NEED FOR MEASURING HORIZONTAL GRADIENTS IN DETERMINING VERTICAL EDDY TRANSFERS OF HEAT AND MOISTURE

Abstract

Contrary to the usual assumption that shear stress is constant with height for a short distance above the ground, wind-tunnel tests with change in floor roughness by Jacobs (1939) and with flow over change of level by Tani (1957) show a long carry-over of irregular shear-stress profile. Such irregularity produces distortions in velocity and temperature profiles as a function of distance from the upwind discontinuity in surface roughness. Shear-stress irregularity, when Jacobs' tests are re-interpreted in per cent transition, is found to reach a distance downwind of more than 100 times the height of the roughness elements located before the prepared smooth ground, the delay in transition increasing with height above ground. Excluding large-scale vertical circulations, the ultimate eddy condition near the top of masts seems to depend on the the average roughness of the surface for several miles upwind, Since there is change with horizontal distance, thorough interpretation of vertical profiles will require consideration of horizontal gradients.