Intercomparison of ultrasonic anemometers during the MAP Riviera project

As part of the MAP-Riviera Project (Mesoscale Alpine Project) a field intercomparison of ultrasonic anemometers (sonics) was carried out at the San Vittore Airfield in the Mesolcina Valley in Southern Switzerland. The motivation for this study was to find out how these sonics compare under ideal field conditions, and use that information for the interpretation of the measurements obtained afterwards under complex conditions at the slopes. From July 12 to 16, 1999, we compared altogether 19 sonics including 4 different designs: 14 Gill Solent R2, 1 Gill Solent HS, 3 Campbell CSAT-3 and 1 METEK USA-1. All sonics were mounted at a height of 1.8m in two arrays which were 10 m apart. The spacing between single instruments was 1m and they were exposed towards the valley wind. Fetch in that direction consisted of homogeneous grassland which extended up to 800m. Under the influence of a large valley wind system the wind speeds from the favorable sector were up to 6 m/s. Raw data (20Hz) were collected continuously using a synchronous digital data acquisition system. Comparability of means and turbulent fluxes, spectral characteristics and acoustic temperature measurements is shown. Effects of different correction and calibration methods are illustrated and discussed. Standard manufacturer calibration is compared to a matrix calibration obtained from sensor-specific wind tunnel measurements, which were carried out with 8 sonics just before the field experiment.