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OCO Science Team Meeting March 22, 2006

A Two Orders of Scattering Approach to Account for Polarization in the OCO RT Model. OCO Science Team Meeting March 22, 2006 Vijay Natraj (Caltech), Hartmut B ö sch (JPL), Yuk Yung (Caltech). Importance of Polarization. Polarization is a result of scattering.

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OCO Science Team Meeting March 22, 2006

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  1. A Two Orders of Scattering Approach to Account for Polarization in the OCO RT Model OCO Science Team Meeting March 22, 2006 Vijay Natraj (Caltech), Hartmut Bösch (JPL), Yuk Yung (Caltech)

  2. Importance of Polarization • Polarization is a result of scattering. • The Earth’s atmosphere contains molecules, aerosols and clouds, all of which contribute to scattering. • Surfaces can also polarize, in some cases significantly (e.g., ocean). • Polarization depends on solar and viewing angles and will therefore introduce spatial biases in XCO2 if unaccounted for. • The OCO instrument measures only one component of polarization.

  3. Polarization in the O2A Band SZA = 10° (solid); 40° (dotted); 70° (dashed) continuum gas absorption od ~ 1 line core

  4. Proposed Solution: Two Orders of Scattering Approximation • Full multiple-scattering vector ARTM codes (e.g. VLIDORT) are too slow to meet large-scale OCO processing requirements. • Scalar computation causes two kinds of errors. • polarized component of the Stokes vector is neglected. • correction to intensity due to polarization is neglected. • Major contribution to polarization comes from first few orders of scattering (multiple scattering is depolarizing). • Single scattering does not account for the correction to intensity due to polarization.

  5. Polarization Approximation Overview • XCO2 retrievals will only be applied to optically thin scattering (τ<0.3). • Intensity will still be calculated with full multiple scattering scalar model. • S = Isca+Icor-Q2 • Fast correction to standard scalar code • Exact through second order • Simple model, easily implemented • Supports analytic Jacobians

  6. 45 geometries 9 scenarios Scenarios for Testing Proposed Method • SZA: 10°, 40°, 70° • VZA: 0° (OCO nadir mode), 35°, 70° • Azimuth: 0° (OCO nadir mode), 45°, 90°, 135°, 180° • Surface Albedo: 0.01, 0.1, 0.3 • Aerosol OD: 0 (Rayleigh), 0.01, 0.1 • Dusty continental aerosol (Kahn et al., JGR 106(D16), pp. 18219-18238, 2001)

  7. Rayleigh Aerosol OD = 0.01 Aerosol OD = 0.1 Increasing Surface Albedo Forward Model Radiance Errors: O2A Band Asterisks refer to different geometries; The red triangles refer to OCO nadir viewing geometry.

  8. Rayleigh Aerosol OD = 0.01 Aerosol OD = 0.1 Increasing Surface Albedo Forward Model Radiance Errors: 1.61 µm CO2 Band Asterisks refer to different geometries; The red triangles refer to OCO nadir viewing geometry.

  9. Rayleigh Aerosol OD = 0.01 Aerosol OD = 0.1 Increasing Surface Albedo Forward Model Radiance Errors: 2.06 µm CO2 Band Asterisks refer to different geometries; The red triangles refer to OCO nadir viewing geometry.

  10. Residuals: Best Case Scenario (O2A Band) SZA = 10°; VZA = 0°; Azimuth = 0°; Surface Albedo = 0.3; No Aerosol

  11. Residuals: Best Case Scenario (1.61 µm CO2 Band) SZA = 10°; VZA = 0°; Azimuth = 0°; Surface Albedo = 0.3; No Aerosol

  12. Residuals: Best Case Scenario (2.06 µm CO2 Band) SZA = 10°; VZA = 0°; Azimuth = 0°; Surface Albedo = 0.3; No Aerosol

  13. Residuals: Worst-Case Scenario (O2A Band) SZA = 70°; VZA = 70°; Azimuth = 90°; Surface Albedo =0.01; Aerosol OD = 0.1

  14. Residuals: Worst-Case Scenario (1.61 µm CO2 Band) SZA = 70°; VZA = 70°; Azimuth = 90°; Surface Albedo =0.01; Aerosol OD = 0.1

  15. Residuals: Worst-Case Scenario (2.06 µm CO2 Band) SZA = 70°; VZA = 70°; Azimuth = 90°; Surface Albedo =0.01; Aerosol OD = 0.1

  16. Timing Results: No Aerosol 16 half-space streams for Gaussian quadrature

  17. Timing Results: Aerosol Present 2 scat approx. adds only 50% to scalar calculation (for simulating 45 geometries). For OCO retrievals, overhead is expected to be around 10%.

  18. Linear Error Analysis • 6 scenarios considered • Surface Albedo: 0.01, 0.1, 0.3 • Aerosol OD: 0.01, 0.1 • SZA = 45°;VZA = 0°;Azimuth = 0° (OCO Nadir Mode) • 8 half-space streams, 11 layers • Number of spectral points: 8307 (O2 A band), 3334 (CO2 bands)

  19. Summary • Ignoring polarization could lead to significant (as high as 10 ppm) errors in XCO2 retrievals. • A two orders of scattering approach to account for the polarization works very well, giving XCO2 errors that are much smaller than other biases. • The approach is two orders of magnitude faster than a full vector calculation. • The additional overhead is in the range of 10% of the scalar computation . • More tests need to be done to test the effect of the approximation for glint viewing over ocean.

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