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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Jul 2018

Research article | 09 Jul 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Hydrology and Earth System Sciences (HESS).

Understanding model biases in the diurnal cycle of evapotranspiration: a case study in Luxembourg

Maik Renner1, Claire Brenner2, Kaniska Mallick3, Hans-Dieter Wizemann4, Luigi Conte1, Ivonne Trebs3, Jianhui Wei4, Volker Wulfmeyer4, Karsten Schulz2, and Axel Kleidon1 Maik Renner et al.
  • 1Max-Planck Institute for Biogeochemistry, Jena, 07745, Germany
  • 2Institut für Wasserwirtschaft, Hydrologie und konstruktiven Wasserbau, Universität für Bodenkultur (BOKU), Wien, 1190, Austria
  • 3Department Environmental Research and Innovation, Luxembourg Institute of Science and Technology (LIST), Belvaux, L-4422, Grand-duchy of Luxembourg
  • 4Institut für Physik und Meteorologie, Universität Hohenheim, 70599 Stuttgart, Germany

Abstract. The diurnal forcing of solar radiation is the largest signal within the Earth system and dominates the diurnal cycle of the turbulent heat fluxes and evapotranspiration (λE) over land. Incoming solar radiation (Rsd) also shapes temperature, vapor pressure deficit and wind speed known as important controls on λE. Current process-based λE schemes used in remote sensing and land–surface modeling differ in how these controls on λE are represented and which input variables are required. Here, we analyze how well different surface energy balance schemes are able to reproduce the diurnal cycle and how the diurnal signals of observed input variables actually influence the resulting diurnal pattern of λE. As additional constraint for model evaluation we estimate a linear and a non-linear phase shift component of a surface variable (e.g. λE) to incoming solar radiation. We illustrate our analysis with observations from an eddy covariance station at a temperate grassland site in Luxembourg with a focus on clear sky conditions. During the field campaign in 2015 a summer drought led to a dry-down of soil moisture which allows for studying the effect of wet and dry conditions on the diurnal cycle.

We found a remarkable, almost linear relationship of λE with Rsd, which exhibits a significant positive phase lag during wet periods. This phase lag in λE was compensated by a preceding phase lag of the sensible heat flux. Vapor pressure deficit (Da, often used as input for Penman–Monteith based approaches) exhibits a strong phase lag, which is driven by air temperature reflecting large diurnal heat storage changes in the lower atmosphere. This large phase lag in Da, which is not seen in λE, explains why actual and potential evapotranspiration approaches can show systematic deviations from observations at the sub-daily time scale and highlight the need for a time-dependent non-linear compensation through the conductance parameterization. The surface to air temperature gradient used as input in energy balance residual approaches corresponds rather well with its linear response and phase lag to the observed sensible heat flux under both, wet and dry conditions. This simplifies the conductance parameterization and explains the better correlation of these models at the sub-daily time scale.

We conclude that the analysis of phase lags at the sub-daily time-scale provides valuable information on the drivers of the surface–atmosphere exchange, which can be used to evaluate and improve the representation of land–atmosphere coupling in land–surface schemes.

Maik Renner et al.
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Maik Renner et al.
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Short summary
We estimate the phase shift of surface states and heat fluxes to incoming solar radiation at the sub-daily timescale. While evapotranspiration reveals a minor phase lag, vapor pressure deficit used as input by Penman–Monteith approaches shows a large phase lag. The surface-to-air temperature gradient used by energy balance residual approaches shows a small phase shift in agreement with the sensible heat flux and thus explains the better correlation of these models at the sub-daily time scale.
We estimate the phase shift of surface states and heat fluxes to incoming solar radiation at the...