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Simulation of Basin Topography Impacts on Rainstorm in Sichuan
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摘要: 利用WRF-Chem模拟了2012年7月20日一次四川盆地暴雨降水过程，并基于控制试验设置填充四川盆地地形的敏感性试验。利用大气动力-热力学和云降水物理学对两试验差异进行诊断分析，与敏感性试验相比，控制试验虽然延迟强降水出现时间，却增强了降水强度。研究表明：偏南气流自南向北经过盆地时，在四川盆地南部形成正涡度扰动中心，延迟水汽、能量到达盆地北部的时间，使强降水出现时间偏晚；地形高度及动力差异使控制试验近地面累积大量水汽、能量，低层能量到达盆地北部迎风坡后受地形抬升与正涡度扰动共同作用激发了强烈的对流；控制试验中，盆地北部大气强烈对流运动及其携带盆地内大量水汽有利于云系的垂直发展，雨滴、雪晶、霰粒子质量浓度明显增大，使降水强度增强至大暴雨量级。Abstract: Topography, especially the height and shape conditions have significant effects on precipitation. Previous studies focus on effects of mountain topography upon precipitation, while influencing mechanisms of the basin topography are not widely discussed. The Weather Research and Forecasting (WRF) with Chemistry model is used to simulate a heavy rain event which occurs on 20 July 2012, over Sichuan Basin. A sensitive test is designed in which the topography of Sichuan Basin is uplifted, with other conditions the same as the control test. The topography in the sensitivity test shows a trend of slow decline from west to east, eliminating the role of basin topography, but keeping the influence of the Tibetan Plateau around the Basin.From the atmospheric dynamics, thermal and cloud micro-physics standpoints, diagnostic analysis is used to analyze results of these two tests, and differences between two experiments are discussed. Results show that the time of heavy rainstorm in the control test is later than that in the sensitivity test, and the rainfall intensity in control test is strongly enhanced. From the point of atmospheric dynamics, when southwesterly airflow through the basin from south, a stronger positive vorticity center forms in the south of the Basin in the lower layer of troposphere in the control test, and southern wind is weakened. Therefore, the water vapor and energy reach the northern part of the Basin later, leading to the precipitation delaying. At the same time, with the southward wind transport, the positive relative vorticity of the lower layer in the northern part of the Basin is continuously strengthened. Favorable dynamic structure strengthens the vertical motion and thus increases the precipitation intensity. From the thermodynamic view, there is more heat and water vapor in control test due to its lower height. Besides, these variables accumulate subjected to topographic dynamics, and are less likely to diffuse, providing sufficient water vapor for the rainstorm. In addition, the high temperature and high humidity condition makes the low level of the control test accumulates moist static energy. When airflow carrying moist static energy reaches the northwest of the Basin, strong upward is stimulated under the influence of topography and the positive relative vorticity in the lower troposphere. From the opinion of micro-physics, the stronger vertical motion provides advantage for the vertical development of the cloud system, and more water vapor provides greater supersaturation for precipitation particles in the control test. Under these conditions, precipitation particles, especially rain water, snow crystals and graupel, are generated and transformed in large quantities, enhancing the precipitation intensity to heavy rainstorm.
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Key words:
- topographic effects;
- rainstorm;
- numerical simulation;
- Sichuan Basin
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图 1 2012年7月20日20：00—21日11：00累积降水量(填色)和地形高度(等值线，单位：m)
(a)观测，(b)控制试验，(c)敏感性试验，(d)控制试验与敏感性试验差值(图 1a中地形高度数据来自控制试验)
Fig. 1 Accumulated precipitation(the shaded) and topography(the contour, unit:m) from 2000 BT 20 July to 1100 BT 21 July in 2012
