|Title||Spectral characteristics and correction of long-term eddy-covariance measurements over two mixed hardwood forests in non-flat terrain|
|Publication Type||Journal Article|
|Year of Publication||2004|
|Authors||Su H-B, Schmid HPeter, Grimmond C.SB, Vogel CS, Oliphant AJ|
We present turbulence specrtra 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 ----. In stable conditions, a non-linear function 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 long-tubes for CO2 and water vapour fluxes were studied empirically. This was done by fitting the ratio between normalized cospectra of CO2 or water vapour fluxes and those of sensible heat to the transfer function of a first-order sensor. The characteristic time constant for CO2 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 CO2 fluxes are about 3-4% during the daytime and 6-10% at night on average. For the daytime latent heat flux, the attenuations are found to vary from less than 10% for newer tubes to over 20% for aged tubes. Corrections to Qe led to increases in the ratio (Qh +Qe)/Q* - Qg) by about 0.05 to 0.19 (Qh is sensible heat flux, Q* is net radiation and Qg is soil heat flux), and thus are expected to have an important impact on the assessment of energy balance closure.