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Mosaic Land Surface Model

The AGCM is coupled to the Mosaic Land Surface Model (LSM) of Koster and Suarez (1996), a well-established soil-vegetation-atmosphere transfer (SVAT) model originally derived from SiB. The Mosaic LSM computes area-averaged energy and water fluxes from the land surface in response to meteorological forcing. The model allows explicit vegetation control over the computed surface energy and water balances, with environmental stresses (high temperatures, dry soil, etc.) acting to increase canopy resistance and thus decrease transpiration. The scheme includes a canopy interception reservoir and three soil reservoirs: a thin layer near the surface, a middle layer that encompasses the remainder of the root zone, and a lower “recharge” layer for long term storage. Bare soil evaporation, transpiration, and interception loss occur in parallel, and runoff occurs both as overland flow during precipitation events and as ground water drainage out of the recharge layer. A complete snow budget is included.

The model's main innovation is its attempt to account for subgrid variability in surface characteristics through the "mosaic" approach. A grid square area containing several different vegetation regimes is divided into relatively homogeneous sub-regions ("tiles" of the mosaic), each containing a single vegetation or bare soil type. Observed vegetation distributions are used to determine the partitioning. A separate energy balance is calculated for each tile, and each tile maintains its own prognostic soil moisture content and temperature.

A general description of the model is given in Koster and Suarez (1992), and a detailed description of the model's heat and energy balances is given in Koster and Suarez (1996). The model performance is documented in the PILPS analyses (e.g., Wood et al., 1998).