Asymmetric Removal of Temperature Inversions in a High Mountain Valley

Abstract

During July 1985 the transition from nighttime to daytime wind regimes was studied in a steep-sided, broad mountain valley at about 2200 m MSL, in southeastern Wyoming. An array of surface weather stations and plot balloon releases from several sites were used to measure the boundary layer (BL) and surface winds, starting before sunrise and ending about midday. On nights with clear skies and strong radiational cooling, downslope, drainage winds occurred at the surface and through the depth of the BL; by midday, after complete removal of the BL inversion, regional winds dominated. The horizontal and vertical patterns of wind change, as regional winds replaced drainage winds, were determined by the pattern of inversion removal within the valley. In this wide mountain valley, erosion of the BL inversion starts on the west side and progress eastward, lagging 1?2 h after the onset of surface heating and resulting in an asymmetric pattern of inversion removal that is apparently independent of regional wind direction but may depend on valley orientation. The general process of inversion removal along the east-facing, west flank of the valley is similar to that observed in the South Park Basin and along the front range of Colorado. The asymmetry observed in the Laramie case was noted only in limited cases in South Park, perhaps since South Park is not entirely flanked by mountains to the east. Other observations, also in Colorado, show that inversion removal in steep-sided but narrow mountain valleys may be a symmetric process that is independent of both valley orientation and above-valley wind direction.