3D Cloud Effects and CO2 Retrievals

The measurement of CO2 by space based sensors is difficult due to the measurement accuracy requirements and due to the complexity of radiative transfer in an atmosphere that contains aerosols, clouds, and surface variations. 3D radiative transfer is one physical effect that is not included in current retrievals. We demonstrate that 3D effects are present in sensed fields. We analyze OCO2 V7 XCO2 uncertainties as a function of cloud fraction data, as measured by the MODIS experiment, to empirically determine how the XCO2 uncertainties are related to clouds outside of the OCO2 observation grid. The 3D effect upon XCO2 retrievals is small, and mitigated in retrievals due to the fact that the 3D effects are manifested primarily in retrieval variables other than the XCO2 mixing ratios. This situation is discussed in a general manner that is applicable to many of the space based CO2 sensors.