Flow Structures over Inhomogeneous Rough Surface under Near-Neutral Conditions

Flow structures in an inhomogeneous neutrally stratified atmospheric boundary layer flow, obtained from Large-Eddy Simulation, are analyzed and compared with homogeneous case counterparts. The inhomogeneity is imposed in the streamwise direction by using two different surface roughness heights zo, each covering a streamwise distance of 4.8 km, to produce internal boundary layers. Unlike homogeneous surface cases, adjustments of the mean velocity profiles are primarily confined to the lowest 100 m of the overlying boundary layer, with parcels being accelerated in the rough-to-smooth transition (smooth region) and decelerated in the smooth-to-rough transition (rough region). For large fetches the mean velocity profiles close to the surface are approximately logarithmic, but fitting to Monin-Obukhov similarity profiles using the zo and stress of the underlying surface requires that the von Karman constant be k=0.4 in the smooth region and k=0.37 in the rough region; much of this difference is attributed to velocity accelerations created by locally induced pressure gradient forces within the boundary layer, requiring k to be adjusted when horizontally homogeneous similarity expressions are utilized. Unlike streak spacing in the homogeneous surface cases, which increases with zo, average streak spacing in the smooth region is larger than that in the rough region. Quadrant analysis indicates that ejection and sweep intensities differ from those of homogeneous surface cases but the occurrence frequencies are similar. Budget analysis of momentum flux indicates that the shear-production and pressure-destruction terms roughly balance, which is consistent with previous homogeneous surface findings. Flow visualization and conditional sampling demonstrate that these two terms are physically associated with ejections, sweeps and vortical structures.