RE试验结果;(b)ZFE减去RE试验结果;(c)OFE减去RE试验结果。黑色椭圆形框代表飑线的位置
Figure 8. Difference in the divergence between simulated 850 hPa sensitivity tests and the control test(shaded, units: 10−5 s−1) at 1800 UTC on April 12, 2016: (a) ZE minus RE; (b) ZFE minus RE; (c) OFE minus RE;The black oval represent the position of the squall lines
图 11 2016年4月12日18:00模式模拟的假相位温线(θse,等值线,间隔:4 K)和垂直运动速度(阴影,单位:m s−1)过图3c中直线AB的垂直剖面(垂直速度扩大20倍):(a)ZE试验;(b)ZFE试验;(c)RE试验;(d)OFE试验
Figure 11. Simulated pseudo-equivalent potential temperature θse (isoline, interval: 4 K) and vertical velocity (shadings, units: m s−1) along AB straight line at 1800 UTC April 12, 2016 (vertical wind speed enlarged 20 times): (a) ZE test; (b) ZFE test; (c) RE test; (d) OFE test
图 12 2016年4月12日19:00 模拟的雷达组合反射率(阴影,单位:dBZ)和0.5~3 km垂直风切变(风羽,单位:m s−1)分布:(a)ZE试验;(b)ZFE试验;(c)RE试验;(d)OFE 试验。椭圆D1和椭圆D2表示β中尺度飑线的位置
Figure 12. Distributions of simulated combined radar reflectivity (shaded, units: dBZ) and 0.5–3 km vertical wind shear (barbs, units: m s−1) at 1900 UTC April 12, 2016: (a) ZE test; (b) ZFE test; (c) RE test; (d) OFE test. The oval d1 and the oval d2 represent the position of meso-β-scale squal lines
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