Parameterization Study Of Atmospheric Boundary Layer On Flow Filed Over Complex Terrain

The interaction between the atmospheric boundary layer and the land surface is quite dependent on the physical properties of the underlying surface.The heterogeneous surface affects the structure and morphology of the atmospheric boundary layer in time and space,and further affects the regional and even global atmospheric climate.Most of present researches on the boundary layer flow of the uneven underlying surface caused by terrain undulations mostly focus on simplified 2D terrain or 3D isolated mountains,but in reality,mountain terrain is mostly complex terrain composed of multiple randomly distributed mountains.Therefore,it is necessary to further study the flow field structure of complex terrain and propose corresponding parameterization schemes,which can provide technical support for small and medium-scale atmospheric models and improve the accuracy of simulation.This paper first combines multiple large-scale randomly distributed mountains into a complex mountain terrain model.By using the ARPS(Advanced Regional Prediction System) large eddy simulation code,the mean flow field and turbulence characteristics of the atmospheric boundary layer under various complex terrain conditions are simulated.The effects of the changes of geomorphological parameters such as random location,mountain density,the mean and standard deviation of mountain height or half-width on the flow field structure of the atmospheric boundary layer under complex terrain are studied.Then,through nonlinear fitting of the mean wind speed profile,the aerodynamic parameters under different complex terrains,such as friction velocity_*u,aerodynamic roughness length z₀ and zero plane displacement height d,are studied.Finally,the frontal area index is used as an independent variable to characterize aerodynamic parameters,and a parameterization scheme of atmospheric boundary layer flow field structure suitable for complex terrain is proposed.The main contents are as follows:1.Among various geomorphological parameters,the random distribution of mountain locations and the changes in the standard deviation of mountain height and width have little effect on the flow field of complex terrain.But,the mountain density and the changes in the mean of mountain height and width has a more obvious influence.This influence is mainly reflected in the change of the slope of the logarithmic profile of the mean wind speed in the flow direction and the change of the maximum Reynolds stress.2.The friction velocity u_* of complex terrain shows an approximately linear increasing trend with the increase of the incoming wind speed,and the normalized friction velocity u_*/u_ref doesn't change with the increase of the incoming wind speed.3.The friction velocity u_*,aerodynamic roughness length z₀ and zero plane displacement height d all show an increasing trend with the increase of the mountain density or the average mountain height of the complex terrain.In addition,_*u and z₀ decrease with the increase of the mean of mountain width,on the contrary,d increases with the increase of the mean width.