Surface roughness, energy distribution, and scale of structures for the atmospheric boundary layer

A wall-modeled large-eddy simulation is conducted to investigate the energy distribution and structural characteristics of atmospheric boundary layer over various surface roughness types, such as homogeneous, abrupt, and alternating roughness. The results show that increasing the roughness enhances the energy of small-scale motions, while large-scale motions exhibit the opposite behavior. A single energy peak is observed for homogeneous roughness, whereas abrupt roughness transition leads to double energy peaks, with very-large-scale motions (VLSMs) appearing near the wall. As the roughness varies alternately at an interval of 2 pi delta, the energy of VLSMs increases. While the interval is further reduced to pi delta, the energy of VLSMs will be transferred to the large-scale motions and small-scale motions. The scale of streamwise structures depends on the roughness, height, and interval of alternating roughness. Meanwhile, the structure inclination angle is proportional to the surface roughness. We conclude that the high-energy region near the wall, generated by abrupt and alternating roughness, will play a key role in material transport.