Numerical Simulation of the Lake Breeze Impact on Thunderstorm over the Taihu Area

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.