Small-scale advection and the neutral wind profile

Abstract

The flow of air in the lowest few metres of the atmosphere is examined in the case when a neutrally stratified logarithmic profile encounters a sudden change in surface roughness. Examination of the available evidence suggests that the ratio of new to old friction velocity is given by the ratio of the roughness lengths raised to a power about equal to 0.09. Some field observations and wind-tunnel measurements over a change in roughness indicate that vertical displacement of the streamlines at all heights within the range considered commences very near the surface transition. Calculations are made of the fetch necessary for the new wind profile to be established up to a given height and indicate that its ratio to the height is not constant but varies with the height and amount of change in roughness. Over a fairly wide range this ratio is about 100-150.