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Published January 2007 | public
Journal Article

Evaluation of errors from neglecting polarization in the forward modeling of O_2 A band measurements from space, with relevance to CO_2 column retrieval from polarization-sensitive instruments

Abstract

Sensitivity studies have been performed to evaluate the errors resulting from ignoring polarization in analyzing spectroscopic measurements of the O_2A band from space, using the Orbiting Carbon Observatory (OCO) as a test case. An 11-layer atmosphere, with both gas and aerosol loading, and bounded from below by a lambertian reflecting surface, was used for the study. The numerical computations were performed with a plane-parallel vectorized discrete ordinate radiative transfer code. Beam and viewing geometry, surface reflectance and aerosol loading were varied one at a time to evaluate and understand the individual errors. Different behavior was observed in the line cores and the continuum because of the different paths taken by the photons in the two cases. The errors were largest when the solar zenith angle was high, and the aerosol loading and surface reflectance low. To understand the effect of neglecting polarization on CO_2 column retrievals, a linear error analysis study was performed on simulated measurements from the OCO spectral regions, viz. the 1.61 and 2.06 μm CO_2 bands and the O_2A band. It was seen that neglecting polarization could introduce errors as high as 10 ppm, which is substantially larger than the required retrieval precision of ∼2 ppm. A variety of approaches, including orders of scattering, spectral binning and the use of lookup tables are being explored to reduce the errors.

Additional Information

© 2006 Elsevier Ltd. Received 8 November 2005; accepted 6 February 2006. Available online 20 March 2006. This work was supported in part by NASA Grant NAG1-1806 and the Orbiting Carbon Observatory (OCO) Project at JPL. We would like to thank Joop Hovenier and Johan de Haan for providing us with a doubling-adding vector RT code to verify the results from VLIDORT. We would also like to acknowledge David Crisp, Run-Lie Shia, Jack Margolis and Xin Guo for reviewing the manuscript, and Charles Miller, Geoff Toon and Bhaswar Sen for helpful comments.

Additional details

Created:
August 22, 2023
Modified:
October 17, 2023