Effects of Soil Moisture Heterogeneity on Boundary Layer Flow with Coupled Groundwater, Land-surface, and Mesoscale Atmospheric Modeling

Mesoscale atmospheric models currently rely on an integrated land-surface model to provide fluxes of heat, momentum, and moisture from the land surface to the atmosphere. While improvements have been made by tuning land-surface models for a variety of test cases, current models are limited to vertical transport in a shallow soil column. They are thus unable to capture lateral transport of soil moisture and limited in their ability to provide spatial variability in predicted land surface fluxes. Current mesoscale atmospheric models are therefore not provided with realistic fluxes at the surface because land-use models cannot represent runoff and subsurface lateral transport that is present when terrain or moisture gradients exist. This can lead to errors in model predictions during periods when thermal forcing dominates the diurnal development of the boundary layer. Starting with a so-called leaky-bucket parameterization (Manabe et al. 1965), climate models have steadily evolved to use a more sophisticated lower boundary condition into what is commonly known as the land surface model (LSM). LSMs play an important role in determining fluxes from the land surface to the atmospheric boundary layer (see e.g. the review by Betts et al. 1996). A large number of LSMs have been developed, with differing parameterizations and levels of sophistication. This led to a number of intercomparison studies, the Project for Intercomparison of Land-Surface Parameterization Schemes (PILPS), for a range of climatic conditions (Henderson-Sellers and Henderson-Sellers 1995; Shao and Henderson-Sellers 1996; Chen and Coauthors 1997; Qu and Coauthors 1998; Lohmann and Coathors 1998; Pitman and Coauthors 1999; Schlosser and Coathors 2000; Luo et al. 2003). As LSMs ignore the deeper soil moisture processes and the saturated zone (i.e. groundwater), there has been recent interest in incorporating a groundwater component into LSMs (Liang et al. 2003; Maxwell and Miller 2005; Yeh and Eltahir 2005). While LSMs have grown in sophistication, until this current study, lateral subsurface and overland flow have not been explicitly accounted for.