Analyze the Surface Thermodynamic and Dynamic Effects on the Atmospheric Boundary Layer Structure by Large Eddy Simulation

Abstract

Atmospheric boundary layer structure affects the exchange of material and energy between the earth and air， the dispersion of pollutants and human health.The boundary layer structure is significantly affected by surface heating and dynamics， but it is difficult to separate the effects of each factor in practical observations， which causes difficulties in parameterizing the boundary layer structure and atmospheric turbulence.Therefore， using the large eddy module， WRF-LES， in the Weather Research and Forecasting Model （WRF）， set up control test （Test1）， surface heat flux （Test2）， low-level wind shear （Test3）， surface roughness （Test4） tests and test that change the above variables simultaneously （Test5） to investigate the effects of surface heating， low-level wind shear and surface roughness on the boundary layer structure on the flat surface.The results were as follows： （1） The surface heating， enhanced by 5 times， warmed up the boundary layer， enhanced convection， increased the thickness of the entrainment layer by 120.8%， increased the turbulent kinetic energy of the full boundary layer by 45.7%， and increased the height of the thermodynamic and dynamic boundary layers by 28.3% and 29.9%， respectively.The strong vertical mixing facilitated the downward momentum transfer， which increased the wind speed in the lower boundary layer and decreased in the middle and upper boundary layers.（2） The 0.5 times increase in wind shear in the lower layer increased the thermodynamic and dynamic boundary layer heights by 11.9% and -6%， respectively， and increased the entrainment zone thickness by 31.2% and the full boundary layer turbulent kinetic energy by 25.7%.（3） The 4 times increase in roughness affected the turbulent kinetic energy mainly in the lower layers.The change in height of the dynamic boundary layer was small， but the friction loss effect reduced the wind speed in the dynamic boundary layer.Meanwhile， it increased the entrainment effect near the top of the boundary layer， which increased the potential temperature in the boundary layer.In a comprehensive view， the effects of surface heating and wind shear on turbulent kinetic energy and boundary layer structure are more significant， while the effects of roughness are smaller and mainly concentrated in the lower boundary layer.The simulation results can reflect to some extent the changing characteristics of the boundary layer structure under ideal conditions and are indicative for understanding scientific issues such as the influence of meteorological factors on pollution dispersion.

Keywords

• large eddy simulation （les）
• atmospheric boundary layer turbulence
• surface heat flux
• wind shear
• roughness