Abstract
Abstract. Accurate measurements of turbulence statistics in the atmosphere are important for eddy-covariance measurements, wind energy research, and the validation of atmospheric numerical models. Sonic anemometers are widely used for these applications. However, these instruments are prone to probe-induced flow distortion effects, and the magnitude of the resulting errors has been debated due to the lack of an absolute reference instrument under field conditions. Here, we present the results of an intercomparison experiment between a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar, which is inherently free of any flow distortion. This novel remote sensing instrument has otherwise very similar spatial and temporal sampling characteristics to the sonic anemometer and hence served as a reference for this comparison. The presented measurements were carried out over flat homogeneous terrain at a measurement height of 30 m. We provide a comparative statistical analysis of the resulting mean wind velocities, the standard deviations of the vertical wind speed and the friction velocity and investigate the reasons for the observed deviations based on the turbulence spectra and co-spectra. Our results show an agreement of the mean wind velocity measurements and the standard deviations of the vertical wind speed with a comparability of 0.082 and 0.020 m s−1, respectively. Biases for these two quantities were 0.003 and 0.012 m s−1, respectively. Slightly larger differences were observed for friction velocity. Analysis of the corresponding co-spectra showed that the CSAT3B underestimates this quantity systematically by about 3 % on average as a result of co-spectral losses in the frequency range between 0.1 and 5 s−1. We also found that an angle-of-attack-dependent transducer-shadowing correction does not improve the agreement between the CSAT3B and the Physikalisch-Technische Bundesanstalt (PTB) lidar effectively.
Highlights
Accurate fast-response measurements of the threedimensional wind vector are of great importance to fundamental research in micrometeorology for flux measurements using the eddy-covariance methods in ecological studies (Aubinet et al, 2012)
Scatter plots and regression parameters for u, w w 0.5, and u∗ generally show a good agreement between the CSAT3B and the Physikalisch-Technische Bundesanstalt (PTB) lidar measurements (Fig. 5, Table 1)
We found that the H15 flow distortion correction improves the u∗ comparison with the PTB lidar considerably, but why is only u∗ improved and not u and w w 0.5? An analysis of the Couw co-spectra shows that the CSAT3B deviates from the expected −7/3 power law behaviour in the inertial subrange at frequencies f > 0.1 s−1 (Fig. 8)
Summary
Accurate fast-response measurements of the threedimensional wind vector are of great importance to fundamental research in micrometeorology for flux measurements using the eddy-covariance methods in ecological studies (Aubinet et al, 2012). In recent years, several studies found that most, if not all, sonic anemometers may be afflicted by a systematic underestimation of turbulent fluctuations due to probe-induced flow distortion errors (Frank et al, 2013, 2016; Wyngaard, 1988). These errors can be further classified into errors due to transducer selfshadowing caused by cross-shadowing and influences of the support structure. An intercomparison experiment between six different commercially available sonic anemometers showed that all participating instruments agreed very well (Mauder and Zeeman, 2018)
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