Comparison of sonic anemometer performance under foggy conditions

A sonic anemometer comparison was performed at a mountain cloud forest site in Taiwan to evaluate the effect of fog on sonic anemometers performance, with particular emphasis to their employment in eddy-covariance applications. Four sonic anemometers (Campbell CSAT3, Gill R3-50, METEK USA-1, and R.M. Young 81000VRE) were tested for 15 consecutive days with an overall fog duration of 86 h.Four aspects were analyzed: (1) spike statistics during foggy and non-foggy conditions, (2) spectral and co-spectral analyses before, during, and after 16 fog events, (3) correlations between turbulence characteristics of wind and temperature, and (4) flux error estimations.All sonic anemometers produce more spikes when the visibility is below 1000 m, compared to conditions with visibilities above 1000 m. However, the overall number of spikes caused by fog is generally low and therefore of no concern for any of the tested sonic anemometers.Spectral analyses showed that for most anemometers fog mostly affects spectra of the sonic temperature. Here, the high frequency range is either damped or amplified. These effects worsen with increasing duration and density of fog. In case of the 81000VRE and the USA-1, all three wind component spectra, sonic temperature spectra as well as the co-spectra of w’T’ and w’u’ show white noise in the high frequency range during dense fog. The CSAT3 shows noise only in the high frequency range of the sonic temperature and the co-spectra of w’T’. Smallest sensitivity to fog was observed for the R3-50. Nevertheless, differences in resulting fluxes are usually smaller than the flux error of the measurements. Due to the results of the spectral analysis and small flux errors the R3-50 seems to be suited best for eddy covariance measurements and process studies under dense foggy conditions.

► The four tested sonic anemometers show different sensitivity to fog.
► Fog duration and characteristics play a key role for the measurements quality.
► Sonic temperature measurements are more sensitive to fog than wind measurements.
► Sites with frequent, dense fog should use the R3-50 (Gill) or the CSAT3 (Campbell).
► Differences between fluxes of individual SATs are smaller than measurement errors of fluxes.