Vegetation controls on surface heat flux partitioning, and land-atmosphere coupling

Abstract:

The relationship between soil moisture and surface evaporative fraction (latent heat flux normalized by the sum of sensible and latent heat flux) is critical in linking land surface forcing to clouds and precipitation. The evaporative fraction (EF) can influence precipitation directly through precipitation recycling, and indirectly through convective triggering. Several studies have shown that the observed correlation between soil moisture and EF is weaker than the correlation predicted in climate models, suggesting that land-atmosphere coupling is overestimated in models. Here, we present evidence that observed land-atmosphere coupling is underestimated when defined by the correlation between soil moisture and EF. Land-atmosphere coupling is three times stronger when using leaf area index as a correlate of EF instead of soil moisture, in the Southern Great Plains. This stronger coupling results from transpiration of relatively deep soil moisture. Furthermore, transpiration makes the relationship between EF and soil moisture nonlinear, and gives rise to the false appearance of weak coupling when using linear soil moisture metrics. The geographical extent of land-atmosphere coupling is not as small as in previous estimates. Regions of substantial land-atmosphere coupling extend to semi-arid and humid continental climates across the United States, in terms of correlations between vegetation metrics and EF. A comparison of the Community Land Model (CLM4.5) to observations in the Southern Great Plains reveals that the model overestimates correlations between soil moisture and EF, but underestimates correlations between leaf area index and EF by 30%, even when using observed daily leaf area index and meteorological forcing. It appears that the hydrological cycle is more tightly constrained by vegetation in observations than in climate models.