Comparison of spatial pattern and mechanism between convexity and gap winds

Abstract

This study uses a numerical model to examine how a convex feature and a gap feature in a mountain range affect the leeward wind field. In the “convexity case”, the mountain ridge has a convex feature (viewed from above). In the “gap case”, the mountain ridge has a gap. The results show that both cases have local winds at the surface exceeding 8 m s⁻¹, and both have similar spatial flow-patterns. However, the momentum budgets at the strong-wind regions differ between the cases. In the convexity case, the downdrafts are important in the momentum balance, whereas in the gap case, both the downdrafts and the pressure-gradient force are important. Thus, although their spatial patterns of surface wind are similar to each other, their mechanisms for producing a strong local wind differ.

Sensitivity experiments of Fr_m show that strong-wind appears in both the convexity and gap cases when Fr_m is between 0.42 and 1.04. In contrast, when Fr_m is 0.21, strong winds only appear in the gap case because the flow can go around the gap. When Fr_m exceeds 1.25, strong surface winds appear in the entire leeward plain.