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

[摘要] Sonic anemometers simultaneously measure the turbulent fluctuations ofvertical wind (w') and sonic temperature (T_s'), and arecommonly used to measure sensible heat flux (H). Our study examines 30-minheat fluxes measured with a Campbell Scientific CSAT3 sonic anemometer abovea subalpine forest. We compared H calculated with T_s to Hcalculated with a co-located thermocouple and found that, for horizontal windspeed (U) less than 8 m s⁻¹, the agreement was around±30 W m⁻². However, for U ≈ 8 m s⁻¹, theCSAT H had a generally positive deviation from H calculated with thethermocouple, reaching a maximum difference of ≈250 W m⁻² atU ≈ 18 m s⁻¹. With version 4 of the CSAT firmware, wefound significant underestimation of the speed of sound and thusT_s in high winds (due to a delayed detection of the sonic pulse),which resulted in the large CSAT heat flux errors. Although thisT_s error is qualitatively similar to the well-known fundamentalcorrection for the crosswind component, it is quantitatively different anddirectly related to the firmware estimation of the pulse arrival time. For aCSAT running version 3 of the firmware, there does not appear to be asignificant underestimation of T_s; however, a T_serror similar to that of version 4 may occur if the CSAT is sufficiently outof calibration. An empirical correction to the CSAT heat flux that isconsistent with our conceptual understanding of the T_s error ispresented. Within a broader context, the surface energy balance is used toevaluate the heat flux measurements, and the usefulness of side-by-sideinstrument comparisons is discussed.

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