Land-atmosphere coupling strength over the U.S. Southern Great Plains: A new diagnostic

Description

Land-atmosphere (LA) coupling plays an important role in climate system through its impacts on the energy and water cycles. The United States Southern Great Plains (SGP) is identified as one of the strong LA coupling regions by multi-model ensemble. However, recently Phillips and Klein [2014] shows weak coupling strength estimated by the covariance between top-layer soil moisture (SM) and evaporative fraction (EF) with the DOE Atmospheric Radiation Measurement (ARM) observations at the SGP central facility (CF). Based on observations at two adjacent sites (close to the CF), Williams and Torn [2015] estimated much stronger coupling by replacing SM with the leaf area index (LAI) in the conventional R(SM, EF) metric, highlighting the vegetation impact. This study tries to reconcile the SGP coupling differences in previous studies. We developed a new LA coupling diagnostic based on multi-variable linear regression. This new diagnostic is able to quantify the integrated impacts on the EF from multiple factors, such as soil moisture, vegetation, and clouds. Combined with the sensitivity index [Dirmeyer, 2011], the new method can further quantify the relative contributions from individual drivers. Thus, the new method provides more comprehensive information than existing ones. We analyze measurements from 10 long-term ARM SGP sites as well as MODIS satellites for March-August in years 2004-2011. These sites represent a wide range of surface types, soil types, and atmospheric conditions. Stronger LA coupling is found at all locations by the new multi-variable method than either of the individual correlations between EF and SM or LAI, suggesting both are important factors for EF. Their relative importance, however, varies at different SGP sites. Cloud fraction has little impact on the coupling, which confirms that summertime SGP is in a soil-moisture-limited regime. At the CF, we find moderate coupling and LAI is indeed more important than SM in affecting EF, which is consistent with previous studies. The coupling at the CF can represent a large fraction of the SGP sites as well as the domain mean, providing a basis for model evaluation. Overall, our results show moderate LA coupling at the SGP with large spatial variabilities (R: 0.4--0.7). Although the major driver of the spatial variabilities in the coupling is not clearly revealed by this study, it suggests that the mesoscale circulation might be potentially important.