A micrometeorological study of velocity profiles and surface drag in the region modified by a change in surface roughness

This paper describes experiments in the lower atmosphere in which the wind passes from one surface to another with different roughness. Observations were made of the variation of surface stress, and of the development of velocity profiles in the region of flow modification over the downwind surface. Measurements are compared with the theories of Taylor, Elliot, and Panofsky and Townsend, and with the growth of a boundary layer on a flat plate. A large proportion of the surface stress adjustment occurs rapidly after the transition in agreement with Taylor's assumption. Velocity changes agree fairly well with the Panofsky and Townsend theory in the smooth-rough direction, but not so well in the reverse. Growth of the modified region follows the 4/5 power law of boundary layer growth. It is concluded that the height/fetch criterion for a good micrometeorological site is about 1/200.