Effects of surface-layer and free-atmosphere stratification on ABL flows over steep hill

What is it about?

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 σ coordinate transformation technique.

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Photo by 阿瞒 on Unsplash

Photo by 阿瞒 on Unsplash

Why is it important?

The results of this study have both theoretical significance and practical value. First, it provides new insight into the impact of atmospheric stratification on wind flow characteristics over hilly terrain. Second, a novel analytical model is developed to predict mean wind speed profiles over complex terrain considering the effects of atmospheric stratification. This model will be beneficial for engineering applications such as wind resource assessment and micro-sitting of wind farms in mountainous regions.

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