Evaluating Climate Model Simulations of Land–Atmosphere Coupling on the U.S. Southern Great Plains

Statistically significant coupling between summertime soil moisture and several surface atmospheric variables has been observed at the U.S. Southern Great Plains (SGP) observational facilities managed by the U.S. DOE Atmospheric Radiation Measurement (ARM) program (Phillips and Klein, 2014 JGR). In the current study, we employed several independent measurements of shallow-depth soil moisture (SM) and of the surface evaporative fraction (EF) during 2003-2011 May-August warm seasons, in order to estimate the range of SM-EF coupling strength at seven sites in the SGP region. We also have used these observational estimates to evaluate the representation of SM-EF coupling strength in version 5.1 of the global Community Atmosphere Model (CAM5.1) when coupled to the CLM4 Land Model. Two model experimental cases for the years 2003-2011 were considered:

1) a free-running Atmospheric Model Intercomparison Project (AMIP) simulation with historically observed sea surface temperatures (SSTs) specified, and

2) a constrained Hindcast run with SSTs also specified, but in which the CAM5.1 atmospheric state was initialized each day from the ERA Interim reanalysis, while the daily initial conditions of the CLM4 were obtained from an offline run of the land model using observed surface net radiation, precipitation, and wind as forcings.

We found that the controlled Hindcast simulation deviated less from the observed surface climate than its AMIP counterpart, but in both configurations the SM-EF coupling was much stronger than the SGP observational estimates. The source of the model’s overly strong SGP coupling thus appears to be located mostly in the detailed physics of the land-atmosphere coupling rather than in biases displayed by the simulated soil moisture and surface atmospheric variables.

In order to gain further insight into these coupling processes, we are constructing composite averages of model soil moisture and surface atmospheric variables at different days’ lag relative to the simulated precipitation events. These lagged composites, in turn, will be compared with analogous composites constructed from the ARM observations at SGP. We will present results of this process analysis, in addition to the details of the observational evaluation of the model’s representation of SM-EF coupling strength in the SGP region.

Acknowledgments

The work of the Lawrence Livermore National Laboratory (LLNL) authors was funded by the U.S. Department of Energy Office of Science under its ARM, Atmospheric System Research (ASR), and Regional and Global Modeling (RGCM), and was performed at LLNL under Contract DE-AC52-07NA27344.