A COMPARATIVE-ANALYSIS OF 2 LAND SURFACE HETEROGENEITY REPRESENTATIONS

Two contrasting representations of land surface variability used in ge neral circulation models (GCMs) are compared through an analysis of th eir corresponding surface energy balance equations. In one representat ion (the ''mixture'' approach), different vegetation types are assumed to be homogeneously mixed over a grid square, so that the GCM atmosph ere sees near-surface conditions pertaining to the mixture only. In th e second representation (the ''mosaic'' approach), different vegetatio n types are viewed as separate ''tiles'' of a grid-square ''mosaic,'' and each tile interacts with the atmosphere independently. The mosaic approach is computationally simpler and in many ways more flexible tha n the mixture approach. Analytical solutions to the linearized energy balance equations and numerical solutions to the nonlinear equations b oth demonstrate that the mixture strategy, when applied to two coexist ing vegetation types that differ only in canopy transpiration resistan ce, promotes both total turbulent flux and latent heat flux relative t o the mosaic strategy. The effective differences between the strategie s, however, are small over a wide range of conditions. In particular, the strategies are effectively equivalent when the transpiration resis tances of the different vegetation types are of the same order of magn itude.