Flow Over Changing Terrain

Abstract

The micrometeorologist setting out to find a field site that satisfies the requirements of horizontal homogeneity will soon be reminded that most of the earth’s surface is not flat and that most of the flat bits are inconveniently heterogeneous. This is what forced the location of early pioneering experiments to remote sites such as Kansas, Minnesota, or Hay (Chapter 1), where the elusive conditions could be realized. Vital as these experiments were to the development of our understanding, they are merely the point of departure for applications to arbitrary terrain. The components of arbitrariness are two: changes in the land surface and hills. In this chapter we discuss the first of these, flow over changing surface conditions; in Chapter 5 we look at flow over hills. In the real world, the two conditions often occur together — in farmland it is the hills too steep to plow that are left covered with trees — but we separate them here to clarify the explication of phenomena and because treating them in combination would exceed the state of the art. We simplify the problem of horizontal heterogeneity still further and discuss mainly single changes in surface conditions from one extensive uniform surface to another. Furthermore, the change will typically be at right angles to the wind direction so the resulting flow field is two-dimensional. Although multiple changes are now receiving theoretical attention (Belcher et al., 1990; Claussen, 1991), there exist as yet no experimental data for comparison. Two types of surface change may be distinguished at the outset: change in surface roughness, which produces a change in surface momentum flux with a direct effect upon the wind field, and change in the surface availability of some scalar. Those of most interest are the active scalars, heat and moisture. (These are called active because their fluxes and concentrations affect stability and thereby turbulent mixing and momentum transfer, as we saw in Chapters 1 and 3.) We shall discover significant differences in flow behavior according to whether the wind blows from a smooth to a rough surface or a rough to a smooth surface.