@article {70352,
title = {An assessment of observed vertical flux divergence in long-term eddy-covariance measurements over two Midwestern forest ecosystems},
journal = {Agricultural and Forest Meteorology},
volume = {148},
year = {2008},
month = {02/2008},
pages = {186 - 205},
abstract = {Vertical divergence of CO2 fluxes is observed over two Midwestern AmeriFlux forest sites. The differences in ensemble averaged hourly CO2 fluxes measured at two heights above canopy are relatively small (0.2{\textendash}0.5 μmol m-2 s-1), but they are the major contributors to differences (76{\textendash}256 g C m-2 or 41.8{\textendash}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 CO2 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{\textdegree} lead to 1\% and 5.6\% differences in momentum fluxes and nighttime sensible heat and CO2 fluxes, respectively, but only 0.5\% and 2.8\% differences in daytime sensible heat and CO2 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{\textendash}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 CO2 are more positive at the higher levels of both sites in the frequency range of 0.03{\textendash}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 CO2 flux differences throughout the day at both sites. At night, the vertical CO2 flux differences between two measurement heights are 20{\textendash}30\% and 40{\textendash}60\% of co-spectral corrected CO2 fluxes measured at the lower levels of the two sites, respectively. Vertical differences of CO2 flux are relatively small in the daytime. Vertical differences in estimated mean vertical advection of CO2 between the two measurement heights generally do not improve the closure of the 1D (vertical) CO2 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.},
keywords = {NET ECOSYSTEM EXCHANGE},
doi = {10.1016/j.agrformet.2007.08.009},
author = {Su, H and Schmid, Hans Peter and Grimmond, C. S. B. and Vogel, Christoph S. and Curtis, Peter S.}
}
@article {70346,
title = {Effects of canopy morphology and thermal stability on mean flow and turbulence statistics observed inside a mixed hardwood forest},
journal = {Agricultural and Forest Meteorology},
volume = {148},
year = {2008},
month = {06/2008},
pages = {862 - 882},
abstract = {The influences of thermal stability and seasonal changes in canopy morphology on mean flow and turbulence statistics in a mixed hardwood forest are presented from a year long field experiment at the University of Michigan Biological Station AmeriFlux site. A secondary wind speed maximum at z/h = 0.07 (z is height above ground and h is mean canopy height) below the level of peak vegetation area density (VAD) in the understory (young white pines) is observed more frequently and is more pronounced in fully leafed (closed) canopy than defoliated (open) canopy, and in stable than near-neutral and unstable conditions. A secondary wind speed maximum at z/h = 0.58 is observed only in the closed canopy below the level of peak VAD in the upper canopy (crowns of mature aspen trees), which occurs less frequently and is less pronounced than that at z/h = 0.07. Horizontal mean winds in the forest are observed to flow to the left (counter-clockwise) of that at the canopy top. The degrees of turning of the mean winds increase with increasing depth into the forest except a reversal (clockwise) near the forest floor in the closed canopy. The degrees of turning are greater in the closed canopy than the open canopy but smaller in near-neutral than unstable and stable conditions. The attenuations of Reynolds stress, correlation coefficient and velocity variances with increasing depth into the forest are more rapid in the closed canopy and in stable conditions. But the relative turbulence intensities are greater in the closed canopy than in the open canopy and decrease with increasing stability. In near-neutral stability, the zero-plane displacement height (d) for the closed canopy decreases with increasing wind speed (not, vert, similar0.81h on average), while d for the open canopy does not show a clear dependence on wind speed (not, vert, similar0.65h on average). The bulk drag coefficient (View the MathML source) measured at the canopy top is much greater over the closed canopy than the open canopy, contrary to earlier observations over a deciduous forest. But View the MathML source (VAI is vegetation area index) is about the same over the closed and open canopies (not, vert, similar0.03 in near-neutral stability). The drag coefficient (Cd) for the parameterization of drag force in mean momentum budget equations in closure models increases with decreasing wind speed and varies with height. The drag coefficient (View the MathML source) for the parameterization of drag force in prognostic momentum equations in large-eddy simulations of airflow in plant canopies is smaller than Cd, and the ratio View the MathML source is greater in the open canopy than closed canopy and in stable than near-neutral and unstable conditions due to smaller relative turbulence intensities. All drag coefficients decrease and the displacement height increases with increasing stability, which indicates that these estimated aerodynamic parameters are not entirely the properties of vegetation elements, but are influenced by vertical turbulent mixing of momentum. Both eddy-diffusivity and mixing-lengths for momentum transfer decrease with increasing stability. An evidence of non-local transport is shown by peak values in estimated eddy-diffusivity and mixing-lengths below the crowns of mature aspen trees in the closed canopy. Otherwise, the eddy-diffusivity decreases with increasing depth into the forest, while the mixing-lengths above the level of the peaks are greater in the open canopy and the opposite is true below the level of the peaks.},
keywords = {WINDS},
doi = {10.1016/j.agrformet.2007.12.002},
author = {Su, H and Schmid, Hans Peter and Vogel, Christoph S. and Curtis, Peter S.}
}