Some aspects of the energy balance closure problem

International audienceAfter briefly discussing several reasons for the energy balance closure problem in the surface layer, the paper focuses on the influence of the low frequency part of the turbulence spectrum on the residual. Changes in the turbulent fluxes in this part of the turbulence spectrum were found to have a significant influence on the changes of the residual. Using the ogive method, it was found that the eddy-covariance method underestimates turbulent fluxes in the case of ogives converging for measuring times longer than the typical averaging interval of 30 min. Additionally, the eddy-covariance method underestimates turbulent fluxes for maximal ogive functions within the averaging interval, both mainly due to advection and non-steady state conditions. This has a considerable influence on the use of the eddy-covariance method

Energy balance closure for the LITFASS-2003 experiment

In the first part, this paper synthesises the main results from a series of previous studies on the closure of the local energy balance at low-vegetation sites during the LITFASS-2003 experiment. A residual of up to 25% of the available energy has been found which cannot be fully explained either by the measurement uncertainty of the single components of the surface energy balance or by the length of the flux-averaging period. In the second part, secondary circulations due to heterogeneities in the surface characteristics (roughness, thermal and moisture properties) are discussed as a possible cause for the observed energy balance non-closure. This hypothesis seems to be supported from the fluxes derived from area-averaging measurement techniques (scintillometers, aircraft)

The energy balance experiment EBEX-2000. Part III: Behaviour and quality of the radiation measurements

An important part of the Energy Balance Experiment (EBEX-2000) was the measurement of the net radiation and its components. Since the terrain, an irrigated cotton field, could not be considered homogeneous, radiation measurements were made at nine sites using a variety of radiation instruments, including pyranometers, pyrgeometers and net radiometers. At several of these sites multiple instruments were employed, which enabled us to compare instruments and assess accuracies. At all sites the outgoing longwave and shortwave radiation and the net radiation were measured, while the incoming radiation was supposed to be uniformly distributed over the field and was therefore measured at three sites only. Net radiation was calculated for all sites from the sum of its four components, and compared with the direct measurement of net radiometers. The main conclusions were: (a) the outgoing shortwave radiation showed differences of up to 30 W m-2 over the field; the differences were not clearly related to the irrigation events; (b) the outgoing longwave radiation showed differences of up to 50 W m-2; the differences increased during the periods of irrigation; (c) the net radiation showed differences of several tens of W m-2 across the field, rising to 50 W m-2 or more during the periods of irrigation; (d) the net radiation is preferably to be inferred from its four components, rather than measured directly, and (e) attention should be paid to the characteristics of pyranometers that measure the outgoing radiation, and thus are mounted upside down, while they are commonly calibrated in the upward position. The error in the net radiation at EBEX-2000 is estimated at max (25 W m-2, 5%) per site during the day and 10 W m-2 at nigh

Attempt to close the energy balance for the LITFASS-2003 experiment

For the LITFASS-2003 experiment conducted through May and June 2003 in a heterogeneous landscape around the Meteorological Observatory Lindenberg of the German Meteorological Service, different aspects relevant to close the energy balance at the surface based on radiation, soil and turbulent flux measurements were investigated. The careful correction of the eddy-covariance data (and the comparison of all instruments including net radiometers) can reduce the residual of the energy balance closure, but still a significant closure gap remains. The same result was found when carefully calculating the ground heat flux including all storage terms. The use of the ogive test to estimate the energy loss for long wavelengths showed an effect of up to 5 % on the residual. Finally, energy balance closure could not be achieved for the surface layer measurements in LITFASS-2003. Recent findings of large-eddy simulations (LES) have shown organised turbulence structures in the lower boundary layer. With the application of LES and the calculation of the horizontally averaged fluxes of sensible and latent heat closure of the energy balance was possible for one selected day, when the LES was available. This result was supported by large aperture scintillometer measurements as well as by averaging of the eddy-covariance measurements over longer time periods up to ten hours. It is therefore concluded, that the energy balance closure problem is a scale problem and structures of the atmospheric boundary layer, which develop over heterogeneous surfaces, must also be taken into account

Attempt to close the energy balance for the LITFASS-2003 experiment

For the LITFASS-2003 experiment conducted through May and June 2003 in a heterogeneous landscape around the Meteorological Observatory Lindenberg of the German Meteorological Service, different aspects relevant to close the energy balance at the surface based on radiation, soil and turbulent flux measurements were investigated. The careful correction of the eddy-covariance data (and the comparison of all instruments including net radiometers) can reduce the residual of the energy balance closure, but still a significant closure gap remains. The same result was found when carefully calculating the ground heat flux including all storage terms. The use of the ogive test to estimate the energy loss for long wavelengths showed an effect of up to 5 % on the residual. Finally, energy balance closure could not be achieved for the surface layer measurements in LITFASS-2003. Recent findings of large-eddy simulations (LES) have shown organised turbulence structures in the lower boundary layer. With the application of LES and the calculation of the horizontally averaged fluxes of sensible and latent heat closure of the energy balance was possible for one selected day, when the LES was available. This result was supported by large aperture scintillometer measurements as well as by averaging of the eddy-covariance measurements over longer time periods up to ten hours. It is therefore concluded, that the energy balance closure problem is a scale problem and structures of the atmospheric boundary layer, which develop over heterogeneous surfaces, must also be taken into account