The influence of 3D cloud radiative effects on Orbiting Carbon Observatory measurements of CO2

The Orbiting Carbon Observatory (OCO-2) makes global measurements of CO2. This seminar reviews how the OCO-2 experiment retrieves CO2, and discusses physics (due to 3D cloud radiative effects) which is not included in the operational retrieval. Clouds outside of the OCO-2 footprints can scatter light towards observation footprints, and thereby increase observed radiances. Sun-cloud-observation footprint geometry can also lead to cloud shadowing (a decrease in radiance). Analysis of OCO-2 data files indicates that there are many more “cloud brightening” than “cloud shadowing” events. 3D and 1D radiative transfer calculations, and analysis of several years of OCO-2 and Moderate Resolution Imaging Spectrometer (MODIS) data files, indicate that a) retrievals are impacted when observations are within 4 km of clouds, and b) 40% of OCO-2 retrievals are within 4 km of clouds. 3D cloud effect biases are calculated over ocean and land for the northern and southern hemispheres. Mitigation of the 3D cloud effect biases is discussed in terms of a simple empirical look-up table algorithm.