Evaluation of Two Energy Balance Closure Parametrizations
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A general lack of energy balance closure indicates that tower-based eddy-covariance (EC) measurements underestimate turbulent heat fluxes, which calls for robust correction schemes. Two parametrization approaches that can be found in the literature were tested using data from the Canadian Twin Otter research aircraft and from tower-based measurements of the German Terrestrial Environmental Observatories (TERENO) programme. Our analysis shows that the approach of Huang et al. (Boundary-Layer Meteorol 127:273–292, 2008), based on large-eddy simulation, is not applicable to typical near-surface flux measurements because it was developed for heights above the surface layer and over homogeneous terrain. The biggest shortcoming of this parametrization is that the grid resolution of the model was too coarse so that the surface layer, where EC measurements are usually made, is not properly resolved. The empirical approach of Panin and Bernhofer (Izvestiya Atmos Oceanic Phys 44:701–716, 2008) considers landscape-level roughness heterogeneities that induce secondary circulations and at least gives a qualitative estimate of the energy balance closure. However, it does not consider any feature of landscape-scale heterogeneity other than surface roughness, such as surface temperature, surface moisture or topography. The failures of both approaches might indicate that the influence of mesoscale structures is not a sufficient explanation for the energy balance closure problem. However, our analysis of different wind-direction sectors shows that the upwind landscape-scale heterogeneity indeed influences the energy balance closure determined from tower flux data. We also analyzed the aircraft measurements with respect to the partitioning of the “missing energy” between sensible and latent heat fluxes and we could confirm the assumption of scalar similarity only for Bowen ratios \(\approx \)1.
KeywordsAirborne measurements Eddy covariance Energy balance closure Surface heterogeneity TERENO programme
The authors would like to thank Baltasar Trancón y Widemann for providing the wavelet routine. Some aspects of this manuscript are part of the master’s thesis of Katrin Kohnert at the University of Bayreuth, which was supervised by Thomas Foken. Funding for TERENO & TERENO-ICOS was provided by BMBF. The support by the land owners of the TERENO sites and technical staff of KIT/IMK-IFU is appreciated. This work was conducted within the Helmholtz Young Investigator Group “Capturing all relevant scales of biosphere-atmosphere exchange—the enigmatic energy balance closure problem”, which is funded by the Helmholtz-Association through the President’s Initiative and Networking Fund, and by KIT.
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