A study on atmospheric boundary layer structure on a clear day in the arid region in northwest China.

The climate environment and atmospheric circulation background affecting the formation and development of atmospheric boundary layer are relatively particular in Northwest China. Vertical structural characters of main physical factors in the arid desert of Northwest China in summer are analyzed based on the field intensive observation data in Dunhuang during May-June 2000. Whether the convection boundary layer(CBL) in the daytime or the stable boundary layer (SBL) in the nighttime in this area are thicker than those in other areas. In the nighttime of a summer clear sky day, the depth of SBL generally exceeds 900 m and its maximum may reach 1750 m. The top of the residual layer (RL) atop the SBL can touch a height of about 4000 m. In the daytime, the CBL develops rapidly after entering into the RL, and its depth exceeds 4000 m. The results show that a deep CBL in the daytime is a precondition for maintaining a deep RL in the nighttime. Meanwhile, a deep RL in the nighttime provides a favorable thermal condition for the development of the CBL in the daytime. The frequent continuous clear days in this area make it possible for the accumulated effect of RL to build up persistently, which creates a favorable thermal condition for the development of atmospheric CBL. At the same time, the characteristics of land surface processes and atmospheric motion in the surface layer in this area also provide positive support for the particularly deep structure of atmospheric thermal boundary layer. In regard to formation mechanism for the deep CBL, the dramatic increase in surface temperature in the daytime is a powerful external thermal forcing, the strong sensible heat flux in surface layer is a sufficient energy source, larger convection and turbulent motion velocities are a necessary kinetic condition, and the accumulation effect of RL provides a favorable thermal environment