1. Global Characterization of X CO2 as Observed by the OCO (Orbiting Carbon Observatory) Instrument H. Boesch 1, B. Connor 2, B. Sen 1,3, G. C. Toon 1, C. E. Miller 1, D. Crisp 1, R. Salawitch 1 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA, 2 National Institute of Water and Atmospheric Research, Lauder, New Zealand 3 Now at Northrop Grumman Space Technology, Redondo Beach, CA, USA Contact: Hartmut Boesch, Phone: 818-393-5976, Email: hartmut.boesch@jpl.nasa.gov Motivation OCO will acquire space-based CO 2 observations with very high precision and accuracy (0.3 - 0.5%) which puts unprecedented demands on both instrument and analysis Here, we present a retrieval characterization for OCO nadir and sunglint observations which allows for verification and quantification of the precision of our retrieval. Furthermore, a good understanding of the sensitivity and the errors of the space-based measurements is critical for inverse modeling of carbon sources and sinks. Characterization of Nadir Soundings The sensitivity of X CO2 retrievals to the CO 2 volume mixing ratios (vmr) x at different heights z is given by the column averaging kernel: ak(z) = X CO2 /x(z) In first order, OCO averaging kernels and X CO2 retrieval errors depend on solar zenith angle (sza), surface albedo or type and aerosol optical depth OCO averaging kernels as a function of solar zenith angle for 4 different surface types (aerosol optical depth of 0.1) The OCO Mission OCO is a space-based mission solely dedicated to CO 2 measurements with precision, accuracy and resolution needed to quantify CO 2 sources and sinks (regional, monthly averaged precision of 1-2 ppm without significant coherent biases) [Crisp et al., 2004, Miller et al., 2006] The OCO instrument incorporates 3 bore-sighted high-resolution grating spectrometers which measure spectra of reflected sunlight in the near IR CO 2 bands at 1.61 m and 2.06 m and in the O 2 A-band OCO will be launched in Sep. 2008 and will fly ahead of the A-Train constellation with a 1:26 PM equator crossing time and a 16 day ground track repeat cycle OCO will switch from nadir observations (small footprint size of 3 km 2 ) to glint observations (high signal over oceans) every 16 days Local Nadir Glint Spot Ground Track OCO observation strategy OCO ground track for nadir observations Retrieval Algorithm X CO2 (dry air, column averaged, mole fraction of CO 2 ) will be retrieved from a simultaneous fit of O 2 and CO 2 bands using Optimal Estimation [Rodgers, 2000] The forward model is based on multiple scattering radiative transfer model Radiant [Spurr and Christi, 2006] and 2 orders of scattering polarization approximation [Natraj and Spurr, 2006]. OCO retrieval algorithm retrieves profiles of CO 2, H 2 O, temperature and aerosol optical depth as well as surface pressure, surface albedo and spectral dispersion [Kuang et al., 2002; Boesch et al., 2006] X CO2 is computed from the retrieved state after the iterative retrieval has converged Simulated nadir radiance spectra for the 3 OCO spectrometers with resolving power of ~20,000 References H. Boesch et al., Space-based Near-infrared CO 2 Measurements: Testing the OCO Retrieval Algorithm and Validation Concept Using SCIAMACHY Observations over Park Falls, Wisconsin, JGR, in press, 2006 B. Connor, H. Boesch et al., The Orbiting Carbon Observatory (OCO): Inverse Method and Prospective Error Analysis, JGR, submitted, 2006 D. Crisp et al., The orbiting carbon observatory (OCO) mission, Adv. Space Res., 2004 C. E. Miller et al., Precision requirements for space-based X CO2 data, JGR, accepted, 2006 V. Natraj and Spurr, R.J.D., A Linearized Approximately Spherical Two Orders of Scattering Model to Account for Polarization in Vertically Inhomogeneous Scattering-Absorbing Media, JQSRT, submitted, 2006 C. D. Rodgers, Inverse Methods for Atmospheric Sounding, World Scientific, Singapore, 2000 R. Spurr and M. Christi, Linearization of the interaction principle: Analytic Jacobians in the ‘‘Radiant’’ model, JQRST, 2007 See also related presentation: B42A-08 - D. Crisp, talk, Thursday 12.05 pm Characterization of Sunglint Soundings Sun glint mode of OCO provides spectra with high signal-to-noise over ocean (and snow) which are ‘dark’ in nadir mode Discussion Glint/nadir observations provide global X CO2 retrievals with high precision (< 2 ppm single sounding ) and large sensitivity to CO 2 near surface for all surface types Averaging kernels and retrieval errors vary significantly between different geophys. regimes (surface type, SZA and aerosol optical depth) Comparing X CO2 retrieved from OCO to models requires OCO column averaging kernel, a priori CO 2 profile and error estimates [Connor, Boesch et al., 2006], Standard OCO data product includes all quantities that are required to infer accurate surface fluxes using inversion or data assimilation methods Single-sounding X CO2 retrieval errors and degrees of freedom (dof) as a function of solar zenith angle for 4 different surface types (aerosol optical depth of 0.1) OCO averaging kernels as a function of solar zenith angle for aerosol optical depths of 0.05 (solid), 0.1 (dashed) and 0.3 (dotted) OCO averaging kernels as a function of solar zenith angle in sunglint mode over ocean (aerosol optical depth of 0.1) Comparison of single-sounding X CO2 retrieval errors and degrees of freedom between nadir and sunglint observations over ocean (aerosol optical depth of 0.1 and wind speed of 5 m/s)