Using sonic anemometer temperature to measure sensible heat flux in strong winds

Abstract

Abstract. Sonic anemometers simultaneously measure the turbulent fluctuations of vertical wind (w ) and sonic temperature (T s ), and are commonly used to measure sensible heat flux (H ). Our study examines 30-min heat fluxes measured with a Campbell Scientific CSAT3 sonic anemometer above a subalpine forest. We compared H calculated with T s to H calculated with a co-located thermocouple and found that, for horizontal wind speed (U ) less than 8 m s −1 , the agreement was around ± 30 W m −2 . However, for U >≈ 8 m s −1 , the CSAT H had a generally positive deviation from H calculated with the thermocouple, reaching a maximum difference of ≈ 250 W m −2 at U ≈ 18 m s −1 . With version 4 of the CSAT firmware, we found significant underestimation of the speed of sound and thus T s in high winds (due to a delayed detection of the sonic pulse), which resulted in the large CSAT heat flux errors. Although this T s error is qualitatively similar to the well-known fundamental correction for the crosswind component, it is quantitatively different and directly related to the firmware estimation of the pulse arrival time. For a CSAT running version 3 of the firmware, there does not appear to be a significant underestimation of T s ; however, a T s error similar to that of version 4 may occur if the CSAT is sufficiently out of calibration. An empirical correction to the CSAT heat flux that is consistent with our conceptual understanding of the T s error is presented. Within a broader context, the surface energy balance is used to evaluate the heat flux measurements, and the usefulness of sideby-side instrument comparisons is discussed.

Analysis of the PKT correction for direct CO<sub>2</sub> flux measurements over the ocean

“…The latent heat flux Hl was calculated from the covariance of the vertical wind speed w with the water vapour mixing ratio x v from wetA and wetB after correction for the time delay. H sonic was corrected with the latent heat flux to derive the sensible heat flux H s , following Burns et al (2012). The measured CO 2 density was converted to a mixing ratio (as described in Sect.…”

Effects of precipitation on sonic anemometer measurements of turbulent fluxes in the atmospheric surface layer

“…By employing the simultaneous observation from a sonic anemometer, a Lyman-α hygrometer and a fast response thermocouple, Schotanus et al (1983) assessed the errors of the sonic anemometer for measuring temperature variance and turbulent flux and proposed expressions for correcting them. Recently, Burns et al (2012) showed that the error of sensible heat flux calculated from measurements of sonic anemometer tends to be enlarged with an increasing wind velocity, which could be connected with the shake of sensors caused by strong wind. Nakai and Shimoyama (2012) made a detailed analysis for the errors caused by the angle of attack, which tended to result in the increasement of eddy flux.…”

Field intercomparison of prevailing sonic anemometers

“…The differences due to different firmware versions are quite well documented for the CSAT3. According to Burns et al (2012), discrepancies between firmware versions 3 and 4 occur mostly for the sonic temperature measurement and they become significant for wind speeds larger than 8 m s −1 . During our field campaign, wind speeds were mostly lower than 5 m s −1 (Fig.…”