Cospectral analysis of high frequency signal loss in eddy covariance measurements

Abstract

The cospectra of momentum (M), sensible heat (H), latent heat (LE), and carbon diox-ide (Fc) fluxes measured by eddy covariance (EC) over a shortgrass steppe are calcu-lated 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 5 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% 10 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 15 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.