Stephen Klein — Lawrence Livermore National Laboratory
Thomas J. Phillips — Lawrence Livermore National Laboratory
Category
General Topics
Description
Scatter plot illustrating the covariation of 2008-2009 summertime CAM5 surface relative humidity versus 10-cm soil moisture index, both down-scaled to the ARM Southern Great Plains site. The corresponding correlation coefficient R and sensitivity index I are also shown.
In a previous study of observed features of land-atmosphere coupling (LAC) at the ARM SGP site (Phillips and Klein, 2014 JGR), we identified statistically significant covarations of 1997-2008 summertime daily averages of near-surface soil moisture with atmospheric variables, such as surface evaporation, relative humidity, and temperature, as well as the boundary layer's lifting condensation level.
Here we report on an evaluation of similar features of LAC simulated by version 5 of the global Community Atmosphere Model (CAM5), coupled to its native CLM4 land model, and downscaled to the ARM SGP site. In these case studies, the CAM5 was initialized from a 6-hourly atmospheric reanalysis for each day of the years 2008 and 2009 (where the CLM4 land state was equilibrated to the atmospheric model state), thus permitting a close comparison of the modeled and observed summer daily average features of LAC in these years. Correlation coefficients R and “sensitivity indices” I (a measure of the comparative change of an atmospheric variable for a one-standard-deviation change in soil moisture) provided quantitative measures of the respective coupling strengths.
Our comparison of observed versus modeled LAC is complicated by differences in atmospheric forcings of the land. For example, the CAM5’s summertime precipitation is too scant, and thus the model’s upper soil layer often is drier than observed. The modeled daily average covariations of soil moisture with lower atmospheric variables also display less coherence (lower R values), but sometimes greater “sensitivity” (higher I values) than are observed at the ARM SGP site. Since the observational estimate of LAC will be sensitive to soil moisture measurement biases, we also will report on an investigation of the dependence of LAC on several alternative choices of soil moisture data sets that are available at the ARM SGP site.
Acknowledgments
This work was funded by the U.S. Department of Energy Office of Science and was performed at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
