Isolating land-atmosphere feedbacks through coupled hydrologic modeling with idealized terrain

Both field observations and simulations indicate strong sensitivity of atmospheric dynamics to land-surface conditions, in particular surface soil moisture. This paper describes the effects of coupled groundwater-atmosphere modeling on simulations of the atmospheric boundary layer. Our coupled model [1] connects subsurface, surface, and atmospheric dynamics to allow a complete representation of the hydrologic cycle. We can therefore capture feedbacks in the land-atmosphere system that occur through precipitation events, evapotranspiration, surface runoff, and infiltration. Use of a three-dimensional variably saturated/unsaturated groundwater model explicitly accounts for lateral moisture transport in the subsurface. These lateral variations provide spatial heterogeneity in the land-surface soil moisture distribution and hence surface forcing in our three-dimensional atmospheric model. The soil moisture distributions naturally correspond to variations in topography, soil types, and vegetation. A land-surface model provides the interface between the atmosphere and the subsurface, passing moisture and heat fluxes between the models.

Here we use idealized sinusoidal terrain to perform a detailed investigation of land-atmosphere feedback processes in our fully-coupled model framework. Using the coupled groundwater-atmosphere model, we demonstrate correlations of soil moisture, land-surface heat fluxes, and boundary layer depth with groundwater levels over short, diurnal time scales. The resulting spatial variations in surface moisture distribution have large impacts on the moisture and temperature structure in the atmosphere, leading to changes in boundary layer depth and convective motions, as compared to standard land-surface models. The results of our coupled simulations show the importance of the groundwater-atmosphere connection in determining soil moisture distributions and land-surface fluxes. The effects of realistic, spatially-varying soil moisture forcing on boundary layer development can be equal to or greater than the effects from heterogeneous land-cover (soil and vegetation types), thus pointing to the need for improved soil moisture representations in current mesoscale atmospheric models.

[1] Maxwell, R.M., Chow, F.K., and S.J. Kollet. 2007. “The groundwater-land-surface-atmosphere connection: soil moisture effects on the atmospheric boundary layer in fully-coupled simulations,” Advances in Water Resources 30(12), 2447-2466.