A Wavelet-Based Correction Method for Eddy-Covariance High-Frequency Losses in Scalar Concentration Measurements

Eddy-covariance (EC) scalar-flux measurements suffer from unavoidable biases introduced by high-frequency losses in the sampled scalar concentration fluctuations. This bias alone leads to an underestimation of scalar fluxes by as much as 20% in some cases, especially when a closed-path gas analyzer is used to sample concentration far from the inlet location. A novel method that directly corrects for these high-frequency losses using only the sampled scalar-concentration time series is proposed and tested. The sampled concentration fluctuation time series is adjusted, point-by-point, in the wavelet half-plane for each EC averaging interval (≈30 min). Similarity between scalars (and temperature) is not necessary and a pre-defined theoretical shape of the cospectrum is not required, making this method attractive at meteorologically non-ideal sites. When closed-path gas analyzers are used to measure H2O concentration fluctuations, the method is shown to reproduce the dependence of the attenuation on air relative humidity. Nevertheless, the method is not able to account for excessively large spectral attenuation that occurs close to the spectral peak, as might be the case with long tubes and high relative humidity. Since the method corrects the original scalar concentration time series and not the cospectrum, other flow statistics-such as variances and integral time scales-are also adjusted. The proposed method can be used synergistically with conventional high-frequency cospectral correction methods given the differences in assumptions and approaches among these methods. When the conventional and the proposed methods agree, added confidence to the estimate of the high frequency correction is gained, and vice versa. © 2012 Springer Science+Business Media B.V.

Duke Scholars

Author Gabriel G. Katul Civil and Environmental Engineering