The Influence of Horizontally Heterogeneous Soil Moisture on a Coupled PBL / Land-Surface System

Land-atmosphere coupling is widely recognized as a crucial component of regional, continental and global scale numerical models. However, predictions from these large-scale models are sensitive to small scale surface layer processes like heat and moisture fluxes at the air-soil-vegetation interface as well as boundary layer treatments. Particularly, the soil moisture boundary condition has a considerable influence on medium-to-long range weather forecasts and on simulated monthly mean climatic states. The resolution used in most large scale models is however relatively coarse so that the turbulent processes in the planetary boundary layer (PBL) which control the surface fluxes are not resolved but are determined by a parameterization. In our view, the shortcomings and sensitivities exhibited by large scale numerical models are partly a consequence of inadequate modeling of the PBL and its interaction with the land surface. In order to improve existing parameterizations, a more complete understanding of the mechanics and thermodynamics of air-soil interaction and the transport of water vapor by turbulent processes in the PBL is required. The importance of the atmospheric planetary boundary layer in land-atmosphere interactions is well known. Turbulent processes in the PBL regulate the exchange of momentum and scalars between the land surface and overlying atmosphere. Furthermore, concentrations of the important elements in the surface energy balance, heat and moisture, influence the fluxes themselves, in a feedback loop. The equilibrium state of concentrations and fluxes depends on surface conditions, entrainment and the entire temporal and spatial history of the PBL. The current understanding of the coupling between land surfaces and the PBL is, however, largely based on the study of idealized homogeneous land surfaces and cloud-free PBLs. An important next step is to examine the coupling between land surfaces and clear PBLs. In this study, we examine the interactions between land surfaces and the atmosphere by coupling a large-eddy simulation (LES) model for the PBL to a land surface model. Typical LES investigations of heterogeneous surfaces utilize prescribed surface fluxes and use relatively coarse grid resolution. In the work described here, fine grids and large computational domains are used to examine the impact of large scale heterogeneity on PBL turbulence and the surface heterogeneity is not imposed but dictated by coupling with a land surface model.