An assessment of observed vertical flux divergence in long-term eddy-covariance measurements over two Midwestern forest ecosystems

Abstract

Vertical divergence of CO 2 fluxes is observed over two Midwestern AmeriFlux forest sites. The differences in ensemble averaged hourly CO 2 fluxes measured at two heights above canopy are relatively small (0.2–0.5 μmol m −2 s −1), but they are the major contributors to differences (76–256 g C m −2 or 41.8–50.6%) in estimated annual net ecosystem exchange (NEE) in 2001. A friction velocity criterion is used in these estimates but mean flow advection is not accounted for. This study examines the effects of coordinate rotation, averaging time period, sampling frequency and co-spectral correction on CO 2 fluxes measured at a single height, and on vertical flux differences measured between two heights. Both the offset in measured vertical velocity and the downflow/upflow caused by supporting tower structures in upwind directions lead to systematic over- or under-estimates of fluxes measured at a single height. An offset of 1 cm s −1 and an upflow/downflow of 1° lead to 1% and 5.6% differences in momentum fluxes and nighttime sensible heat and CO 2 fluxes, respectively, but only 0.5% and 2.8% differences in daytime sensible heat and CO 2 fluxes. The sign and magnitude of both offset and upflow/downflow angle vary between sonic anemometers at two measurement heights. This introduces a systematic and large bias in vertical flux differences if these effects are not corrected in the coordinate rotation. A 1 h averaging time period is shown to be appropriate for the two sites. In the daytime, the absolute magnitudes of co-spectra decrease with height in the natural frequencies of 0.02–0.1 Hz but increase in the lower frequencies (<0.01 Hz). Thus, air motions in these two frequency ranges counteract each other in determining vertical flux differences, whose magnitude and sign vary with averaging time period. At night, co-spectral densities of CO 2 are more positive at the higher levels of both sites in the frequency range of 0.03–0.4 Hz and this vertical increase is also shown at most frequencies lower than 0.03 Hz. Differences in co-spectral corrections at the two heights lead to a positive shift in vertical CO 2 flux differences throughout the day at both sites. At night, the vertical CO 2 flux differences between two measurement heights are 20–30% and 40–60% of co-spectral corrected CO 2 fluxes measured at the lower levels of the two sites, respectively. Vertical differences of CO 2 flux are relatively small in the daytime. Vertical differences in estimated mean vertical advection of CO 2 between the two measurement heights generally do not improve the closure of the 1D (vertical) CO 2 budget in the air layer between the two measurement heights. This may imply the significance of horizontal advection. However, a reliable assessment of mean advection contributions in annual NEE estimate at these two AmeriFlux sites is currently an unsolved problem.