Abstract
Abstract. We propose a method to assess the accuracy of atmospheric turbulence measurements performed by sonic anemometers and test it by analysis of measurements from two commonly used sonic anemometers, a Metek USA-1 and a Campbell CSAT3, at two locations in Denmark. The method relies on the estimation of the ratio of the vertical to the along-wind velocity power spectrum within the inertial subrange and does not require the use of another measurement as reference. When we correct the USA-1 to account for three-dimensional flow-distortion effects, as recommended by Metek GmbH, the ratio is very close to 4∕3 as expected from Kolmogorov's hypothesis, whereas non-corrected data show a ratio close to 1. For the CSAT3, non-corrected data show a ratio close to 1.1 for the two sites and for wind directions where the instrument is not directly affected by the mast. After applying a previously suggested flow-distortion correction, the ratio increases up to ≈1.2, implying that the effect of flow distortion in this instrument is still not properly accounted for.
Highlights
Accurate observations of atmospheric flow velocities, turbulence, and turbulence fluxes are critical for our understanding of all physical processes that occur in the atmospheric boundary layer and for the improvement of atmospheric modeling
For the velocity spectra at all three locations, we observe that the high-frequency w noise is the lowest of the three velocity components and is proportionally lower for the CSAT3 than for the USA1, which is consistent with its larger path elevation angle as explained theoretically in Appendix B
The accuracy of atmospheric turbulence measurements performed by sonic anemometers was investigated using two instruments, a CSAT3 and a USA-1, at two locations in Denmark
Summary
Accurate observations of atmospheric flow velocities, turbulence, and turbulence fluxes are critical for our understanding of all physical processes that occur in the atmospheric boundary layer and for the improvement of atmospheric modeling. Sonic anemometer measurements suffer from flow distortion due to the effects of both the structure(s) where the anemometer is mounted on, i.e., booms, clamps, and the bulk of the mast itself (e.g., Dyer, 1981; McCaffrey et al, 2017), and the anemometer itself. Statistical isotropy of the second order means that no second-order statistics change if the coordinate system is rotated in any way This would imply that the variances of the three velocity components would be identical and the covariances would be zero. The cross-spectrum between u and w, where w is the vertical velocity component, decreases like k1−7/3, which is more rapid than Fu and the bulk of the momentum flux u w , where the prime indicates fluctuations, is located at a wavenumber lower than the inertial subrange.
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