Regional Scale Surface CO2 Exchange Estimates Using a Boundary Layer Budget Method over the Southern Great Plains
| Williams, Ian | University of Chicago |
| Riley, William | Lawrence Berkeley National Laboratory |
| Berry, Joseph | Carnegie Inst.of Washington |
| Torn, Margaret | Lawrence Berkeley National Laboratory |
| Fischer, Marc | Lawrence Berkeley National Laboratory |
Category: Atmospheric State and Surface
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 rate 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 rate with large-scale subsidence velocities obtained from (1) NCEP reanalysis, (2) the Rapid Update Cycle (RUC) atmospheric model, (3) ECMWF reanalysis, and (4) 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 soundings taken above the tower. Fluxes were calculated from diurnally-averaged concentration data for 2003 and 2004, and averaged over each month. The model exhibited monthly trends in CO2 fluxes that matched trends in eddy covariance data, with the exception of October through December. From May through September, differences between model and observed fluxes were less than 1 µmol m-2 s-1. From October 2004 through April 2004, the model differed significantly from measured fluxes, by about 1 to 3 µmol m-2 s-1. The 2003 results showed similar differences in winter. Increasing the length of the lag mean decreased differences between model and observations, and brought different versions of the model into closer agreement with each other. Increasing the time average beyond 10 days did not significantly change results. The model is sensitive to changes in the entrainment parameterization, but these changes had little effect on the qualitative agreement between model results and eddy covariance measurements. We hypothesize that the differences between model and observations during winter may be a result of advection of CO2, nearby sources of CO2, or land surface heterogeneity, rather than parameter sensitivity in the model.
This poster will be displayed at the ARM Science Team Meeting.
