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

Yeonuk Kim,Monica Garcia,Ulrich Weber,Laura Morillas,T. Andrew Black,Mark S. Johnson

doi:10.5194/hess-25-5175-2021

Abstract

Abstract. Earth's climate and water cycle are highly dependent on terrestrial evapotranspiration and the associated flux of latent heat. Although it has been hypothesized for over 50 years that land dryness becomes embedded in atmospheric conditions through evaporation, underlying physical mechanisms for this land–atmosphere 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 to partition observed evaporation into diabatic vs. adiabatic thermodynamic processes. Vertical RH gradients were widely observed to be near zero on daily to yearly timescales 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.

Highlights

Latent heat flux (LE) associated with plant transpiration and evaporation from soil and intercepted water links the water cycle with the terrestrial energy budget
We found that evaporative fraction (EF) variability is mostly determined by LEG variability since the diurnal and seasonal signals of Q are removed from LE in EF
LEG was near zero during both daytime and nighttime periods due to near-saturated atmospheric and land surface conditions. These two cases show that “dry land–dry air” or “wet land–wet air” conditions can each lead to daily scale land–atmosphere equilibrium, the diurnal pattern of LEG is starkly different for dry land–dry air vs. wet land–wet air conditions

Summary

Introduction

Latent heat flux (LE) associated with plant transpiration and evaporation from soil and intercepted water (i.e., evapotranspiration, ET) links the water cycle with the terrestrial energy budget. More than half of the incoming radiation energy at the land surface is consumed as LE, making ET the second largest flux in the terrestrial water balance after precipitation (Oki and Kanae, 2006). Improvement of the theoretical understanding of LE still remains an essential cornerstone to correctly simulate and predict climate and hydrological cycles (Emanuel, 2020). Climatic conditions over the land surface have been getting warmer and drier in recent decades (i.e., decrease in relative humidity)

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Results

Discussion

Conclusion