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Vijay Natraj (Caltech), Hartmut B ö sch (Leicester), Rob Spurr (RT Solutions), Yuk Yung (Caltech)

Glint and Target Mode Simulations for the Orbiting Carbon Observatory. Vijay Natraj (Caltech), Hartmut B ö sch (Leicester), Rob Spurr (RT Solutions), Yuk Yung (Caltech) AGU Fall Meeting December 17, 2008. Project and Mission Overview. The O rbiting C arbon O bservatory ( OCO )

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Vijay Natraj (Caltech), Hartmut B ö sch (Leicester), Rob Spurr (RT Solutions), Yuk Yung (Caltech)

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  1. Glint and Target Mode Simulations for the Orbiting Carbon Observatory Vijay Natraj (Caltech), Hartmut Bösch (Leicester), Rob Spurr (RT Solutions), Yuk Yung (Caltech) AGU Fall Meeting December 17, 2008

  2. Project and Mission Overview The Orbiting Carbon Observatory (OCO) Watching The Earth Breathe…Mapping CO2 From Space • Salient Features: • High-resolution, three-channel grating spectrometer • Partnership with HS (Instrument) and OSC (Spacecraft) • High heritage spacecraft, flies in formation with the A-Train • Launch date: 15 January 2009 on Taurus XL from VAFB • Operational life: 2 years • Principal Investigator: Dr. David Crisp, Deputy: Dr. Charles Miller • Project Manager: Thomas Livermore, Deputy: Dr. Ralph Basilio • Earth Science Flight Projects Office Manager: Dr. Steven Bard, JPL • ESSP Program Manager : Edward Grigsby, LaRC • Program Scientist: Dr. William Emanuel, NASA HQ • ESSP Program Executive: Eric Ianson, NASA HQ • Science: • Collect the first space-based measurements of atmospheric CO2 with the precision, resolution, and coverage needed to characterize its sources and sinks on regional scales and quantify their variability over the seasonal cycle. • Use independent data validation approaches to ensure high accuracy (1-2 ppm, 0.3% - 0.5%) • Reliable climate predictions require an improved understanding of CO2 sinks • What human and natural processes are controlling atmospheric CO2? • What are the relative roles of the oceans and land ecosystems in absorbing CO2?

  3. Mission System Description 3-channel Spectrometer (JPL/HS) Ground Validation Sites Dedicated Spacecraft (OSC) Data Processing Center (JPL) Taurus XL 3110 (KSC) Mission Ops (OSC) NASA GN (GSFC) and SN (TDRSS) Please visit http://oco.jpl.nasa.gov for more information

  4. R I Glint Spot Ground Track OCO Glint Mode • Glint Observations: views “glint” spot • Angle of reflection equals angle of incidence of sunlight at surface: R = I • Improves SNR over oceans • 70% time spent over oceans Spacecraft Coordinates Azimuth Orientation

  5. Geolocation Accuracy Spatial Direction Along Slit Scan Direction OCO Target Mode • Tracks a stationary surface target (calibration site) to collect large numbers of soundings • Uplooking ground-based FTS data acquired simultaneously through same slant column • Acquire Target data over 1 surface validation site each day 447-m WLEF Tower

  6. Polarization Characteristics of OCO Spectrometers • Transmits light with polarization parallel to slit • I: intensity; Q, U: components of linear polarization; : angle between slit axis and principal plane (polarization angle) • Nadir and glint modes: • Target mode: measurement not restricted to principal plane

  7. Scenario 1 Scenario 2 Scenario 3 Scenario 4 scatterer scatterer scatterer 1 scatterer 2 2OS Model Schematic Natraj and Spurr, JQSRT, 107, 263–293, 2007

  8. Glint Mode: Scenarios • Solar Zenith Angle (SZA): 15°, 45°, 60°, 65°, 70°, 75° • Aerosol Optical Thickness (AOT): 0 (Rayleigh), 0.01, 0.05, 0.1, 0.3 • Dusty maritime aerosol (Kahn et al., JGR, 2001) • Background stratospheric aerosol • Ocean surface reflectance simulated using Cox-Munk model • Wind Speed: 4 m/s, 8 m/s, 12 m/s

