Characteristics of Secondary Circulations in the Convective Boundary Layer over Two-Dimensional Heterogeneous Surfaces

Large-eddy simulations are conducted to investigate the impacts of the scale of chessboard-like heterogeneous surface heating and the background wind on secondary circulations (SCs) in the convective boundary layer (CBL). When the wind blows along the diagonal of the chessboard pattern, the cases with different heterogeneity length scales (=1.2, 2.4, and 4.8 km) and weak background wind (U=2.5 m s-1) suggest that there exists a threshold for the roll-like SCs, which is satisfied when the heterogeneity length scale is 1.6 times the boundary layer height (=1.6zi). During the CBL development, the SC intensity increases before this threshold is met, whereas it decreases thereafter. The cases with different background wind speeds (U=2.5, 5.0, and 10.0 m s-1) and relatively large heterogeneity length scale (=4.8 km) show that the SCs are strengthened by larger wind speeds when the heterogeneity length scale is so large that the threshold cannot be met during the CBL development. Another case with wind direction along neither the diagonal nor the side of the chessboard pattern shows that the roll-like SCs can still be triggered, but the roll axes are orientated along the diagonal of the chessboard pattern rather than along the wind direction.