A measurement of mountain drag.

Abstract

The pressure drag on the Blue Ridge Mountain in the central Appalachians has been determined by measuring the pressure on each side of the ridge. During the first two weeks of January 1974 several periods with significant drag were observed with pressure differences typically 0.5 mb (50 Pa) across the ridge. This corresponds to a mountain drag which is a least equal to the skin friction drag in the Appalachian region if the other ridges in the area experience a similar drag. As expected, the drag seems to occur when the environmental wind has a component perpendicular to the ridge. Such a cross flow appears necessary but not sufficient for the production of drag. The measured mountain drag seems also to be associated with blocking of the surface flow upstream, a condition which occurs most frequently in stable nighttime conditions. While none of the possible drag mechanisms can be absolutely excluded or verified, drag due to flow separation appears unlikely and the mechanism of wave drag (of some sort) appears a likely candidate. The sources of error in mountain drag measurements and ways to improve the measurement techniques are also discussed. (A)