Roughness effects in a stratified atmospheric boundary layer (ABL)

We examine roughness effects in the ABL by conducting Direct Numerical Simulation (DNS) of a canonical problem, the Ekman boundary layer (EBL), at moderate Reynolds number (Re_*=700). The roughness takes the form of periodic two-dimensional bumps whose nondimensional amplitude and aspect ratio are changed among cases. The EBL is in a transitionally rough regime and, without stratification, the effect of roughness is found to be mild in contrast to the stratified case. Stratification is imposed through a stabilizing cooling flux that is applied for approximately an inertial period. The flat-bottom case exhibits initial collapse of turbulence which eventually recovers, albeit with large-amplitude oscillations in turbulent kinetic energy. There is a strong thermal inversion and a low-level jet characteristic of a very stable (Mahrt 1998) regime. In contrast, the 4 bump EBL belongs to the weakly stable regime. Relative to the flat surface, the 4 bump case has a weaker thermal inversion, lower bulk Richardson number, and stronger near-surface turbulence. The role of coherent structures will be discussed through visualizations and statistics obtained by triple decomposition.