Performance of a three-dimensional ultrasonic anemometer/thermometer for turbulence measurements

The performance of a three-dimensional ultrasonic anemometer/thermometer (Solent HS from Gill Instruments Ltd.) has been investigated in laboratory and field experiments. The device has a sampling frequency of 100 Hz. Noise-floor measurements in the laboratory under no-wind conditions resulted in uncorrelated-noise standard deviations of 40 mK and 2 cm/s for temperature and velocity, respectively. A second experiment was performed in the field at wind speeds of about 10 m/s and at altitudes of 2.8 m and 5.5 m above ground level. While the spectra of all three velocity components show a clean inertial range up to the Nyquist frequency, the temperature spectrum flattens out between 1 and 10 Hz. This temperature noise floor is much higher than the one obtained under quiet conditions in the laboratory. (Unfortunately, temperatures are not available for the three measurement paths separately, which makes an error analysis of the temperature measurements difficult.) Ratios of lateral versus streamwise velocity spectra (classical turbulence theory predicts 4/3) are presented. Co-spectra of temperature and vertical velocity are presented and compared with standard models. Detectability thresholds for energy dissipation rates, temperature structure parameters, and heat fluxes are given.