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Comparison of turbulence measurements by a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar

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.