LES study on inversion-capped atmospheric boundary layer flows over steep topography considering the effects of free-atmosphere lapse rate

Abstract

The capping temperature inversion is a common atmospheric phenomenon that strongly influences the mean flow and turbulence structures within the atmospheric boundary layer (ABL). In this study, full-scale large eddy simulations are utilized to shed light on the characteristics of inversion-capped ABL flows over steep hilly terrain. As atmospheric stratification increases, the vertical wind veer becomes stronger, creating asymmetric flow patterns in the hill wake, and the buoyancy force acts to resist turbulent wake motions. In contrast to the conventionally neutral boundary layer (CNBL) and the convective boundary layer (CBL) cases, the stable boundary layer (SBL) exhibits pronounced flow acceleration at the hilltop and a faster wake recovery on the lee side of the hill. Based on the quadrant analysis, flow separation and vortex shedding are found to enhance organized motions downstream of the hill crest. In the wake region, sweep and ejection motions are identified at different heights. Furthermore, a wind speed prediction approach is developed for inversion-capped ABL flows over steep hilly terrain under both stable and unstable stratifications. The proposed approach incorporates the effect of the free-atmosphere lapse rate. Overall, it shows satisfactory agreement in predicting mean wind speed profiles over steep hills under both CBL and SBL conditions. The overestimation of mean wind speed at the hilltop in the SBL case can be corrected using the $\sigma$ coordinate transformation technique.