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

Sonic anemometers simultaneously measure the turbulent fluctuations of vertical wind ( w ') and sonic temperature (T '), 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 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 in high winds (due to a delayed detection of the sonic pulse), which resulted in the large CSAT heat flux errors. Although this T 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 ; however, a T 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 error is presented. Within a broader context, the surface energy balance is used to evaluate the heat flux measurements, and the usefulness of side-by-side instrument comparisons is discussed.