A simple method for estimating frequency response corrections for eddy covariance systems

A simple analytical formula is developed for estimating the frequency attenuation of eddy covariance fluxes due to sensor response, path-length averaging, sensor separation, signal processing, and flux averaging periods. Although it is an approximation based on flat terrain cospectra, this analytical formula should have broader applicability than just flat-terrain providing the peak frequencies of the logarithmic cospectra are known. Comparing the integral and analytical formulations for momentum flux, heat flux, vapor flux, and closed-path and open-path CO2 eddy covariance systems demonstrates that, except for a relatively uncommon atmospheric condition, the absolute difference between the integral and approximate correction factors is less than ±0.06 for both stable and unstable atmospheric conditions (0≤z/L≤2). Because closed-path systems can have the tube entrance separated longitudinally from the sonic anemometer, a cospectral transfer function is developed for the phase shift caused by the intrinsic time constant of a first-order scalar instrument and the longitudinal separation of the mouth of the tube and the sonic anemometer. The related issues of tube lag time and other spectral transfer functions are also discussed. In general, it is suggested that the simple formula should be quite useful for experimental design and numerical correction of eddy covariance systems for frequency attenuation.