A New Look at Modeling Surface Heterogeneity: Extending its Influence in the Vertical

Andrea Molod, Haydee Salmun and Darryn W. Waugh

Heterogeneities in the land surface exist on a wide range of spatial scales and make the coupling between the land surface and the overlying boundary layer complex. In this study we investigate the vertical extent to which the surface heterogeneities affect the boundary layer turbulence. We present a technique called `extended mosaic' which models the coupling between the heterogeneous land surface and the atmosphere by allowing the impact of the subgrid scale variability to extend throughout the vertical extent of the planetary boundary layer. Simulations with extended mosaic show that there is a GCM level at which the distinct character of the turbulence over different land scene types is homogenized, which we call the model blending height. The behavior of the model blending height is an indicator of the mechanism by which the surface heterogeneities extend their direct influence upwards into the boundary layer, and exert their influence on the climate system. Results are presented which show the behavior of the model blending height and the relationships to atmospheric and surface conditions. The model blending height is generally one third to one half of the planetary boundary layer height, although the exact ratio varies with local and climatological conditions and the distribution of the underlying vegetation. The model blending height also increases with canopy temperature and sensible heat flux, and is influenced by the amount of variability in the surface vegetation and the presence of deciduous trees.

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