Simulations of Space-Based Near IR CO 2 Observations over Ground Target

1. Simulations ofSpace-Based Near IR CO2 Observations over Ground Target Vijay Natraj (Caltech), Hartmut Bösch (University of Leicester), Rob Spurr (RT Solutions), Yuk Yung (Caltech) EGU General Assembly, Vienna, Austria April 21, 2009

2. Outline • CO2 from space • Introduction to target mode • 2OS polarization model • Scenarios • XCO2 Errors • Conclusions

3. CO2 from Space • Space-based CO2 measurements can improve source/sink estimates • First dedicated CO2 missions launched this year • GOSAT (JAXA): successful • OCO (NASA): failed • Both OCO and GOSAT measure reflected sunlight in NIR to infer total column CO2 (XCO2) • Precision and accuracy < 1% required on regional scales and monthly time scales

4. Spacecraft Coordinates 447-m WLEF Tower Target Mode: Validation Tool

5. GOSAT Observation Schematics 5 cross track patters with 1, 3, 5, 7, 9 points/cross track scan 88 – 800 km Satellite Direction (along track) Cross track

6. 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) • Satellite instruments could be polarization sensitive • Polarization depends on solar and viewing angles and will therefore introduce spatial biases in XCO2 if not accounted for

7. Polarization Characteristics of Different Viewing Modes • Light with polarization parallel to slit (OCO-like instrument) • I: intensity; Q, U: components of linear polarization; : angle between slit axis and principal plane • Nadir and glint modes: • Target mode: measurement not restricted to principal plane

8. 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

9. Scenarios • Location: Bremen (validation site for space-based CO2 measurements) • Solar Zenith Angle (SZA): 50.4° • Scattering Angle: 85°–150° • Scatterer scenarios • Aerosol only: (OD) 0.05, 0.3, 0.3 (high) • Cirrus only: (OD) 0.05, 0.3, 0.3 (low) • Aerosol and cirrus: • AOD - 0.05, COD - 0.25 (at 750 nm) • AOD - 0.25, COD - 0.05 (at 750 nm) • Surface: Lambertian

10. Sample Radiance Spectrum O2A band Weak CO2 band Strong CO2 band

11. XCO2 Errors: Example

12. XCO2 Errors: Example Two orders of magnitude worse results for scalar model

13. XCO2 Errors: Summary • Scalar • High altitude scattering always gives large errors • Underestimation of photon path length • Highly polarized single scattering from higher altitudes • For similar scattering altitudes, higher scattering OD better • Multiple scattering depolarizes • 2OS • Thin cirrus modeled well • Largest errors for large amounts of high altitude scattering • Polarized multiple scattering from higher altitudes • Aerosol and cirrus have opposite polarization • Different spectral extinction and scattering behavior for aerosol and cirrus

14. XCO2 Precision

15. Averaging Kernels

16. Summary • Target mode is important for validating space-based XCO2 retrievals (such as those from GOSAT) • Ignoring polarization could lead to significant errors in retrieved XCO2 • 2OS approach to account for polarization works very well • Careful scene and geometry selection necessary to do proper validation • 2OS model can be applied directly to GOSAT data to take advantage of polarization measurements

17. Backup Slides

18. RT Model • Multiple scattering model: LIDORT (L) – scalar; VLIDORT (VL) - vector • Discrete ordinate solution for Stokes vector • Linearized: derivatives of intensity w.r.t. optical depth and single scattering albedo obtained analytically • Polarization: 2 Orders of Scattering (2OS) • Polarization approximated by two orders of scattering • Analytic integration over optical depth • Fast invariant imbedding approach to add individual layers • Linearized

19. 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

20. Degrees of Freedom