A Large-Eddy Simulation Study of Convective Boundary Layers over Flat and Urban-Like Surfaces

The effects of urban-like surface with a block array on the dry convective boundary layer (CBL) are numerically investigated using the parallelized large-eddy simulation model (PALM). Five cases representing a free CBL, a sheared CBL, and a strongly sheared CBL over flat surfaces, a sheared CBL over a block array, and a sheared CBL over a modeled canopy layer are simulated and compared. In the sheared CBL over a block array, mixed-layer flow is decelerated due to increased surface friction and horizontal convective rolls appear in the mixed layer. In contrast, convection cells and intermediate structures between cells and rolls occur in the free and sheared CBLs over flat surfaces, respectively. Horizontal convective rolls and the traces of block-induced turbulent eddies are detected in the spectral density fields of vertical velocity and in the vertical profiles of vertical velocity skewness in the sheared CBL over a block array. The decelerated mixed-layer flow in the sheared CBL over a block array leads to stronger wind shear in the entrainment zone than in the other cases, resulting in streamwise-alternating updrafts and downdrafts there. While the contributions to vertical turbulent heat flux by rising cool air and sinking cool air are dominant in the free CBL, the contributions to vertical turbulent heat flux by sinking warm air and rising warm air increase as the wind shear in the entrainment zone increases. Due to enhanced turbulence activity and wave-like motions in the entrainment zone, the magnitude of entrainment heat flux in the CBL over a block array is larger than that in the other cases.