The Effect of Line Averaging on Scalar Flux Measurements with a Sonic Anemometer near the Surface

Abstract

We present a theoretical analysis of the effect of line averaging by a sonic anemometer on scalar fluxes and an observational study of this phenomenon in the atmospheric surface layer. The theoretical analysis rests on an axisymmetric model for the cross-spectral tensor of vertical velocity and scalar fluctuations, limiting the validity of the derived transfer function to a constant-flux layer. Observations of temperature flux in the unstable surface layer confirm the theoretical prediction that line averaging does not significantly affect the flux estimate down to heights only several times the sonic path length. However, the observations exhibit large scatter at small height, indicating that problems with the representativity of the measurement and not with line averaging may become a limiting factor. Based on the analysis of the Kansas data and the characteristics of the transfer function, we infer that temperature flux measurements will, in general, be affected by some line averaging under stable conditions at all heights in the surface layer. In these cases, the theoretical transfer function may be used to correct the measured heat fluxes.