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, the models are limited to vertical transport in the soil column. They are thus unable to capture lateral variations in 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 forcing 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 serious errors in model predictions during periods when thermal forcing dominates the diurnal development of the boundary layer.

This work incorporates a 3D groundwater flow model with surface runoff capabilities within a mesoscale atmospheric model to investigate the effects of soil moisture heterogeneity on boundary layer processes. In particular, we have coupled ParFlow, a 3D parallel unsaturated/saturated groundwater flow model, with the Advanced Regional Prediction System (ARPS, mesoscale model). ParFlow also includes an overland flow or runoff component, and thus provides ARPS with soil moisture information that includes the effects of ponding, runoff, and seepage that occur. In turn, ARPS, through its land-surface model, provides ParFlow with precipitation and evapotranspiration. This leads to a fully coupled model which can represent spatial variations in land-surface forcing driven by 3D atmospheric and subsurface components. Our test case is the Little Washita watershed in Oklahoma, which has been the subject of numerous studies and provides a unique source of both subsurface, surface, and atmospheric data for validation.

This work was conducted under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory (LLNL) under contract W-7405-Eng. This work was funded by the LLNL LDRD program.