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The Orbiting Carbon Observatory Mission: Effects of Polarization on Retrievals Vijay Natraj

The Orbiting Carbon Observatory Mission: Effects of Polarization on Retrievals Vijay Natraj Advisor: Yuk Yung Collaborators: Robert Spurr (RT Solutions, Inc.), Hartmut Boesch (JPL), Yibo Jiang (JPL). Outline. Introduction Retrieval Strategy Radiative Transfer Essentials

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The Orbiting Carbon Observatory Mission: Effects of Polarization on Retrievals Vijay Natraj

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  1. The Orbiting Carbon Observatory Mission: Effects of Polarization on Retrievals Vijay Natraj Advisor: Yuk Yung Collaborators: Robert Spurr (RT Solutions, Inc.), Hartmut Boesch (JPL), Yibo Jiang (JPL)

  2. Outline • Introduction • Retrieval Strategy • Radiative Transfer Essentials • O2 A Band Results • Sensitivity Analysis • Outlook

  3. Introduction Since 1860, global mean surface temperature has risen ~1.0 °C with a very abrupt increase since 1980. Atmospheric levels of CO2 have risen from ~ 270 ppm in 1860 to ~370 ppm today. Does increasing atmospheric CO2 drive increases in global temperature? Do increasing temperatures increase atmospheric CO2 levels?

  4. Where are the Missing Carbon Sinks? • Only half of the CO2 released into the atmosphere since 1970 has remained there. The rest has been absorbed by land ecosystems and oceans • What are the relative roles of the oceans and land ecosystems in absorbing CO2? • Is there a northern hemisphere land sink? • What are the relative roles of North America and Eurasia? • What controls carbon sinks? • Why does the atmospheric buildup vary with uniform emission rates? • How will sinks respond to climate change? • Reliable climate predictions require an improved understanding of CO2 sinks • Future atmospheric CO2 increases • Their contributions to global change

  5. Why Measure CO2 from Space?Improved CO2 Flux Inversion Capabilities • Current State of Knowledge • Global maps of carbon flux errors for 26 continent/ocean-basin-sized zones retrieved from inversion studies • Studies using data from the 56 GV-CO2 stations • Flux residuals exceed 1 GtC/yr in some zones • Network is too sparse • Inversion tests • global XCO2 pseudo-data with 1 ppm accuracy • flux errors reduced to <0.5 GtC/yr/zone for all zones • Global flux error reduced by a factor of ~3. Flux Retrieval Error GtC/yr/zone Rayner & O’Brien, Geophys. Res. Lett. 28, 175 (2001)

  6. OCO Mission • First global, space-based observations of atmospheric CO2 • high accuracy, resolution and coverage • geographic distribution of CO2 sources and sinks and variability • High resolution spectroscopic measurements of reflected sunlight • NIR CO2 and O2 bands • Remote sensing retrieval algorithms • estimates of column-averaged CO2 dry air mole fraction (XCO2) • accuracies near 0.3% (1 ppm) • Chemical transport models • spatial distribution of CO2 sources and sinks • two annual cycles

  7. Spectroscopy • Column-integrated CO2 abundance => Maximum contribution from surface • High resolution spectroscopic measurements of reflected sunlight in near IR CO2 and O2 bands O2A band Clouds/Aerosols, Surface Pressure “weak” CO2 band Column CO2 “strong” CO2 band Clouds/Aerosols, H2O, Temperature

  8. Retrieval Strategy

  9. Radiative Transfer Essentials Fundamental Equation of RT Beer’s Law Source Function (Emission, Scattering)

  10. Polarization and the Stokes Vector • Electromagnetic radiation can be described in terms of the Stokes Vectors: I, Q, U & V • I - total intensity • Q & U - linear polarization • V - circular polarization • Degree of Polarization (for OCO)

  11. Atmospheric and Surface Setup • 11-layer plane-parallel atmosphere (4 in stratosphere) • Urban, tropospheric and stratospheric aerosols • Lambertian surface: albedos of 0.05, 0.1, 0.3 • SZA: 10°, 40°, 70° • VZA: 0°, 35°, 70° • Azimuth: 0°, 45°, 90°, 135°, 180° • Aerosol extinction optical depth: 0, 0.025, 0.25

  12. Extinction Processes

  13. Results for O2 A Band with Rayleigh Scattering 0.0113 0.818 103.539

  14. Varying Solar Zenith Angle

  15. Varying Aerosol Loading

  16. Varying Surface Albedo

  17. Linear Sensitivity Analysis • Park Falls, Wisconsin • Geometry • Nadir viewing • SZA: 75.1° (Jan), 34.8° (Jul) • Azimuth: 210.9° (Jan), 240.0° (Jul) • Lorentzian ILS • Resolving Powers • O2A Band: 17000 • CO2 Bands: 20000 • Errors • July: 0.3 ppm • January: 10 ppm

  18. Outlook • Polarization: significant part of retrieval error budget • Full vector retrieval too time-consuming and not practical • Ways to handle polarization? • Orders of Scattering • Spectral Binning • Look-up tables

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