Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 6 2008 10.5194/acpd-8-19917-2008 http://www.atmos-chem-phys-discuss.net/8/19917/2008/ http://www.atmos-chem-phys-discuss.net/8/19917/2008/acpd-8-19917-2008.html http://www.atmos-chem-phys-discuss.net/8/19917/2008/acpd-8-19917-2008.pdf 19917 19955 2008-11-28 Estimating surface CO₂ fluxes from space-borne CO₂ dry air mole fraction observations using an ensemble Kalman Filter L. Feng lfeng@staffmail.ed.ac.uk P. I. Palmer H. Bösch S. Dance School of Geosciences, University of Edinburgh, King's Buildings, Edinburgh EH9 3JN, UK Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK Department of Mathematics and Department of Meteorology, University of Reading, Reading RG6 6BB, UK We have developed an ensemble Kalman Filter (EnKF) to estimate 8-day regional surface fluxes of CO₂ from space-borne CO₂ dry-air mole fraction observations (X<sub>CO_2</sub) and evaluate the approach using a series of synthetic experiments, in preparation for data from the NASA Orbiting Carbon Observatory (OCO). The 32-day duty cycle of OCO alternates every 16 days between nadir and glint measurements of backscattered solar radiation at short-wave infrared wavelengths. The EnKF uses an ensemble of states to represent the prior error covariance to estimate 8-day CO₂ surface fluxes over 144 geographical regions. We use a 12&times;8-day lag window, recognising that X<sub>CO_2</sub measurements include surface flux information from prior time windows. The observation operator that relates surface CO₂ fluxes to atmospheric distributions of X<sub>CO_2</sub includes: a) the GEOS-Chem transport model that relates surface fluxes to global 3-D distributions of CO₂ concentrations, which are sampled at the time and location of OCO measurements that are cloud-free and have aerosol optical depths <0.3; and b) scene-dependent averaging kernels that relate the CO₂ profiles to X<sub>CO_2</sub, accounting for differences between nadir and glint measurements, and the associated scene-dependent observation errors. We show that OCO X<sub>CO_2</sub measurements significantly reduce the uncertainties of surface CO₂ flux estimates. Glint measurements are generally better at constraining ocean CO₂ flux estimates. 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