Analysis of Changes in the Structure of Atmospheric Boundary Layer from Non-monsoon Zone to Monsoon Zone

Sounding data observed at 0700 BJT (Beijing time), 1300 BJT and 1900 BJT at seven sounding stations in central Northwest China on clear days in the summer of 2013 are used in the present study. The seven sounding stations are distributed in the non-monsoon zone, the summer monsoon transition zone and the monsoon zone, respectively. Vertical structures of potential temperature, specific humidity, wind speed, and atmosphere boundary layer thickness over different zones are analyzed. The results show that there are differences in the structure characteristics and thickness of the atmosphere boundary layer in different zones. The stable boundary layer thickness, the residual layer height and the convective boundary layer thickness decrease significantly from the non-monsoon zone to the summer monsoon transition zone and from the summer monsoon transition zone to the monsoon zone. The convective boundary layer thickness decreases by 25.6% from the non-monsoon zone to the summer monsoon transition zone, and the decrease is 81.8% from the summer monsoon transition zone to the monsoon zone. The stable boundary layer thickness decreases by 58.3% from the non-monsoon zone to the summer monsoon transition zone, and the decrease is 41.8% from the summer monsoon transition zone to the monsoon zone. Low-level jets can be observed in stable boundary layer, and the height of the low-level jet in the non-monsoon zone is higher than that in the summer monsoon transition zone and the monsoon zone. A similar phenomenon is found for the wind speed of the low-level jet. Based on analysis of the distribution of land surface thermal factors in different climatic regions, it is found that the distributions of the net radiation, the daily maximum difference between surface temperature and surface air-temperature (T_s-T_a) and the sensible heat fluxes are similar to the distribution of the convective boundary layer thickness. These land surface thermal factors in non-monsoon zone are larger than those in the summer monsoon transition zone and the monsoon zone. Results of the present paper provide a theoretical basis for the study of differences in the structure and thickness of the atmospheric boundary layer in different zones.