HR: 13:40h
AN: B43C-01 INVITED     [Abstracts]
TI: A Method for Measuring Subcanopy CO2 Advection
AU: * Staebler, R M
EM: ralf.staebler@ec.gc.ca
AF: Meteorological Service of Canada, 4905 Dufferin St., Toronto, ON M3H 5T4 Canada
AU: Fitzjarrald, D R
EM: fitz@asrc.cestm.albany.edu
AF: Atmospheric Sciences Research Center, 252 Fuller Road, Albany, NY 12203 United States
AB: Underestimation of nocturnal CO2 respiration under calm conditions remains an unsolved problem at many forest flux stations, and several groups are currently investigating the direct measurement of horizontal advection of CO2. This presentation will describe a systematic, relatively low-cost methodology developed to determine whether horizontal mean transport of CO2 accounts for the missing CO2 at the Harvard Forest (Petersham, MA). This methodology includes the characterization of subcanopy motions, determining the appropriate size of the subcanopy network required to make the measurements, developing a method of integrating the measurements in the vertical, and determining the required averaging time. Measurements were conducted over 4 years and produced data for 310 nights covering all seasons. Subcanopy flows were decoupled from the flows aloft 75% of the time. Conditions conducive to the generation of negative buoyancy near the forest floor, necessary for drainage flows to develop, were given in 92% of all nights. The occurrence of nocturnal drainage flows correlated well with "missing flux" problems ("deficit nights"), prompting us to propose an improvement on the commonly used friction velocity criterion (which requires u* to be larger than some empirical cut-off for the eddy fluxes to be considered credible). The "negative buoyancy forcing fraction", i.e. negative buoyancy as a fraction of the sum of the dynamic driving forces, can be shown to predict deficit nights significantly better than the u* cut-off. The appropriate horizontal size of the network of wind and CO2 sensors at the Harvard Forest was shown to be on the order of 100 m, ensuring that sensors were generally observing coherent processes on this scale or larger and thus displaying some correlation. Horizontal transport of CO2 was found to be restricted to the bottom ~10 m of the forest, facilitating the development of a method of integrating the horizontal CO2 gradients in the vertical. Including the measured horizontal transport terms did not, on average, fully account for the observed difference in NEE of 1.2 +/- 0.3 umoles/m2/s between deficit and non-deficit nights, but decreased the difference to 0.7 +/- 0.5 umoles/m2/s.
DE: 1694 Instruments and techniques
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0394 Instruments and techniques
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting