![]() This assessment is based, however, on the surface layer turbulence models derived from the Kansas meteorological experiment (Kaimal et al., 1990). They suggest that, for typical sonic anemometers measuring at ≥20 Hz high-frequency losses in turbulence spectra may become significant when deployed too close to the ground. (2004), however, reviewed limitations to the resultant turbulence power spectra as a function of the height of the instrument above the surface, the path length between the transducers, and the average wind speeds involved, to establish minimum measurement run durations for restricting low-frequency losses and minimum sampling frequencies for restricting high-frequency losses. Physical limitations on positioning of the instrument are few, apart from the size of the transducer framework, and they have been deployed close to the ground and just above marram grass vegetation on coastal foredunes. (2011) used a total of 24 Gill-HS sonic anemometers distributed over a sequence of 16 m tall masts to explore the role of off-shore winds on sand transport on the beach, also enabling a detailed comparison with computational fluid dynamics (CFD) modeling. In most experiments the deployment has been restricted to a small number of sonics (and commonly a single one), although recently a comprehensive study of wind over coastal foredunes by Jackson et al. In the recent decade sonic anemometers have been deployed in a number of aeolian field investigations on beaches (most recently: Walker et al., 2009) as well as in desert and semi-arid environments ( Wiggs et al., 1996 Leenders et al., 2005 Weaver and Wiggs, 2010) in a variety of configurations. Furthermore, the sensor does not contain any moving parts and is therefore not prone to fouling by airborne dust or sediment. ![]() The precision and measuring frequency make these anemometers particularly suitable for investigating many scales of turbulence and for determining shear stresses from Reynolds’ decomposition (see eqns – in Section 14.17.4). Entry models start at 4 to 10 Hz measurements, whereas research grade sensors like the Gill Instruments HS-range can measure at 50 or even 100 Hz, with a speed resolution on the order of 0.01 m s −1 and a directional resolution of less than 1°. Sonic anemometers are commercially available over a range of design specifications, principally as a function of measuring frequency and resolution. The advantage of the latter design is that the 3D wind vector is obtained from a single, relatively small, sampling volume. ![]() Second, the pairs can be arranged so that they measure along paths crossing through a single sampling volume, usually optimized for a specific orientation to the wind so that the transducer clusters are positioned above and below the horizontal plane of the flow. ![]() An example is the ‘K’ probe manufactured by Applied Technologies Inc., which is based on a design by Kaimal to minimize potential flow distortions of the mounting frame ( Kaimal et al., 1990). First, the pairs can be mounted independently, measuring along individual path lengths, in a mutually orthogonal framework. Two basic arrangements of transducer pairs are in use. Three pairs of emitters and receivers are combined in a 3D anemometer to resolve the complete wind vector. As air density is principally controlled by air temperature the sonic measurement of C therefore also yields an estimation of the temperature in conjunction with wind component speed, which can be useful for atmospheric stability assessment. The measurement of transit times across both directions of the path length allows the explicit determination of the speed of sound, which is dependent on the density of the air. (2004), and Walker (2005), and is based on the transit times, t 1 and t 2, in each direction between the transducers of the ultrasonic sound signal over the path length, L, where the sonic speed in still air, C, is offset by the speed of the airflow component, V. ![]() The operating principle has been described in detail by Kaimal and Finnigan (1994), van Boxel et al. The basic element of a sonic anemometer consists of an ultrasonic sound emitter and receiver at opposite ends of a sampling volume. Sonic anemometers work on the principle that the travel time of sound waves through air is affected by the wind speed component parallel with the direction of travel. Baas, in Treatise on Geomorphology, 2013 14.17.3.1.2 Sonic anemometers ![]()
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