四川暴雨过程中盆地地形作用的数值模拟
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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。
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.
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.
引文
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[19]张元春,孙建华,傅慎明.冬季一次引发华北暴雪的低涡涡度分析.高原气象,2012,31(2):387-399.
[20]赵宇,高守亭.对流涡度矢量在暴雨诊断分析中的应用研究.大气科学,2008,32(3):444-456.
[21]吴乃庚,林良勋,曾沁,等.南海季风爆发前罕见连续3场暴雨特征及成因.应用气象学报,2013,24(2):129-139.
[22]杨晓霞,万丰,刘还珠,等.山东省春秋季暴雨天气的环流特征和形成机制初探.应用气象学报,2006,17(2):183-191.
[23]苏涛,苗峻峰,王语卉.辐射参数化对海南岛海风雷暴结构模拟的影响.地球物理学报,2017,60(8):3023-3040.
[24]Fan J,Rosenfeld D,Yang Y,et al.Substantial contribution of anthropogenic air pollution to catastrophic floods in Southwest China.Geophys Res Lett,2015,42(14):606 6-6075.
[2]倪允琪,周秀骥,张人禾,等.我国南方暴雨的试验与研究.应用气象学报,2006,17(6):690-704.
[3]杨薇,苗峻峰,谈哲敏.太湖地区湖陆风对雷暴过程影响的数值模拟.应用气象学报,2014,25(1):59-70.
[4]王学锋,郑小波,黄玮,等.近47年云贵高原汛期强降水和极端降水变化特征.长江流域资源与环境,2010,19(11):1350-1355.
[5]胡豪然,毛晓亮,梁玲.近50年四川盆地汛期极端降水事件的时空演变.地理学报,2009,64(3):278-288.
[6]谌芸,李强,李泽椿.青藏高原东北部强降水天气过程的气候特征分析.应用气象学报,2006,17(增刊I):98-103.
[7]楼小凤,胡志晋,王广河.对流云降水过程中地形作用的数值模拟.应用气象学报,2001,12(增刊I):113-121.
[8]何立富,陈涛,孔期.华南暖区暴雨研究进展.应用气象学报,2016,27(5):559-569.
[9]Miglietta M M,Rotunno R.Numerical simulations of sheared conditionally unstable flows over a mountain ridge.J Atmos Sci,2009,66(7):1865-1885.
[10]姚文清,徐祥德.一次特大暴雨形成中天气尺度和次天气尺度系统的作用.应用气象学报,2003,14(3):287-298.
[11]廖菲,胡娅敏,洪延超.地形动力作用对华北暴雨和云系影响的数值研究.高原气象,2009,28(1):115-126.
[12]刘洋,钱贞成,朱宇宁,等.“8·31”云阳特大暴雨地形动力作用数值研究.高原山地气象研究,2015,35(3):9-17.
[13]Wang D,Miao J,Tan Z.Impacts of topography and land cover change on thunderstorm over the Huangshan(Yellow Mountain)area of China.Natural Hazards,2013,67(2):675-699.
[14]何光碧,屠妮妮,张利红.一次低涡暴雨过程发生机制及其模式预报分析.暴雨灾害,2014,33(3):239-246.
[15]刘新超,陈永仁.两次高原涡与西南涡作用下的暴雨过程对比分析.高原山地气象研究,2014,34(1):1-7.
[16]卢萍,郑伟鹏,赵兴炳.川西西南涡加密探空资料分析及数值模拟试验.高原山地气象研究,2012,32(1):1-7.
[17]苏涛,苗峻峰,蔡亲波.海南岛海风雷暴结构的数值模拟.地球物理学报,2016,59(1):59-78.
[18]张小玲,程麟生.“96.1”暴雪期中尺度切变线发生发展的动力诊断Ⅰ:涡度和涡度变率诊断.高原气象,2000,19(3):285-294.
[19]张元春,孙建华,傅慎明.冬季一次引发华北暴雪的低涡涡度分析.高原气象,2012,31(2):387-399.
[20]赵宇,高守亭.对流涡度矢量在暴雨诊断分析中的应用研究.大气科学,2008,32(3):444-456.
[21]吴乃庚,林良勋,曾沁,等.南海季风爆发前罕见连续3场暴雨特征及成因.应用气象学报,2013,24(2):129-139.
[22]杨晓霞,万丰,刘还珠,等.山东省春秋季暴雨天气的环流特征和形成机制初探.应用气象学报,2006,17(2):183-191.
[23]苏涛,苗峻峰,王语卉.辐射参数化对海南岛海风雷暴结构模拟的影响.地球物理学报,2017,60(8):3023-3040.
[24]Fan J,Rosenfeld D,Yang Y,et al.Substantial contribution of anthropogenic air pollution to catastrophic floods in Southwest China.Geophys Res Lett,2015,42(14):606 6-6075.