HR: 0800h
AN: B51C-0225    [Abstracts]
TI: Regional-Scale Surface CO₂ Exchange Estimates Using a Boundary Layer Budget Method Over the Southern Great Plains
AU: * Williams, I N
EM: inw@uchicago.edu
AF: Lawrence Berkeley National Laboratory, Earth Sciences Division MS 90-1116 1 Cyclotron Road, Berkeley, CA 94720 United States
AU: Riley, W J
EM: wjriley@lbl.gov
AF: Lawrence Berkeley National Laboratory, Earth Sciences Division MS 90-1116 1 Cyclotron Road, Berkeley, CA 94720 United States
AU: Berry, J A
EM: joeberry@stanford.edu
AF: Carnegie Institution of Washington, Stanford University 260 Panama Street, Stanford, CA 94305-1297 United States
AU: Torn, M S
EM: mstorn@lbl.gov
AF: Lawrence Berkeley National Laboratory, Earth Sciences Division MS 90-1116 1 Cyclotron Road, Berkeley, CA 94720 United States
AU: Fischer, M L
EM: mlfischer@lbl.gov
AF: Lawrence Berkeley National Laboratory, Earth Sciences Division MS 90-1116 1 Cyclotron Road, Berkeley, CA 94720 United States
AB: Concentration gradients of CO₂ and H₂O at the transition between the atmospheric boundary layer and free troposphere are linked to land surface exchanges at the regional scale. We used atmospheric concentration measurements and a boundary-layer budget model to estimate regional-scale CO₂ fluxes, address uncertainties in the model, and suggest approaches to improve model accuracy. The budget model CO₂ fluxes were compared against eddy covariance measurements from a 60 m tower at the Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) near Lamont, Oklahoma. The model assumes a slowly evolving boundary layer in which CO₂ concentrations are quasi-steady on diurnal and longer temporal scales. We used the mass conservation equation to equate CO₂ fluxes across the boundary layer top to surface fluxes. Fluxes across the boundary layer top were calculated as the product of the entrainment velocity and the CO₂ concentration gradient between the free atmosphere and the well-mixed boundary layer. We neglected advective and local changes in the diurnally-averaged boundary layer height, and equated entrainment velocity with large-scale subsidence velocities obtained from (1) NCEP reanalysis, (2) the Rapid Update Cycle (RUC) atmospheric model, and (3) a water vapor tracer method. CO₂ concentration measurements were made on the 60 m tower and on aircraft at 3700 m, and water vapor data was obtained from meteorological balloon soundings taken above the tower. Fluxes were calculated from diurnally-averaged concentration data from January through November 2004, and averaged over each month. Model flux estimates exhibited monthly trends in CO₂ fluxes that matched trends in eddy covariance data. However, during peak surface CO₂ exchange in April, model estimates differed from monthly-averaged eddy covariance measurements by 2.3 and 3.2 μmol m^-2 s^-1 using the water vapor derived vertical velocities and the RUC vertical velocities, respectively. During a less active vegetation period in August, model estimates differed by 1.7 to 3.0 μmol m^-2 s^-1. Preliminary results indicate that neglect of advection and cloud venting may be significant sources of error in the approach at the ARM site. We also attribute the differences between concentration-based model fluxes and eddy covariance fluxes to heterogeneity in the land surface vegetation cover.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0428 Carbon cycling (4806)
DE: 0490 Trace gases
DE: 1610 Atmosphere (0315, 0325)
DE: 1631 Land/atmosphere interactions (1218, 1843, 3322)
SC: Biogeosciences [B]
MN: Fall Meeting 2005