ELSEVIER [31 Journal of Hydrology 166 (1995) 353-369 Journal of Hydrology Estimation of regional heat and moisture fluxes in homogeneous terrain with bluff roughness elements Martin Claussen Max-Planck-lnstitut ffir Meteorologic, Bundesstr. 55, 20146 Hamburg, Germany Received 6 May 1993; accepted 10 February 1994 Abstract Recently, models of regional heat and moisture fluxes in terrain with bluff roughness elements have been developed, based on observations which indicate that small areas of large roughness or obstacles dominate the regional momentum flux, whereas the regional heat and moisture fluxes are determined by the dominant surface cover. One model (Claussen and Klaassen, Beitr. Phys. Atmos., 65, 243-248, 1992) deduces the regional heat and moisture fluxes from local surface parameters, whereas the other (Beljaars and Holtslag, J. Appl. MeteoroL, 30, 327-341, 1991) uses effective parameters. Here, the two models are compared; it is shown that they are equivalent when applied to homogeneous terrain with bluff roughness elements. Moreover, they yield the same results when used in numerical simulations. Both models are tested against an earlier proposal by Brutsaert (Water Resour. Res., 11,543-550, 1975). 1. Introduction In numerical models of atmospheric flow, it is necessary to consider the properties of boundary-layer flow as averaged over the grid size of the model. In heterogeneous terrain, this leads to the problem of estimating areal averages of surface fluxes and associated roughness lengths, the latter being defined only for homogeneous conditions. Recently, the concept of blending height has become a useful approach to the parameterization of areally averaged surface fluxes over heterogeneous terrain (e.g. Wieringa, 1986; Mason, 1988; Claussen, 1990,1991a). Implicit in this concept is the assumption that at sufficiently large heights above a heterogeneous surface, the modification of air flow as a result of changes in surface conditions will not be recognizable individually, and an overall stress or heat flux profile will exist, repre- senting the surface conditions of a large area. Consequently, regional momentum and 0022-1694/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved SSDI 0022-1694(94)05089-G