Wind Tunnel and Field Test of Three 2D Sonic Anemometers

The Royal Netherlands Meteorological Institute (KNMI) uses conventional cup anemometers and wind vanes to measure wind speed and direction. Although the KNMI cup and vane meet WMO and ICAO requirements concerning the accuracy of wind measurements, the sensors require a large amount of maintenance and occasionally some anemometer freeze during calm winter situations. Therefore, the use of alternative wind sensors is considered. Sonic anemometers, in this report referred to as sonics, have no moving part, which makes them robust and almost maintenance free. In addition, the sonics have virtually no detection limit and detect changes almost instantly, whereas cup and vane have a threshold speed and need some time to adjust to the prevailing conditions. Furthermore sonic anemometers can easily be equipped with a heater in order to prevent malfunctioning due to icing. Sonic anemometers have been available for several years. These sensors are generally used for scientific research particularly because the 3-D sonics also measure the small vertical wind component very accurately and with a high temporal resolution so that turbulence can be measured. Furthermore, sonics are used on wind turbines and on buildings - e.g. for operating sun blinds - since they require little maintenance. The first sonic sensors already measured wind speed and direction accurately, but the sensors did not perform well during precipitation. Using more advanced measurement techniques and especially improvements in the processing of the raw data makes the new sonic anemometers reliable under all conditions. Several tests of sonic anemometers in various environments have been performed (cf. e.g. Gregoire and Oualid, 1997; Gouveia and Lockhart, 1998; Wastrack et al, 2001; Gilhousen and Hervey, 2001; Tammelin et al, 2003; Borstnik and Knez, 2003; Lewis and Dover, 2004; Larre et al, 2005). The results of the sonic anemometers show that they can be considered for operational use. Therefore KNMI considered it a good moment to perform a test of sonic anemometers in order to find out whether they are suitable for use in the operational meteorological network. The test is restricted to commercially available 2D sonic anemometers since the more expensive 3D sensors are not required in the climatological, synoptical and aeronautical meteorological network.