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

Abstract

The sensible heat flux (<i>H</i>) is a significant component of the surface energy balance (SEB). Sonic anemometers simultaneously measure the turbulent fluctuations of vertical wind (<i>w</i>') and sonic temperature (<i>T</i><sub>s</sub>'), and are commonly used to measure <i>H</i>. Our study examines 30-min heat fluxes measured with a Campbell Scientific model CSAT3 sonic anemometer above a subalpine forest. We compare <i>H</i> calculated with <i>T</i><sub>s</sub> to <i>H</i> calculated with a co-located thermocouple and find that for horizontal wind speed (<i>U</i>) less than 8 m s<sup>−1</sup> the agreement is ≈±30 W m<sup>−2</sup>. However, for <i>U</i> >≈ 8 m s<sup>−1</sup>, the CSAT3 <i>H</i> becomes larger than <i>H</i> calculated with the thermocouple, reaching a maximum difference of ≈250 W m<sup>−2</sup> at <i>U</i> ≈ 18 m s<sup>−1</sup>. <i>H</i> calculated with the thermocouple results in a SEB that is relatively independent of <i>U</i> at high wind speeds. In contrast, the SEB calculated with <i>H</i> from the CSAT3 varies considerably with <i>U</i>, particularly at night. Cospectral analysis of <span style="border-top: 1px solid #000;color:#000;"><i>w</i>'<i>T</i><sub>s</sub>'</span> suggest that spurious correlation is a problem during high winds which leads to a positive (additive) increase in <i>H</i> calculated with the CSAT3. At night, when <i>H</i> is typically negative, this CSAT3 error results in a measured <i>H</i> that falsely approaches zero or even becomes positive. Within a broader context, the usefulness of side-by-side instrument comparisons are discussed

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

“…In our discussions, the simple SEB closure fraction will be designated as CF (e.g., Barr et al, 2006) and refers to the ratio of the sum of the turbulent fluxes to (R net − G), i.e., CF ≡ (H + LE)/(R net − G). Similarly to Turnipseed et al (2002), we find nocturnal CF with R net from the Q*7.1 sensor is about 15 % closer to closing the SEB than with the CNR1 sensor (see Burns et al, 2012 for details). For simplicity, only results with the Q*7.1 R net sensor are presented here.…”

“…For f > 1 Hz, the f S T s noise appears to follow the f +1 slope that is typical of white noise, and indicative of the true temperature signal dropping below the sensor noise threshold (Kaimal and Gaynor, 1991;Kaimal and Finnigan, 1994). Low-pass filtering T s to remove this noise did not significantly change H CSAT (Burns et al, 2012). The temperature spectra from the thermocouples are attenuated at frequencies above ≈ 1 Hz, because the thermal mass of the thermocouple wire limits the response time.…”

“…The number of spikes was also affected by the relative humidity. Further details of the despiking can be found in the on-line discussion of Burns et al (2012). Any CSAT data sample deemed questionable by the CSAT diagnostic flag was replaced with a linear fit between valid samples.…”

Empirical assessment of uncertainties of meteorological parameters and turbulent fluxes in the AmeriFlux network

“…Offsets between T a and T s do not affect the estimation of buoyancy fluxes (as long as T a and T s are linearly related) but application of T s to other calculations (e.g., air density, WPL terms) can introduce additional error [ Loescher et al , 2005]. While systematic errors are rare, Burns et al [2012] recently reported discrepancies between sonic temperature and a collocated thermocouple for high wind speeds (>8 m s −1 ) for one specific anemometer. Another possible source for discrepancies in T s comparisons is in the application of crosswind corrections [ Liu et al , 2001] as some manufacturers apply these corrections internally while others leave this to the user.…”

Evaluation of Turbulence Stability Schemes of Land Models for Stable Conditions

“…In particular, differences in model representations of the stability feedback explain much of the large range of simulated surface temperatures in uncoupled modeling studies (Lapo et al, ; Slater et al, ). Moreover, turbulent fluxes during stable conditions affect the timing of snow melt (Burns et al, ; Mott et al, ; Pohl et al, ), sublimation and mass loss (MacKay et al, ; Reba et al, ), and substantial flooding through snow melt during rain‐on‐snow events (Garvelmann et al, ; Marks et al, ; Wayand et al, ). Errors in the sensible heat flux during stable conditions also play an important role in the evolution of the boundary layer in atmospheric models, in particular, leading to large errors in 2‐m air temperatures (Atlaskin & Vihma, ) and in the simulated diurnal temperature of the atmosphere (Holtslag et al, ; Jin & Wen, ; Wei et al, ).…”