Numerical investigation of neutral atmospheric boundary layer flows over flat terrain and three-dimensional hills considering the effects of Coriolis force

Abstract

The characteristics of neutral atmospheric boundary layer (ABL) flows can be strongly affected by terrain and earth rotation-induced Coriolis force. The purpose of this study is to ascertain the effects of Coriolis force on neutral ABL flows over flat terrain and isolated three-dimensional hills. Based on large-eddy simulations (LES), a new formula for the maximum turbulence length scale lmax in the limited-length-scale (LLS) k-ε model is proposed to reproduce neutral ABL flows over flat terrain with different exposure conditions. It is demonstrated that lmax can be reasonably expressed as a function of the geostrophic wind speed Ug, the Coriolis parameter fc and the roughness length z0. The effects of Coriolis force on atmospheric flows over single isolated hills are subsequently elucidated from the perspective of flow patterns and turbulent statistics. The wake trajectory deflection and asymmetric vortex structures are identified on the lee side of hills owing to the wind veer. Additionally, lateral wind shear associated with the Coriolis force contributes to a faster wake recovery and larger turbulence fluctuations downstream of steep hills. Moreover, the speed-up factor at the hilltop is significantly enhanced with increasing hill slopes and height ratios, while it is not sensitive to the Coriolis effects.