Since the early 1960s, sonic anemometer thermometers (typically referred to as sonic anemometers or sonics) have been employed by atmospheric scientists to measure three-dimensional wind vectors, temperature, and surface-sensible heat and momentum fluxes quickly and accurately (Kaimal and Businger 1963; Kaimal et al. 1968). Sonic anemometers can measure the wind speed between 0–60 m s-1, at rates up to 100 hertz (Hz), allowing turbulent structures on scales of a few centimeters to be resolved. One popular model, the Campbell Scientific CSAT3 sonic anemometer, is pictured in Figure 3. Sonic anemometers operate by measuring the time required for a pulse of sound to travel between a pair of transducers. This time depends on the distance between the transducers, the speed of sound, and the air speed along the axis of the transducers. In turn, the speed of sound depends on air temperature, moisture, and pressure along the path. A sonic anemometer thermometer sequentially transmits and receives a pulse of sound so that air speed and direction may also be inferred. The temperature measured by a sonic anemometer is not exactly the air temperature, but is a function of both temperature and moisture and is not strictly identical to the virtual temperature (Burns et al. 2012). However, the fluctuating component of the sonic temperature may be considered equivalent to the fluctuating component of the air temperature, and so the high-rate measurements of sonic temperature may be used in Reynolds decomposition to measure a surface heat flux.
Figure 3. A three-dimensional sonic anemometer. Image from Campbell Scientific (2018b) Atmospheric field campaigns have deployed sonic anemometers at several altitudes within the surface layer and boundary layer. Measurements have been as low as 0.5 m (Poulos et al. 2002), but the lowest measurement is typically between 2 and 10 m to observe the development of eddies within the surface layer (Oncley et al. 1996). Comprehensive guidance for the use of sonic anemometers to estimate turbulence fluxes using the eddy covariance method, profile method, flux-variance method, and accumulation methods is provided by Foken (2008). An intercomparison of sonic anemometer approaches was presented by Loescher et al. (2005).
Sonic anemometers (see Section 18.104.22.168) use the change of frequency of a beam of sound along a short (20 cm or less) path to determine the wind speed along the path. There are a range of sonic anemometer designs that measure in one, two, or three dimensions. Sonic anemometers have high sampling rates compared to cup or propeller anemometers. Together with the small measurement volume, they have a higher frequency response than other anemometers. Sonic anemometers have a TRL >8.
Sonic Anemometer is generally considered to have a TRL of at least 8.