Marine-Atmospheric Boundary Layer Characteristics over the South China Sea During the Passage of Strong Typhoon Hagupit

The structures and characteristics of the marine-atmospheric boundary layer over the South China Sea during the passage of strong Typhoon Hagupit are analyzed in detail in this paper. The typhoon was generated in the western Pacific Ocean, and it passed across the South China Sea, finally landfalling in the west of Guangdong Province. The shortest distance between the typhoon center and the observation station on Zhizi Island (10 m in height) is 8.5 km. The observation data capture the whole of processes that occurred in the regions of the typhoon eye, two squall regions of the eye wall, and weak wind regions, before and after the typhoon's passage. The results show that: (a) during the strong wind (average velocity ū10 m s-1) period, in the atmospheric boundary layer below 110 m, ū is almost independent of height, and vertical velocity w is greater than 0, increasing with ūand reaching 2-4 m s-1 in the squall regions; (b) the turbulent fluctuations (frequency 1/60 Hz) and gusty disturbances (frequency between 1/600 and1/60 Hz) are both strong and anisotropic, but the anisotropy of the turbulent fluctuations is less strong; (c) ūcan be used as the basic parameter to parameterize all the characteristics of fluctuations; and (d) the vertical flux of horizontal momentum contributed by the average flow (ū w) is one order of magnitude larger than those contributed by fluctuation fluxes (u'w' and v'w'), implying that strong wind may have seriously disturbed the sea surface through drag force and downward transport of eddy momentum and generated large breaking waves, leading to formation of a strongly coupled marine-atmospheric boundary layer. This results in w 0 in the atmosphere, and some portion of the momentum in the sea may be fed back again to the atmosphere due to ū w 0.