(a)observation, (b)control test, (c)sensitivity test, (d)the difference between control test and sensitivity test (the topography in Fig. 1a from control test)
图 2 2012年7月20—21日强降水中心降水强度随时间演变
Fig. 2 Rain rate of rainstorm center from 20 Jul to 21 Jul in 2012
图 3 2012年7月20—21日观测与控制试验10 m风速、2 m相对湿度、2 m温度随时间演变
Fig. 3 10 m wind speed, 2 m relative humidity and 2 m temperature for observation and control test from 20 Jul to 21 Jul in 2012
图 4 2012年7月20—21日800 hPa风场(箭头)和涡度场(填色)(差值为控制试验减敏感性试验)
(a)20日21：00控制试验，(b)21日01：00控制试验，(c)21日05：00控制试验，(d)20日21：00差值，(e)21日01：00差值，(f)21日05：00差值
Fig. 4 Wind(the vector) and relative vorticity(the shaded) at 800 hPa from 20 Jul to 21 Jul in 2012 (difference is between control test and sensitivity test)
(a)control test at 2100 BT 20 Jul, (b)control test at 0100 BT 21 Jul, (c)control test at 0500 BT 21 Jul, (d)difference at 2100 BT 20 Jul, (e)difference at 0100 BT 21 Jul, (f)difference at 0500 BT 21 Jul
图 5 2012年7月20—21日控制试验与敏感性试验相对涡度(填色)和风场(箭头)差值沿104.6°E垂直剖面
(a)20日21：00, (b)21日01：00, (c)21日05：00
Fig. 5 Cross-section of relative vorticity(the shaded) and wind(the vector) difference between control test and sensitivity test along 104.6°E from 20 Jul to 21 Jul in 2012
(a)2100 BT 20 Jul, (b)0100 BT 21 Jul, (c)0500 BT 21 Jul
图 6 2012年7月20—21日风场(箭头)、温度场(实线，单位：℃)及比湿(阴影)沿104.6°E垂直剖面
(a)20日21：00控制试验，(b)21日01：00控制试验，(c)21日05：00控制试验，(d)20日21：00敏感性试验，(e)21日01：00敏感性试验，(f)21日05：00敏感性试验
Fig. 6 Cross-section of wind(the vector) temperature(the contour, unit:℃) and specific humidity(the shaded) along 104.6°E from 20 Jul to 21 Jul 2012
(a)2100 BT 20 Jul in control test, (b)0100 BT 21 Jul in control test, (c)0500 BT 20 Jul in control test, (d)2100 BT 20 Jul in sensitivity test, (e)0100 BT 21 Jul in sensitivity test, (f)0500 BT 21 Jul in sensitivity test
图 7 2012年7月20—21日湿静力能(单位：J·kg-1)沿104.6°E垂直剖面(阴影代表地形)
(a)20日21：00控制试验，(b)21日01：00控制试验，(c)21日05：00控制试验，(d)20日21：00敏感性试验，(e)21日01：00敏感性试验，(f)21日05：00敏感性试验
Fig. 7 Cross-section of moist static energy(the contour, unit:J·kg-1) from 20 Jul to 21 Jul in 2012(the shaded denotes the terrain)
(a)2100 BT 20 Jul in control test, (b)0100 BT 21 Jul in control test, (c)0500 BT 21 Jul in control test, (d)2100 BT 20 Jul in sensitivity test, (e)0100 BT 21 Jul in sensitivity test, (f)0500 BT 21 Jul in sensitivity test
图 8 2012年7月20—21日控制试验与敏感性试验对流有效位能、对流抑制能量差值随时间演变
Fig. 8 Time series of convective available potential energy and convective inhibition energy difference between the control test and the sensitivity test from 20 Jul to 21 Jul in 2012
图 9 2012年7月20—21日温度场(实线，单位：℃)、风场(箭头)及降水粒子浓度(填色)沿104.6°E垂直剖面
(a)21日05：00控制试验(云滴)，(b)21日05：00控制试验(雨滴)，(c)21日01：00敏感性试验(云滴)，(d)21日01：00敏感性试验(雨滴)
Fig. 9 Cross-section of temperature(the contour, unit:℃), wind(the vector) and mass concentration(the shade) of precipitation particle along 104.6°E from 20 Jul to 21 Jul in 2012
(a)cloud water in control test at 0500 BT 21 Jul, (b)rain water in control test at 0500 BT 21 Jul, (c)cloud water in sensitivity test at 0100 BT 21 Jul, (d)rain water in sensitivity test at 0100 BT 21 Jul
图 10 2012年7月20—21日温度场(实线, 单位：℃)、风场(箭头)及降水粒子浓度(填色)沿104.6°E垂直剖面
(a)21日05：00控制试验(冰晶)，(b)21日05：00控制试验(雪晶)，(c)21日05：00控制试验(霰粒子)，(d)21日01：00敏感性试验(冰晶)，(e)21日01：00敏感性试验(雪晶)，(f)21日01：00敏感性试验(霰粒子)
Fig. 10 Cross-section of temperature(the contour, unit:℃), wind(the vector) and mass concentration(the shade) of precipitation particle from 20 Jul to 21 Jul in 2012
(a)ice crystals in control test at 0500 BT 21 Jul, (b)snow crystals in control test at 0500 BT 21 Jul, (c)graupel in control test at 0500 BT 21 Jul, (d)ice crystals in sensitivity test at 0100 BT 21 Jul, (e)snow crystals in sensitivity test at 0100 BT 21 Jul, (f)graupel in sensitivity test at 0100 BT 21 Jul
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