Evolution of the Atmospheric Stratification and Mixed Layer before and after Monsoon Onset over the South China Sea
南海季风爆发前后大气层结和混合层的演变特征

Based on the sounding data four times a day observed on R / V "Kexue 1" and R/ V "Shiyan 3" during the South China Sea Monsoon Experiment (SCSMEX), the evolution of the atmospheric stratification and mixed layer before and after monsoon onset over the South China Sea (SCS) were analyzed. The results was as follows: (1) The date of monsoon onset over the northern SCS was on May 17, whereas the date over the southern SCS was on May 21 to 22 or so. The monsoon onset over the northern SCS was greatly different from the southern SCS and its abrupt onset was more prominent. (2) Before and after monsoon onset over the SCS, the height of the mixed layer over the southern SCS was obviously different from northern SCS. Before the monsoon onset, the mixed layer existed on both parts of the SCS, but the height was different. For the southern SCS, the mixed layer varied from 940 to 975 hPa, while for the northern SCS, the height of the mixed layer was a bit higher, about 900 to 980 hPa. With the monsoon onset, the height weakened and even disappeared. (3) Before and after monsoon onset over the SCS, the evident different stratification structure appeared in the middle and low troposphere. Before monsoon onset, the humidity of air was relatively low, the atmosphere was stable and the ceiling height of the mixed layer was relatively high, which suggested an obvious dry layer existed in the middle troposphere. With the monsoon onset, the dry layer gradually weakened or even tended to disappear. After the monsoon onset, the southwesterly monsoon transported the volume of warm and wet air to the SCS and the convection enhanced. So, the stratification was unstable and precipitation was also falling.