Atmos. Chem. Phys. Discuss., 11, 11455-11495, 2011
© Author(s) 2011. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Using boundary layer equilibrium to reduce uncertainties in transport models and CO2 flux inversions
I. N. Williams1, W. J. Riley2, M. S. Torn2, J. A. Berry3, and S. C. Biraud2
1Department of Geophysical Sciences, University of Chicago, Chicago, IL, USA
2Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, CA, USA
3Carnegie Institution of Washington, Stanford, CA, USA

Abstract. This paper reexamines evidence for previously hypothesized errors in atmospheric transport models and CO2 flux inversions by evaluating the diagnostics used to infer vertical mixing rates from observations. Several conventional mixing diagnostics are compared to analyzed mixing using data from the US Southern Great Plains Atmospheric Radiation Measurement Climate Research Facility, the CarbonTracker data assimilation system based on Transport Model version 5 (TM5), and atmospheric reanalyses. The results demonstrate that previous diagnostics based on boundary layer depth and vertical concentration gradients are unreliable indicators of vertical mixing. Vertical mixing rates are anti-correlated with boundary layer depth at some sites, diminishing in summer when the boundary layer is deepest. Vertical CO2 gradients between the boundary layer and free-troposphere are strongly affected by seasonal surface fluxes and therefore do not accurately reflect vertical mixing rates. The finite timescale over which vertical tracer gradients relax toward equilibrium is proposed as an improved mixing diagnostic, which can be applied to observations and model simulations of CO2 or other conserved boundary layer tracers with surface sources and sinks. This diagnostic does not require dynamical variables from the transport models, and is independent of possible systematic biases in prior- and post-inversion seasonal surface fluxes. Results indicate that observations frequently cited as evidence for systematic biases in atmospheric transport models are insufficient to prove that such biases exist. Some previously hypothesized transport model biases, if found and corrected, could cause inverse estimates to further diverge from land-based estimates.

Citation: Williams, I. N., Riley, W. J., Torn, M. S., Berry, J. A., and Biraud, S. C.: Using boundary layer equilibrium to reduce uncertainties in transport models and CO2 flux inversions, Atmos. Chem. Phys. Discuss., 11, 11455-11495, doi:10.5194/acpd-11-11455-2011, 2011.
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