Page 1 1 of 20, EGU General Assembly, Apr 21, 2009 Vijay Natraj (Caltech), Hartmut Bösch (University of Leicester), Rob Spurr (RT Solutions), Yuk Yung (Caltech) EGU General Assembly, Vienna, Austria April 21, 2009 Simulations of Space-Based Near IR CO 2 Observations over Ground Target

Page 2 2 of 20, EGU General Assembly, Apr 21, 2009 Outline CO 2 from space Introduction to target mode 2OS polarization model Scenarios X CO2 Errors Conclusions

Page 3 3 of 20, EGU General Assembly, Apr 21, 2009 CO 2 from Space Space-based CO 2 measurements can improve source/sink estimates First dedicated CO 2 missions launched this year –GOSAT (JAXA): successful –OCO (NASA): failed Both OCO and GOSAT measure reflected sunlight in NIR to infer total column CO 2 (X CO2 ) Precision and accuracy < 1% required on regional scales and monthly time scales

Page 4 4 of 20, EGU General Assembly, Apr 21, 2009 Target Mode: Validation Tool Spacecraft Coordinates 447-m WLEF Tower

Page 5 5 of 20, EGU General Assembly, Apr 21, 2009 GOSAT Observation Schematics 88 – 800 km Satellite Direction (along track) Cross track Configuration2-axes scanner Scanning Cross Cross Track (±35 deg) Along Track (±20 deg) Field of viewIFOV <10.5 km 790 km (scan width) 5 cross track patters with 1, 3, 5, 7, 9 points/cross track scan

Page 6 6 of 20, EGU General Assembly, Apr 21, 2009 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 X CO2 if not accounted for

Page 7 7 of 20, EGU General Assembly, Apr 21, 2009 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

Page 8 8 of 20, EGU General Assembly, Apr 21, OS Model Schematic Scenario 1 Scenario 2 scatterer Scenario 3 scatterer Scenario 4 scatterer 1 scatterer 2 Natraj and Spurr, JQSRT, 107, 263–293, 2007

Page 9 9 of 20, EGU General Assembly, Apr 21, 2009 Scenarios Location: Bremen (validation site for space-based CO 2 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 , COD (at 750 nm) AOD , COD (at 750 nm) Surface: Lambertian

Page of 20, EGU General Assembly, Apr 21, 2009 Sample Radiance Spectrum O 2 A band Weak CO 2 band Strong CO 2 band

Page of 20, EGU General Assembly, Apr 21, 2009 X CO2 Errors: Example

Page of 20, EGU General Assembly, Apr 21, 2009 X CO2 Errors: Example Two orders of magnitude worse results for scalar model

Page of 20, EGU General Assembly, Apr 21, 2009 X CO2 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

Page of 20, EGU General Assembly, Apr 21, 2009 X CO2 Precision

Page of 20, EGU General Assembly, Apr 21, 2009 Averaging Kernels

Page of 20, EGU General Assembly, Apr 21, 2009 Summary Target mode is important for validating space-based X CO2 retrievals (such as those from GOSAT) Ignoring polarization could lead to significant errors in retrieved X CO2 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

Page of 20, EGU General Assembly, Apr 21, 2009 Backup Slides

Page of 20, EGU General Assembly, Apr 21, 2009 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

Page of 20, EGU General Assembly, Apr 21, 2009 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

Page of 20, EGU General Assembly, Apr 21, 2009 Degrees of Freedom