  9. Spectral Residuals (Glint): Scalar Model Wind speed = 4 m/s Residual = Model-Exact(VLIDORT)

  10. Spectral Residuals (Glint): R-2OS Model Wind speed = 4 m/s Residuals from R-2OS model are smaller by 1–2 orders of magnitude

  11. AOT ↑ Glint XCO2 Errors Wind speed = 4 m/s R-2OS Model Scalar Model XCO2 errors from R-2OS model < 1 ppm; scalar model errors as high as 5 ppm

  12. Glint XCO2 and Surface Pressure Errors Retrieval error dominated by incorrect estimation of surface pressure; other effects become more important for large AOTs

  13. Glint XCO2 Errors R-2OS Model Scalar Model XCO2 errors larger when only O2A band contributes to forward model error => CO2 and O2 errors cancel out in the ratio

  14. Target Mode: Scenarios • Location: Bremen (OCO validation site) • Solar Zenith Angle (SZA): 50.4° • Polarization angle: 122.68°, 177.385°, 118.961° • Scatterer scenarios: 0.05 AOT, 0.05 AOT+0.25 Cirrus OT, 0.3 AOT • Surface: Lambertian

  15. Target XCO2 Errors R-2OS Model Scalar Model 0.05 AOT 0.3 AOT 0.05 AOT+0.25 Cirrus OT

  16. Target XCO2 Errors R-2OS Model Scalar Model XCO2 errors from R-2OS model < 1 ppm; scalar model errors as high as 30 ppm

  17. Summary • Ignoring polarization could lead to significant (as high as 40 ppm) errors (that are much larger than the measurement noise) in XCO2 retrievals • 2OS approach to account for polarization works very well (in and out of principal plane), giving XCO2 errors that are typically smaller than 1 ppm, and smaller or comparable to measurement noise • Errors dominated by errors in retrieved surface pressure • R-2OS model two orders of magnitude faster than a full vector calculation • Model needs to be tested for glint over land

  18. Backup Slides

  19. RT Model • Scalar multiple scattering model: Radiant (R) • Discrete ordinate solution for layer reflection and transmission matrices • Adding method to obtain combined matrices for different layers • Linearized: derivatives of intensity w.r.t. optical depth and single scattering albedo obtained analytically • Polarization: 2OS • Polarization approximated by two orders of scattering • Analytic integration over optical depth • Fast invariant imbedding approach to add individual layers • Linearized

  20. Radiance Results: Clear Sky Wind speed = 4 m/s SZA ↑ Glint reflectance ~ 10 times larger at 75° than at 15°

  21. Radiance Results: Cloudy Sky (AOT = 0.3) Wind speed = 4 m/s Q decreases! For large SZA, slant-path attenuation of solar beam very large; large fraction of light comes from atmospheric scattering

  22. Linear Error Analysis • Forward model errors systematic • Bias in retrieved parameters x • Bias can be expressed as follows: • G: gain matrix • Describes mapping of measurement variations into retrieved vector variations • ΔF:forward model error

  23. AOT ↑ Glint XCO2 Errors R-2OS Model Scalar Model Wind speed = 4 m/s Wind speed = 8 m/s Wind speed = 12 m/s XCO2 errors from R-2OS model < 1 ppm; scalar model errors as high as 5 ppm

  24. Glint XCO2 and Surface Pressure Errors Wind speed = 4 m/s Wind speed = 8 m/s Wind speed = 12 m/s Retrieval error dominated by incorrect estimation of surface pressure; other effects become more important for large AOTs

  25. Glint XCO2 Errors R-2OS Model Scalar Model Wind speed = 4 m/s Wind speed = 8 m/s Wind speed = 12 m/s XCO2 errors larger when only O2A band contributes to forward model error => CO2 and O2 errors cancel out in the ratio

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