Cold Air Drainage on a Forested Mountain Slope

Abstract

The nocturnal cold airflow on a small forested slope was studied in an attempt to relate the volume and velocity of flow to the net radiation balance of the slope. The hillside, with an approximately 30% grade, was about 400 m long and covered with an uneven-aged spruce-fir stand of about 20 m height. The location was near Fraser, Colo., at an altitude of 3000 m. The vertical profiles of wind speed and air temperature were measured at various points on a transect approximating a streamline segment down the hill for several nights. Net radiation was measured at the base of the transect. Canopy closure and tree heights and diameters were measured on sample plots along the transect. The vertical profiles of potential temperature deficit and wind speed at the various stations show approximate similarity when scaled by the height of the lowest inversion and by the average velocities and potential temperature deficits below that inversion. The latter varies in a nearly linear manner down the slope, and the inversion heights appear to be relatively constant over the range of net radiation rates observed. Assuming a corresponding analytical similarity for the equations of motion and the sensible heat balance, the last two phenomena are found to imply that Um???, ???R0?, Um/??x, where Um is the average velocity indicated above, ?? the potential temperature drop down the slope, ? the sine of the angle of the slope to the horizontal along the streamline, x the downslope distance from the virtual origin of the flow measured along the streamline, and R0 the average net radiation loss on the slope. The data appear to confirm these results. The constants of proportionality calculated from the topographic contours, the estimated foliage distribution, and the scaled profiles of wind speed and potential temperature deficit are in fair agreement with observations.