Relevance of a Tuning-Fork Effect for Temperature Measurements with the Gill Solent HS Ultrasonic Anemometer–Thermometer

Abstract

The performance of a new type of sonic anemometer?thermometer (called a sonic), the Solent-Research HS, manufactured by Gill Instruments, Lymington, United Kingdom, was investigated. Measurements of the three wind-velocity components u, ?, w, and temperature T were taken in the laboratory under quiet conditions and in the field at wind speeds of about 10 m s?1. The power spectra of u, ?, w, and T measured in the laboratory follow a ?5/3 power law at moderate frequencies. At frequencies higher than u/l (here u is the mean wind speed along a given path of length l), there is a roll-off in all spectra, an expected effect caused by the spatial averaging along the finite pathlength. Over the bandwidth of fs/2 = 50 Hz, the standard deviations due to uncorrelated noise amount to 0.02 m s?1 for u, ?, and w and to 0.02 K for T. In the field, the spectra of u, ?, and w show a clean ?5/3 power law, except for a flattening at frequencies larger than 30 Hz. The ratio of the spectra of the transverse and longitudinal velocity components was close to 4/3, the ratio predicted by classical theory for isotropic turbulence. The T spectra measured in the field were severely contaminated at frequencies larger than about 5 Hz. Closer inspection of the T time series revealed amplitude-modulated artifacts. These artifacts were presumed to be the result of oscillations of the sonic's pathlengths induced by oscillations of the tower, which was exposed to a turbulently changing wind. The artifacts were reproduced in the laboratory by controlled blows on the sonic's attachment. The mechanical oscillations, which the authors refer to as the tuning-fork effect, were measured with a strain gauge attached to the sonic. The tuning-fork effect was observed simultaneously and independently in the strain-gauge measurements and as artifacts in the temperature time series.