A new OCO-2 cloud flagging and rapid retrieval of marine boundary layer cloud properties

The Orbiting Carbon Observatory-2 (OCO-2) carries a hyperspectral A-band sensor that can obtain information about cloud geometric thickness (H). The OCO2CLD-LIDAR-AUX product retrieved H with the aid of collocated CALIPSO lidar data to identify suitable clouds and provide a priori cloud-top pressure (Ptop). This collocation is no longer possible since CALIPSO no longer flies in coordination with OCO-2, so here we introduce a new cloud flagging and a priori assignment using only OCO-2 data, restricted to ocean footprints where solar zenith angle < 45°. Firstly, a multi-layer perceptron network was trained to identify liquid clouds over ocean with sufficient optical depth ( > 1) for a valid retrieval, and agreement with MODIS-CALIPSO is 90.0 %. Secondly, we developed a lookup table to simultaneously retrieve cloud , effective radius (re) and Ptop from A-band and CO2 band radiances, with the intention that these will act as the a priori in a future retrieval. Median Ptop difference versus CALIPSO is 12 hPa with interdecile range [-11,87] hPa, substantially better than the MODIS-CALIPSO [-83,81] hPa. The MODIS-OCO-2  difference is 0.8 (-3.8,6.9) and re is -0.3 [-2.8,2.1] m. The  difference is due to optically thick and horizontally heterogeneous cloud scenes. As well as an improved passive Ptop retrieval, this a priori information will allow a purely OCO-2 based Bayesian retrieval of cloud droplet number concentration (Nd). Finally, our cloud flagging procedure may also be useful for future partial column above-cloud CO2 abundance retrievals.