Page 1 1 of 16, NATO ASI, Kyiv, 9/15/2010

Vijay Natraj (Jet Propulsion Laboratory)

CollaboratorsHartmut Bösch (Univ Leicester)

Rob Spurr (RT Solutions)Yuk Yung (Caltech)

NATO ASI on Polarimetry and Remote SensingNATO ASI on Polarimetry and Remote Sensing

September 15, 2010September 15, 2010

A Two Orders of Scattering Approach to Account for Polarization in CO2 Retrievals from

Space

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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 retrieved trace gas column densities if unaccounted for

• The satellite instrument could be sensitive to polarization

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Orbiting Carbon Observatory (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

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OCO Spectroscopy

O2 A bandClouds/Aerosols, Surface Pressure

“strong” CO2 bandClouds/Aerosols, H2O, Temperature

“weak” CO2 bandColumn CO2

• Column-integrated CO2 abundance => Maximum contribution from surface

• High resolution spectroscopic measurements of reflected sunlight in near IR CO2 and O2 bands

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Polarization in the O2 A Band

continuum

line core

gas absorption od ~ 1

SZA = 10° (solid); 40° (dotted); 70° (dashed)

Natraj et al., JQSRT, 103, 245–259, 2007

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Why Two Orders of Scattering (2OS)?

• Full multiple-scattering vector RT codes (e.g. VLIDORT) are too slow to meet large-scale operational 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

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• 2OS gives a first order approximation to the intensity correction

• Multiple scattering angular integrals involve computations for the following sets of angles:– Solar-quadrature

– Quadrature-viewing

– Quadrature-quadrature

• 2OS does NOT need quadrature-quadrature computations

• O(N) rather than O(N2) computation (N: # of quadrature angles)

Why 2OS?

€

Iv = Iv1 + Iv2 + Iv3 + ...

€

Is = Is1 + Is2 + Is3 + ...

€

Icor = Iv2 − Is2 + ...

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2OS Model Schematic

Scenario 1 Scenario 2

scatterer

Scenario 3

scatterer

Scenario 4

scatterer 1

scatterer 2

Natraj and Spurr, JQSRT, 107, 263–293, 2007

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2OS Approximation Overview

• Retrievals will only be applied to optically thin scattering ( < 0.3)

• Intensity will still be calculated with full multiple scattering scalar model

• (for OCO)

• Fast correction to standard scalar code

• Exact through second order

• Supports analytic Jacobians

€

τ

€

S = Is + Icor −Q2

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Scenarios (Nadir Mode)

Natraj et al., JGR, 113, D11212, 2008

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Scenarios (Nadir Mode)

• Two seasons (Jan/Jul)

• Three AOD (0, 0.01, 0.3)

• Tropospheric aerosol: Kahn et al., 2001

• Stratospheric aerosol: 75% H2SO4, modified gamma size distribution

• Surface reflectances from ASTER library

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Spectral Residuals

Scalar Model 2OS Model

AOT ↑

Scalar model errors order of magnitude larger than 2OS model errors

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Retrieval error dominated by incorrect estimation of surface pressure

XCO2 and Surface Pressure Errors

XCO2 Error (ppm) Surface Pressure Error (Pa)

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XCO2 Errors (Glint Mode)

Scalar Model 2OS Model

XCO2 errors from 2OS model < 1 ppm; scalar model errors as high as 5 ppm

Wind speed = 4 m/s

AOT ↑

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XCO2 Errors (Target Mode)

Scalar Model 2OS Model

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

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Summary

• Ignoring polarization could lead to significant (as high as 30 ppm) errors 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

• Errors dominated by errors in retrieved surface pressure

• 2OS model two orders of magnitude faster than a full vector calculation