Impact of physical representations in CALMET on the simulated wind field over land during Super Typhoon Meranti (2016)

Abstract

A WRF (Weather Research and Forecasting Model)/CALMET (California Meteorological Model) coupled system is used to investigate the impact of physical representations in CALMETon simulations of the near-surface wind field of Super Typhoon Meranti (2016). The coupled system is configured with a horizontal grid spacing of 3 km in WRF and 500 m in CALMET, respectively. The model performance of the coupled WRF/ CALMET system is evaluated by comparing the results of simulations with observational data from 981 automatic surface stations in Fujian Province. The root mean square error (RMSE) of the wind speed at 10 m in all CALMET simulations is significantly less than the WRF simulation by 20%–30%, suggesting that the coupled WRF/CALMET system is capable of representing more realistic simulated wind speed than the mesoscale model only. The impacts of three physical representations including blocking effects, kinematic effects of terrain and slope flows in CALMET are examined in a specified local region called Shishe Mountain. The results show that before the typhoon landfall in Xiamen, a net downslope flow that is tangent to the terrain is generated in the west of Shishe Mountain due to blocking effects with magnitude exceeding 10 m/s. However, the blocking effects seem to take no effect in the strong wind area after typhoon landfall. Whether being affected by the typhoon strong wind or not, the slope flows move downslope at night and upslope in the daytime due to the diurnal variability of the local heat flux with magnitude smaller than 3 m/s. The kinematic effects of terrain, which are speculated to play a significant role in the typhoon strong wind area, can only be applied to atmospheric flows in stable conditions when the wind field is quasi-nondivergent.