Surface length scales and shear stress: Implications for land‐atmosphere interaction over complex terrain

Abstract

Abstract. A large eddy simulation (LES) code of the atmospheric boundary layer (ABL)has been developed and applied to study the effect of spatially variable surface properties on the areally averaged surface shear stress at the land-atmosphere interface. The LES code simulates the space and time evolution of the large-scale turbulent eddies and their transport effects in the ABL. We report here on simulations of flow over spatially variable roughness fields. The dynamics are simulated, and the resulting space-time fields are averaged to explore the effects of the surface variability length scales on the average surface shear stress, as used in large-scale models to estimate scalar fluxes, such as evaporation. We observe asymmetrical response of the smooth-to-rough and rough-tosmooth transitions, such that the effects of the transitions accumulate rather than cancel.It is shown that the presence of abrupt changes in surface roughness and the atmosphere's response to these patches create a marked dependence of the statistical structure of surface shear stress on the length scale of the surface patches. An increase in regionally averaged surface stress for decreasing horizontal patch length scale is found. IntroductionSuccessful modeling of surface hydrologic and atmospheric processes hinges on the ability to describe the exchange of water, heat, and momentum across the land-atmosphere interface. A major concern is how to account for the effect of the spatial variability of surface conditions on scales smaller than the model grid cell. The actual total grid cell exchange is an integration of small-scale exchange processes over the area of the cell, where the physical integration is provided by the turbulent mixing in the atmospheric boundary layer (ABL).

Atmospheric stability effect on subgrid-scale physics for large-eddy simulation

“…Since it was ®rst introduced [11], LES has been used to study the impact of dierent surface hydrologic factors such as the spatial variability of surface heat, moisture and roughness, on the ABL transport of momentum, heat and water vapor [1,2,18,20,25,46,55]. In LES, the unsteady 3D equations governing turbulent transport are numerically solved for scales of motion larger than a given grid size D, usually on the order of meters in simulations of the ABL.…”

Direct and Large-Eddy Simulation VIII

“…The numerical discretization follows the approach used by Moeng [7] and Albertson & Parlange [8], which combines a pseudo-spectral discretization in the horizontal directions and a centered second-order finite differencing in the vertical direction. With the periodic boundary conditions the simulations reproduce the conditions of a fully developed Wind Turbine Array Boundary Layer (WFABL).…”

A Hybrid Spectral/Finite-Volume Algorithm for Large-Eddy Simulation of Scalars in the Atmospheric Boundary Layer

“…Stresses are imposed at the bottom boundary based on a local application of the logarithmic law as described in Bou-Zeid et al (2005). More details on the solution of the velocity field can be found in Albertson and Parlange (1999).…”

“…This combination takes advantage of spectral accuracy in the homogeneous directions and allows flexibility for boundary conditions in the vertical direction and is widely used in a variety of ABL applications (e.g. Khanna and Brasseur 1997;Albertson and Parlange 1999;Kosovic and Curry 2000;Kumar et al 2006;Basu et al 2007;Mao et al 2007;Yue et al 2007).…”