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

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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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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

project and mission overview
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?
mission system description
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

oco glint mode

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

oco target mode

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

polarization characteristics of oco spectrometers
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
2os model schematic

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

glint mode scenarios
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
spectral residuals glint scalar model
Spectral Residuals (Glint): Scalar Model

Wind speed = 4 m/s

Residual = Model-Exact(VLIDORT)

spectral residuals glint r 2os model
Spectral Residuals (Glint): R-2OS Model

Wind speed = 4 m/s

Residuals from R-2OS model are smaller by 1–2 orders of magnitude

glint x co2 errors

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

glint x co2 and surface pressure errors
Glint XCO2 and Surface Pressure Errors

Retrieval error dominated by incorrect estimation of surface pressure; other effects become more important for large AOTs

glint x co2 errors1
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

target mode scenarios
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
target x co2 errors
Target XCO2 Errors

R-2OS Model

Scalar Model

0.05 AOT

0.3 AOT

0.05 AOT+0.25 Cirrus OT

target x co2 errors1
Target XCO2 Errors

R-2OS Model

Scalar Model

XCO2 errors from R-2OS model < 1 ppm; scalar model errors as high as 30 ppm

summary
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
rt model
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
radiance results clear sky
Radiance Results: Clear Sky

Wind speed = 4 m/s

SZA ↑

Glint reflectance ~ 10 times larger at 75° than at 15°

radiance results cloudy sky aot 0 3
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

linear error analysis
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
glint x co2 errors2

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

glint x co2 and surface pressure errors1
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

glint x co2 errors3
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