Analysis of 3D Cloud Effects in OCO-2 XCO2 Retrievals

Abstract:

The presence of 3D cloud radiative effects in OCO-2 retrievals is demonstrated from an analysis of XCO2raw retrievals, bias corrected XCO2bc data, ground based Total Carbon Column Observation Network (TCCON) XCO2, and Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask data. Over 50 % of OCO-2 Quality Flag QF=0 (best quality) and QF=1 (lesser quality) retrievals are within 5 km of clouds. 3D radiative transfer calculations indicate that 3D cloud radiative perturbations at this cloud distance, for an isolated low altitude cloud, are larger in absolute value than those due to a 1 ppm increase in CO₂. OCO-2 measurements are therefore susceptible to 3D cloud effects. Four 3D cloud metrics, based upon MODIS cloud mask and stand-alone OCO-2 measurements, relate XCO2bc-TCCON differences to 3D cloud effects. This analysis indicates that the operational bias correction has a non-zero residual 3D cloud bias for both QF=0 and QF=1 data. XCO2bc –TCCON averages at small cloud distances differ from those at large cloud distances by -0.4 and -2.2 ppm for the QF=0 and QF=1 data over the ocean. Three cloud bias mitigation techniques are applied and discussed: (1) table look-up, (2) data screening, and (3) the addition of additional terms to the operational bias correction equations. Mitigation of 3D cloud biases by the table look-up technique, which utilizes nearest cloud distance (Distkm) and stand-alone OCO-2 Colorratio 3D metrics, reduces QF=1 ocean and land XCO2bc –TCCON averages from -1 ppm to near 0.2 ppm. The addition of more terms to the linear regression equations used in the current bias correction processing, without data screening, however, did not introduce an appreciable improvement in the standard deviations of the XCO2bc-TCCON statistics.