Spectral Characteristics and Correction of Long-Term Eddy-Covariance Measurements Over Two Mixed Hardwood Forests in Non-Flat Terrain

Abstract

We present turbulence spectra and cospectra derived from long-term eddy-covariance measurements (nearly 40,000 hourly data over three to four years) and the transfer functions of closed-path infrared gas analyzers over two mixed hardwood forests in the mid-western U.S.A. The measurement heights ranged from 1.3 to 2.1 times the mean tree height, and peak vegetation area index (VAI) was 3.5 to 4.7; the topography at both sites deviates from ideal flat terrain. The analysis follows the approach of Kaimal et al. (Quart. J. Roy. Meteorol. Soc. 98, 563-589, 1972) whose results were based upon 15 hours of measurements at three heights in the Kansas experiment over flatter and smoother terrain. Both the spectral and cospectral constants and stability functions for normalizing and collapsing spectra and cospectra in the inertial subrange were found to be different from those of Kaimal et al. In unstable conditions, we found that an appropriate stability function for the non-dimensional dissipation of turbulent kinetic energy is of the form φ (ζ) = (1 − b − ζ) −1/4 − c − ζ , where ζ represents the non-dimensional stability parameter. In stable conditions, a non-linear function G xy (ζ) = 1 + b xy ζ c xy (c xy < 1) was found to be necessary to collapse cospectra in the inertial subrange. The empirical cospectral models of Kaimal et al. were modified to fit the somewhat more (neutral and unstable) or less (stable) sharply peaked scalar cospectra observed over forests using the appropriate cospectral constants and non-linear stability functions. The empirical coefficients in the stability functions and in the cospectral models vary with measurement height and seasonal changes in VAI. The seasonal differences are generally larger at the Morgan Monroe State Forest site (greater peak VAI) and closer to the canopy. The characteristics of transfer functions of the closed-path infrared gas analysers through longtubes for CO 2 and water vapour fluxes were studied empirically. This was done by fitting the ratio between normalized cospectra of CO 2 or water vapour fluxes and those of sensible heat to the transfer function of a first-order sensor. The characteristic time constant for CO 2 is much smaller than that for water vapour. The time constant for water vapour increases greatly with aging tubes. Three methods were used to estimate the flux attenuations and corrections; from June through August, the attenuations of CO 2 fluxes are about 3-4% during the daytime and 6-10% at night on average. For the daytime latent heat flux (Q E), the attenuations are found to vary from less than 10% for newer tubes to over 20% for aged tubes. Corrections to Q E led to increases in the ratio (Q H + Q E)/(Q * − Q G) by about 0.05 to 0.19 (Q H is sensible heat flux, Q * is net radiation and Q G is soil heat flux), and thus are expected to have an important impact on the assessment of energy balance closure.

Technical note: Water vapour concentration and flux measurements with PTR-MS

“…7 is 1.2 s. The transfer function found for our system is very similar to empirical transfer functions reported by Goulden et al (1996), Laubach and Teichmann (1996), Leuning and Judd (1996), and Su et al (2004) for water vapour flux measurements with closed path systems using a long inlet tube (materials: polyethylene or Teflon). They all applied a combined CO 2 /H 2 O IRGA instrument for the water vapour detection and observed the strong damping effect only for the H 2 O and not for the CO 2 flux.…”

“…7 for an exemplary day together with the total transfer function of all known damping effects. The observed empirical transfer function can be well described by a first order response function of the following form (Horst, 1997;Su et al, 2004):…”

Two years with extreme and little snowfall: effects on energy partitioning and surface energy exchange in a high-Arctic tundra ecosystem

“…Previous studies have shown large high-frequency flux attenuation of traditional closed-path gas analysers (Haslwanter et al, 2009), such as used in this study at the dry heath site, caused by tube walls and tube age (Su et al, 2004;Massman and Ibrom, 2008) and tube length (Novik et al, 2013). Enclosed-path systems, however, reduce flux attenuation compared to traditional closed-path systems due to shorter tube length (Burba et al, 2010;Novick et al, 2013).…”

Heat storage and anthropogenic heat flux in relation to the energy balance of a central European city centre

“…These uncertainties and errors have been discussed in detail elsewhere (e.g. Goulden et al, 1996;Moncrieff et al, 1996;Vickers and Mahrt, 1997;Wilson et al, 2002;Su et al, 2004). Here, issues of particular importance in the urban environment are highlighted.…”