Multi-scale numerical simulation of the wind field over a coastal complex terrain during a typhoon landfall

The research and forecast of typhoons in China's coastal regions have significant economic and social benefits. A thorough investigation of the wind field over a coastal complex terrain is performed using the multi-scale method which is a combination of the meso-scale WRF (weather research and forecasting) and the micro-scale LES (large eddy simulation). The 24-hour central route of typhoon Kai-tak in 2012, as well as the time series of 10-min averaged wind speed and direction at 100 m above the ground are produced by WRF. Based on the temporal-averaged wind speed profiles from WRF, a cycling turbulence generator is applied to obtain a quasi-stationary inlet condition for the LES simulation, which further yields temporal- and spatial-distributions of wind speeds near the ground. The numerical results agree reasonably well with site observations and show that the streamwise turbulence intensity near the ground doubles as the typhoon boundary layer marches from the ocean to the observation site. Meanwhile, the high-frequency wind velocities, which are related to the terrain effect, have been considerably enhanced. Also, a downward mean wind speed of 0.73 m/s occurs at the leeside of hills. As a result, strong gusts might occur in the surface layer above coastal complex terrain during the landfall of typhoon.