A Climatological Study of the Relationship between Alpine Treeline and Thermally-driven Upslope Winds on Niwot Ridge, Colorado

摘要

Thermally-driven upslope winds are one of the many different wind patterns unique to mountainous terrain. While the general mechanics of these winds have been widely studied, there is a lack of information on how transitions in land cover influence these wind systems. Questions also remain about how these wind systems affect the biological functioning of mountain ecosystems. This two year measurement program indentified the upslope flow regimes common to the southeast face of Niwot Ridge, an east-west running ridge directly east of the Continental Divide in the Colorado Front Range. The study focused on the role that alpine treeline played in the generation of upslope winds, as well as whether the airmass composition of upslope versus downslope winds influenced the spatial distribution of alpine vegetation. Observations showed that thermally-driven upslope winds were most common in summer daytime hours. Differentiation between two different types of thermally-driven upslope winds, land cover-induced flow and anabatic flow, was done through an investigation of surface pressure and sensible heat flux at sites in the subalpine forest and alpine tundra. It was found that anabatic forces, not transitions in land cover from forest to tundra, were responsible for the generation of upslope winds on Niwot Ridge. Analyses on the composition of airmasses showed that upslope winds were generally cooler, more humid, and weaker in strength than downslope, westerly winds. These characteristics, combined with the higher frequency of upslope winds in the subalpine forest compared to the alpine tundra, suggested that upslope winds played an important role in the spatial distribution of vegetation in the alpine treeline ecotone.