| The atmospheric boundary layer is the atmospheric region at the bottom of the troposphere closest to the surface of the atmosphere.Almost all the exchanges of heat,momentum,water vapor,aerosols,trace gases and atmospheric pollutants between the Earth and the atmosphere are achieved through atmospheric boundary layer processes.Therefore,both climate change and atmospheric circulation adjustment are closely related to the contribution of atmospheric boundary layer processes.Among the characteristic physical parameters of the atmospheric boundary layer,the thickness of the atmospheric boundary layer is one of the most important parameters.The atmospheric boundary layer not only determines the atmospheric environmental capacity of a region,but also strongly influences the development and evolution of cloud and convection.It is also one of the most important physical parameters in air quality monitoring and weather prediction,and is a key factor in initialization of numerical weather prediction model.The monsoon system is not only an important role of the global atmospheric circulation system,but also the main mechanism of global atmospheric energy and water vapor transport.According to the influence degree of summer monsoon,China's climate region can be divided into monsoon region,non-monsoon region and transition monsoon region affected by summer monsoon.Because of the different degree of influence of summer monsoon precipitation in each region,the spatial difference of land surface characteristics is obvious,and the thickness of atmospheric boundary layer decreases sharply from northwest to southeast.In this paper,the structure and thickness of atmospheric boundary layer during the transition monsoon region from non-monsoon region to monsoon region are compared and analyzed by using the sounding station altitude observation data distributed in the affected region of summer monsoon,the transition monsoon region of summer monsoon and the non-monsoon region,and the MERRA2 reanalysis data of NASA.The response mechanism of atmospheric boundary layer thickness to the advance and retreat of summer monsoon is discussed.The phenomenon of super-thick convective atmospheric boundary layer in non-monsoon arid area is analyzed and explained through the energy mechanism of atmospheric boundary layer development.The main conclusions are as follows:(1)The thickness of the stable boundary layer,the height of the residual layer top and the thickness of the convective boundary layer decrease in the order of the non-monsoon region,the transition monsoon region affected by the summer monsoon and the monsoon region.The thickness of the stable boundary layer,the height of the residual layer top and the thickness of the convective boundary layer decreased by 58.3%,28.5% and 25.6% respectively from the non-monsoon region to the transition monsoon region of the influence of the summer monsoon.From the transition monsoon region affected by the summer monsoon to the monsoon region,it decreased by 41.8%,75.5% and 81.8% respectively.The thickness of atmospheric boundary layer decreases stepwise from non-monsoon region,transition monsoon region of summer monsoon influence to monsoon region.(2)Besides the seasonal variation of atmospheric boundary layer thickness,the influence of summer monsoon is also very important to the variation of boundary layer thickness.With the onset of the summer monsoon,the northwest coast of the summer monsoon pushes forward,and the atmospheric boundary layer thickness affected by the summer monsoon decreases accordingly.With the weakening and retreating of the summer monsoon,the north edge of the summer monsoon retreats to the southeast,and the thickness of the atmospheric boundary layer in the area previously controlled by the summer monsoon also increases.Because the non-monsoon region is not affected by the summer monsoon,the thickness of atmospheric boundary layer still increases in summer and decreases in autumn.(3)The correlation between entrainment thermal diffusivity and atmospheric boundary layer thickness is very well in non-monsoon and transition monsoon regions,especially in non-monsoon region.The thickness of atmospheric boundary layer in non-monsoon region and transition monsoon region is generally much thicker than that in monsoon region.Thicker atmospheric boundary layer will produce thicker residual layer.The entrainment of residual layer in daytime is the main process of entrainment,which makes the entrainment thermal diffusivity in non-monsoon and monsoon regions have a good correlation with the thickness of atmospheric boundary layer.(4)Wavelet Cross-analysis of Meteorological Elements and Atmospheric Boundary Layer Thickness.The latent heat flux has little effect on the atmospheric boundary layer in the non-monsoon region,and only a small part of the high energy resonance region appears in the midsummer season(June-July).However,the monsoon and transition monsoon region have a larger impact,and they exist in spring and summer(April-September).The sensible heat flux and net radiation have less influence on the monsoon region,but more influence on the non-monsoon region and the monsoon region.Especially during the monsoon prevailing period in the monsoon region,there is no high energy resonance region in sensible heat flux and atmospheric boundary layer thickness.(5)The deep background of near-neutral residual layer plays an important role in the development of super-thick convective boundary layer in non-monsoon arid areas.Entrainment energy from deep residual layer to convective boundary layer through entrainment process is the key energy supplement for the continuous development of convective boundary layer.The sum of entrainment energy and surface sensible heat energy coincides with the energy absorbed for the development of super-thick convective boundary layer.During each continuous clear-sky period in summer,a daily cyclic growth mechanism is formed between the convective boundary layer and the residual layer.It is this positive feedback cycle growth process between the convective boundary layer and the residual layer triggered by strong sensible surface heating.Only in this way can the super-thick convective atmospheric boundary layer structure,which is rare in other regions,be developed in the arid region in summer. |
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