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Numerical Simulation of the Lake Breeze Impact on Thunderstorm over the Taihu Area
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摘要: 利用耦合了NOAH陆面模式的WRF中尺度数值模式，对2010年8月18日发生在太湖地区的一次强雷暴过程进行数值模拟，并将模拟结果与实况进行对比。结果表明:模式能较合理地模拟出雷暴演变过程及近地面要素变化。此次雷暴天气过程发生在湖风发展强盛时期，雷暴沿东岸湖风与背景风形成的辐合线发展。通过两个敏感性试验，研究了太湖地区湖陆风对雷暴过程的影响。湖风锋对雷暴过程起触发和增强作用，湖风锋的阻挡和抬升作用导致此次雷暴的产生。在湖风锋前缘形成的初始对流进一步发展加强为雷暴，发展成熟的雷暴低层出流又与湖风作用形成新的雷暴，湖风的辐合为对流云的发展提供水汽和能量。在雷暴的形成发展过程中，感热通量输送可改变大气边界层结构，使低层不稳定能量较易释放，潜热释放加强上升和下沉气流，使边界层湿度增大，对流进一步发展增强。Abstract: During the afternoon hours of 18 August 2010, thunderstorms struck the Taihu area and cause extensive damage in the vicinity. To investigate the impact of lake land use changes on the evolution of the severe thunderstorms, a coupled Weather Research and Forecasting (WRF) model with the NOAH land surface model is used. The control run and two sensitivity experiments are designed. The control run (CNTL) is carried out with the original surface characteristics; the first sensitivity experiment EXP1 is designed to replace the Taihu with cropland; and in the second sensitivity experiment EXP2 the underlying surface is considered as water. Three experiments employ four nested fixed grid which are set as a two-way run with spacing of 27, 9, 3, 1 km, respectively. The initial and boundary conditions are provided by the NCEP FNL analysis. To verify the simulation, the control run results from 1 km domain are compared with observation.Results show that the control run simulates well both lake-land breeze circulation and remarkable lake-land breeze evolution on 18 August 2010. It is found that the wind speed and depth of the lake breeze are horizontal asymmetries on the east and west coast of the Taihu are affected by southeasterly gradient flows and valley breeze. At the leading edge of lake breeze circulation called lake breeze front, convergence lines spread along the lake shore, and then the ascending motion, moisture air and low-level vertical wind shear triggers the development of thunderstorm at 1200 BT. Characteristics of the diurnal evolution of the thunderstorm are reproduced by WRF model, representing the initiation of convection along the lake breeze front and the formation of thunderstorm, and then the collision between outflow from thunderstorm and lake breeze triggers a new thunderstorm.The convective cloud doesn't develop in EXP1, and the whole area shows cloudless in EXP2. The comparison experiments show that the lake breeze front triggers and strengthens the severe convective weather. In the course of thunderstorm development, the exchange of sensible heat fluxes can change the structure of the boundary layer, and make the atmosphere more unstable. On the other hand, the surface fluxes moisten the boundary layer atmosphere and enhance horizontal convergence and divergence which can accelerate the development of cloud and precipitation.
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Key words:
- lake-land breezes;
- WRF model;
- thunderstorm;
- numerical simulation
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图 1 模拟区域地形分布
(a) 四重嵌套区域，(b) 第4层区域 (AB为本文所取剖面)
Fig. 1 The topography of simulation area
(a) four nested domains, (b) area of domain 4
图 3 2010年8月18日08:00风场 (矢量)、位势高度场 (实线，单位：dagpm)和温度场 (虚线，单位：℃)
(a)500 hPa，(b)850 hPa
Fig. 3 The wind (vector), geopotential height (solid line, unit:dagpm) and temperature (dashed line, unit:℃) at 0800 BT 18 August 2010
(a)500 hPa, (b)850 hPa
图 4 2010年8月18日00:00—24:00 24 h累积降水量 (单位：mm)
(a) 实况，(b) 模拟
Fig. 4 The 24-h accumulated precipitation (unit:mm) from 0000 BT to 2400 BT on 18 August 2010
(a) observed, (b) simulated
图 5 2010年8月18日2 m温度和10 m风速实况与模拟结果对比
Fig. 5 Comparisons between the observed and the simulated of 2-m temperature, 10-m wind speed on 18 August 2010
图 6 2010年8月18日模拟的2 m温度 (等值线，单位：℃) 和10 m风场 (已减去区域平均) 分布
(阴影表示地形，蓝色虚线代表湖风锋位置，下同)
Fig. 6 The simulated 2-m temperature (contour, unit:℃) and 10-m wind (minus area-averaged) on 18 August 2010
(the shaded denotes model terrain, the dashed blue line denotes the position of lake breeze front, the same hereinafter)
图 7 2010年8月18日沿AB的u-w场 (w扩大20倍)、假相当位温 (等值线，单位：K)和垂直速度 (阴影) 的东西向剖面图
(横坐标上黑色粗线表示湖面，下同)
Fig. 7 Vertical cross section of simulated wind field (the w-component is multiplied by a factor of 20), pseudo-equivalent potential temperature (contour, unit:K) and vertical velocity (shaded) along AB on 18 August 2010
(the lake surface is represented by solid black thick line, the same hereinafter)
图 8 2010年8月18日模拟的云水混合比 (阴影) 和雨水混合比 (等值线，单位：g·kg-1) 沿AB东西向剖面图
Fig. 8 Vertical cross section of simulated cloud water mixing ratio (shaded) and rain water mixing ratio (contour, unit:g·kg-1) along AB on 18 August 2010
图 9 2010年8月18日12:00的u-w矢量场 (w扩大20倍)、假相当位温 (等值线，单位：K) 和垂直速度 (阴影) 沿AB东西向剖面图
Fig. 9 Vertical cross section of simulated wind field (w-component is multiplied by a factor of 20), pseudo-equivalent potential temperature (contour, unit:K) and vertical velocity (shaded) along AB at 1200 BT 18 August 2010
图 10 2010年8月18日第4层区域平均的感热通量、潜热通量、净辐射和边界层高度随时间变化
Fig. 10 Hourly area-averaged sensible heat flux, latent heat flux, net radiation and PBL height for simulations of domain 4 on 18 August 2010
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