LARGE-EDDY SIMULATION OF SMALL-SCALE SURFACE EFFECTS ON THE CONVECTIVE BOUNDARY-LAYER STRUCTURE

The effects of small-scale surface inhomogeneities on the turbulence s tructure in the convective boundary layer are investigated using a hig h-resolution large-eddy simulation model. Surface heat flux variations are sinusoidal and two-dimensional, dividing the total domain into a checkerboard-like pattern of surface hot spots with a 500-m wavelength in the x and y directions, or 1/4 of the domain size. The selected wi nd speeds were 1 and 4 m s-1, respectively. As a comparison, a simulat ion of the turbulence structure was performed over a homogeneous surfa ce. When the wind speed is light, surface heat flux variations influen ce the horizontally averaged turbulence statistics, including the high er moments despite the small characteristic length of the surface pert urbation. Stronger mean wind speeds weaken the effects of inhomogeneou s surface conditions on the turbulence structure in the convective bou ndary layer. Results from conditional sampling show that when the mean wind speed is small, weak mean circulations occur, with updraft branc hes above the high heat flux regions and down-draft branches above the low heat flux regions. The inhomogeneous surface induces significant differences in the turbulence statistics between the high and low heat flux regions. However, the effect of the surface perturbations weaken rapidly when the mean wind speed increases. This research has implica tions in the explanation of the large-scale variability commonly encou ntered in aircraft observations of atmospheric turbulence.