Structures and characteristics of the windy atmospheric boundary layer in the South China Sea region during cold surges

An observational analysis of the structures and characteristics of a windy atmospheric boundary layer during a cold air outbreak in the South China Sea region is reported in this paper. It is found that the main structures and characteristics are the same as during strong wind episodes with cold air outbreaks on land. The high frequency turbulent fluctuations (period <1 min) are nearly random and isotropic with weak coherency, but the gusty wind disturbances (1 min<period <10 min) are anisotropic with rather strong coherency. However, in the windy atmospheric boundary layer at sea, compared with that over land, there are some pronounced differences: (1) the average horizontal speed is almost independent of height, and the vertical velocity is positive in the lower marine atmospheric boundary layer; (2) the vertical flux of horizontal momentum is nearly independent of height in the low layer indicating the existence of a constant flux layer, unlike during strong wind over the land surface; (3) the kinetic energy and friction velocity of turbulent fluctuations are larger than those of gusty disturbances; (4) due to the independence of horizontal speed to height, the horizontal speed itself (not its vertical gradient used over the land surface) can be used as the key parameter to parameterize the turbulent and gusty characteristics with high accuracy.