Insights into 3D cloud radiative transfer for OCO-2

Abstract. Clouds impose radiance perturbations upon Orbiting Carbon Observatory (OCO-2) measured spectra. The Spherical Harmonic Discrete Ordinate radiative transfer Method (SHDOM) code is applied in both idealized bar cloud and scene specific calculations of 1D and 3D radiances in order to understand 3D cloud effects for a wide range of gas vertical optical depths, solar and sensor viewing geometries, for ocean and land scenes. We find that OCO-2 measurements are susceptible to 3D cloud effects since 3D radiance enhancements are several times larger than radiance perturbations due to a 1 ppm increase in XCO2, and the 3D radiance perturbations are spectrally distinct (different than the radiance perturbations due to changes in surface pressure, XCO2, aerosol, and surface reflectance). SHDOM calculations for 36 scenes over the Amazon and the Pacific are co-analyzed with Moderate Resolution Imaging Spectroradiometer (MODIS) radiance-based cloud distance data, and OCO-2 Lite file rawXCO2 for both Quality Flag=0, (QF0, best quality) and Quality Flag=1 (QF1, poor quality) data. SHDOM calculations of the ocean and land scenes indicate that the 1 D / 3 D radiance intensity ratios and XCO2 decrease concurrently as nearest cloud distance decreases towards zero, especially for the ocean glint QF1 data, which provide the clearest evidence of 3D cloud effects in OCO-2 retrievals. Average differences in rawXCO2 at cloud distances near 20 and 0 km are an appropriate measure of 3D cloud effect biases (that are introduced by the retrieval), and are calculated for 10° latitude bands for 2015–2018. The ocean glint QF0 (QF1) latitude average 3D rawXCO2 biases are near 0.4 (1.5) ppm.