A Global Assessment of the Mosaic Approach to Modeling Land Surface Heterogeneity

Andrea Molod and Haydee Salmun

Heterogeneities in the land surface type on scales smaller than the typical General Circulation Model (GCM) grid size have long presented a challenge to properly model the impact of the subgrid scale variability on the grid scale. We present here a global offline comparison between two approaches to account for the heterogeneities. These approaches are mosaic, which computes separate energy budgets for each surface type within a grid box, and dominant, which assumes that a grid box can be completely described by the dominant vegetation. The experiments are all conducted using the Goddard Earth Observing System (GEOS) GCM turbulence and boundary layer parameterization coupled to the Koster-Suarez Land Surface Model. The results show a large impact in the high and mid-latitude northern hemisphere climates. At high latitudes, the warming of the surface after the spring snow melt is more rapid for dominant. At mid-latitudes, where the surface is potentially under moisture stress, the mosaic approach results in a drier, warmer climate. This impact is determined to a large extent by the influence of bare soil areas on the grid scale climate. The impact of the choice of approach is less important over more homogeneous terrains, such as deserts, as can be expected in the offline framework. These results support the need for a mosaic-type approach to properly model the coupling at the land surface interface.

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