ACPD Atmospheric Chemistry and Physics Discussions ACPD Atmos. Chem. Phys. Discuss. 1680-7375 Göttingen, Germany 10.5194/acpd-7-13151-2007 Cospectral analysis of high frequency signal loss in eddy covariance measurements Wolf A. 1 Laca E. A. 2 Carnegie Institution of Washington, Department of Global Ecology, Stanford CA, USA Department of Plant Sciences, U.C. Davis, Davis CA 95616, USA 07 09 2007 7 5 13151 13173 Copyright: © 2007 A. Wolf 2007 This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Generic License. To view a copy of this licence, visit https://creativecommons.org/licenses/by-nc-sa/2.5/ This article is available from https://acp.copernicus.org/preprints/7/13151/2007/acpd-7-13151-2007.html The full text article is available as a PDF file from https://acp.copernicus.org/preprints/7/13151/2007/acpd-7-13151-2007.pdf

The cospectra of momentum (M), sensible heat (H), latent heat (LE), and carbon dioxide (Fc) fluxes measured by eddy covariance (EC) over a shortgrass steppe are calculated for over 800 time intervals spanning a range of wind, surface heating, evaporative, and photosynthetic conditions. The power spectrum of the vertical wind clearly shows that the inertial subrange is not sufficiently captured. The cospectra of the different fluxes show that the lack of measurement resolution in the high frequency results in a loss of flux, especially as stability approaches neutral. A procedure is outlined to use statistics from the cospectrum to estimate the amount of high-frequency flux that remains unmeasured for each time interval. The greatest loss of flux was for H (14% on average for 0>z/L>0.001 where z/L is the dimensionless stability), consistent with other studies which indicate temperature fluctuations actively produce turbulence at high frequencies. LE and Fc showed less than half as much loss of flux as H. This differential loss of flux has direct implications for addressing energy balance closure in EC studies, as well as reconciling biases of fluxes measured by EC with the Modified Bowen Ratio technique. It is recommended that the cospectra of fluxes be examined while setting the height of instrumentation in order to insure that high frequency eddies are resolved.

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