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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Here, we present a novel physically-based evaporation model to demonstrate vertical relative humidity gradients from the land surface to the atmosphere tend to evolve towards zero due to land-atmosphere equilibration processes. Collapsing relative humidity gradients on daily to yearly timescales indicate an emergent land-atmosphere equilibrium, making it possible to determine evapotranspiration using only meteorological information, independent of land surface conditions and vegetation controls.
Preprints
https://doi.org/10.5194/hess-2020-643
https://doi.org/10.5194/hess-2020-643

  17 Dec 2020

17 Dec 2020

Review status: this preprint is currently under review for the journal HESS.

Relative humidity gradients as a key constraint on terrestrial water and energy fluxes

Yeonuk Kim1, Monica Garcia2, Laura Morillas3, Ulrich Weber4, T. Andrew Black5, and Mark S. Johnson1,3,6 Yeonuk Kim et al.
  • 1Institute for Resources, Environment and Sustainability, University of British Columbia, Vancouver, V6T1Z4, Canada
  • 2Department of Environmental Engineering, Technical University of Denmark, Lyngby, 2800, Denmark
  • 3Center for Sustainable Food Systems, University of British Columbia, Vancouver, V6T1Z4, Canada
  • 4Max Planck Institute for Biogeochemistry, Hans Knoell Strasse 10, 07745 Jena, Germany
  • 5Faculty of Land and Food Systems, University of British Columbia, Vancouver, V6T1Z4, Canada
  • 6Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, V6T1Z4, Canada

Abstract. Earth's climate and water cycle are highly dependent on terrestrial evapotranspiration and the associated flux of latent heat. Despite its pivotal role, predictions of terrestrial evapotranspiration remain uncertain due to highly dynamic and spatially heterogeneous land surface dryness. Although it has been hypothesized for over 50 years that land dryness becomes embedded in atmospheric conditions, underlying physical mechanisms for this land-atmospheric coupling remain elusive. Here, we use a novel physically-based evaporation model to demonstrate that near-surface atmospheric relative humidity (rh) fundamentally coevolves with rh at the land surface. The new model expresses the latent heat flux as a combination of thermodynamic processes in the atmospheric surface layer. Our approach is similar to the Penman-Monteith equation but uses only routinely measured abiotic variables, avoiding the need to parameterize surface resistance. We applied our new model to 212 in-situ eddy covariance sites around the globe and to the FLUXCOM global-scale evaporation product. Vertical rh gradients were widely observed to be near zero on daily to yearly time scales for local as well as global scales, implying an emergent land-atmosphere equilibrium. This equilibrium allows for accurate evaporation estimates using only the atmospheric state and radiative energy, regardless of land surface conditions and vegetation controls. Our results also demonstrate that the latent heat portion of available energy (i.e., evaporative fraction) at local scales is mainly controlled by the vertical rh gradient. By demonstrating how land surface conditions become encoded in the atmospheric state, this study will improve our fundamental understanding of Earth's climate and the terrestrial water cycle.

Yeonuk Kim et al.

 
Status: open (until 11 Feb 2021)
Status: open (until 11 Feb 2021)
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Yeonuk Kim et al.

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Short summary
Here, we present a novel physically-based evaporation model to demonstrate vertical relative humidity gradients from the land surface to the atmosphere tend to evolve towards zero due to land-atmosphere equilibration processes. Collapsing relative humidity gradients on daily to yearly timescales indicate an emergent land-atmosphere equilibrium, making it possible to determine evapotranspiration using only meteorological information, independent of land surface conditions and vegetation controls.
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