Analyzing the Validity of Similarity Theories in Complex Topographies

Abstract

The dispersion of trace gases in the atmosphere depends on the state of the atmospheric boundary layer (ABL) and one of the most important parameters characterizing it is the intensity of turbulence within the ABL. Hence, the reliability of the atmospheric dispersion models depends on the way turbulent parameters are calculated and related to the structure of the ABL. Similarity theories are the usual tool used to study the ABL structure, and it is able to describe not only the distributions of turbulent statistical variables, but also profiles of the mean variables and spectra. Different kinds of similarity have different similarity scales, similarity relations and application ranges. There is a long history of field experiments contributing to the continuing development of similarity theories (Businger et al., 1971; Niewstadt, 1984; Sorbjan 1986; Mahrt et al. 1998) and the results of such studies lead to a well known description of the structure of turbulence over flat, homogeneous surfaces and various atmospheric conditions. However only in recent years attention has been devoted to the structure of the atmospheric turbulence over complex terrain. Kaimal and Finnigan (1994) addressed the problem of heterogeneity surface on several scales. On the smallest scale the effects are confined to the surface layer and it is related to local advection in micrometeorology, i.e. how far downwind of a change we must go to find a flow in equilibrium with the local surface. Over very complex terrain, however, this equilibrium may never be attained. Generally studies over non-homogeneous topography try to estimate surface fluxes from profile measurements once the one-dimensional conditions, upon which Monin-Obukhov similarity is predicted, are lost. On the other hand information on turbulence response to the surface changes is much less complete than that on the mean fields (Kaimal and Finnigan, 1994). Based on theory, Jackson and Hunt (1975) have suggested the existence of a two-layer to describe