太湖地区湖陆风对雷暴过程影响的数值模拟

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。%During the afternoon hours of 18 August 2010,thunderstorms struck the Taihu area and cause exten-sive damage in the vicinity.To investigate the impact of lake land use changes on the evolution of the se-vere thunderstorms,a coupled Weather Research and Forecasting (WRF)model with the NOAH land sur-face 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 de-signed 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 thun-derstorm 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.