Observational Study On The Boundary Layer Structure And Air Sea Fluxes Of Landfall Typhoon In The Coastal Area Of South China Sea

The coastal area of the South China Sea was the regions that suffered by landfall typhoon most frequently and seriously in China. Lack of observation data, it is still difficult to accurately describe the typhoon boundary layer structure and the air sea fluxes. Based on the data of the Yearbook of tropical cyclone and the data collected by GPS sounding and the eddy covariance data on the offshore platform during typhoon episodes from the Marine Meteorological Science Experiment Base at Bohe, Maoming which was established by Guangzhou Institute of Tropical and Marine Meteorology, China Meteorological Administration, the dynamic and thermodynamic boundary layer structure of typhoon and the exchange characteristics of air sea fluxes are investigated in this study.In this study, the basic characteristics of the tropical cyclones in the coastal area of South China are analyzed. It was found that the offshore area of South China is the most frequently affected area in China with annual average of 5.4, which accounted for 59.2% of the total number of landing tropical cyclones in China, and the most of the area was located in the northwest of Zhongsha islands of Sansha city. The strongtyphoons often occurred in the sea region of 5-20°N, 120-150°E, the super typhoons occurred at southeast of the region of the strong typhoons. The time span of typhoons affecting the offshore area of South China was from April to November, especially in June and September. The typhoons occur most frequently in summer and less in autumn. From 1949 to 2015, the maximum number of typhoons affecting the offshore area of South China each year is eight, and the minimum was two. In the last ten years,the frequency of typhoons affecting the offshore area of South China decreased.The characteristics of boundary layer structure were analyzed for the landfall typhoon "Utor" and "Mujigae",it is found that the dynamic and thermodynamic characteristics of boundary layer structure before typhoon landing were significantly different from that after typhoon landing. We also analyzed the height of the maximum wind (humax)and the inflow layer depth (hinfl) and the mixed layer depth(Zi), and found that Zi was much lower than humax and hut and Ainfl, was lower than 500 m,humax and hut varied from 500 to 2200 m, and hinfl varied from 400 to 1500 m.The humax and hut were above the inflow layer. There are significant differences between before and after typhoon landing in the aspect of the humax and hut varied significantly when the typhoon was approaching and landfall .The value of humax,hut and Zi decreased when the typhoon center was close to observation site.The variations of the air sea fluxes, friction velocity (u*) and drag coefficient(Cd) parameters were analyzed,and the relationship between u* and Cd was also studied. The results indicated that the momentum flux increased with increasing wind speed during the typhoon landing process. The sensible heat flux and latent heat flux wre much larger when the typhoon was landing than thos before and after typhoon landing, and the variation was dramatic, suggesting that more heat fluxes were needed for typhoon landing. It is also found that the latent heat flux was larger than sensible heat flux, which indicated that latent heat flux was the main energy source of supporting the development of typhoon. There was nonlineraty between Cd and wind speed during the landfaal process. It decreased with the increasing wind speed in low wind speed regime, and increased with the increasing wind speed in the moderated wind speed regime, and decreased with the increasing wind speed when the U10 was stronger than 17 m/s,with the corresponding value of 0.003.