HOW LARGE MUST SURFACE INHOMOGENEITIES BE BEFORE THEY INFLUENCE THE CONVECTIVE BOUNDARY-LAYER STRUCTURE - A CASE-STUDY

A high-resolution large-eddy simulation model has been used to investigate the effect of scale in surface inhomogeneities on the turbulence structure of the convective boundary layer (CBL). Surface heat fluxes were varied sinusoidally in two dimensions with wavelengths of 250, 500 and 1000 m, corresponding to about 0.27 z(i), 0.55 z(i) and 1.1z(i) (z(i) is the boundary layer depth). The geostrophic wind speed was 1 m s(-1). Horizontally-averaged statistics of the CBL above surface heat-flux inhomogeneities of different two-dimensional scales were compared. The phase-average method was used to study the horizontal variation of turbulence and the mean circulation in the CBL. Results from horizontally-averaged statistics show that surface inhomogeneities of small horizontal extent (x < z(i)) influence the horizontally-averaged variances, covariances and third moments. The surface heterogeneities of larger scales produce more vigorous energy transport in the CBL and larger temperature variances near the ground than do those of smaller scales. There is more entrainment over the region of high heat flux than over the region of low heat flux. The magnitude of entrainment and the mean circulation both increase with increasing scale of heterogeneities present at the ground. The effects on the turbulence structure of the two-dimensional surface inhomogeneities differ from those in the one-dimensional case reported by Hadfield et al. (1991, 1992). The most pronounced difference occurs when the scale of the surface heterogeneities is comparable to the height of the boundary layer; the v-variance is larger than the u-variance in the boundary layer. Although the scope of this case-study is restricted to a limited number of typical scenarios, it does provide evidence in support of the fact that even small-scale inhomogeneities in the surface thermal forcing can modify the CBL structure very differently than could an equivalent but uniform field. The sensitivity of the CBL turbulence structure to surface thermal perturbations with scales smaller than the boundary layer depth is also discussed.