A new Orbiting Carbon Observatory 2 cloud flagging method 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 (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) lidar data to identify suitable clouds and provide a priori cloud top pressure (P-top). This collocation is no longer possible, since CALIPSO's coordination flying with OCO-2 has ended, 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 degrees. Firstly, a multi-layer perceptron network was trained to identify liquid clouds over the ocean with sufficient optical depth (tau > 1) for a valid retrieval, and agreement with MODIS-CALIPSO (Moderate Resolution Imaging Spectroradiometer) is 90.0 %. Secondly, we developed a lookup table to simultaneously retrieve cloud tau, effective radius (re) and P-top from A-band and CO2 band radiances, with the intention that these will act as the a priori state estimate in a future retrieval. Median P-top difference vs. CALIPSO is 12 hPa with an inter-decile range of [-11; 87] hPa, substantially better than the MODIS-CALIPSO range of [-83,81] hPa. The MODIS-OCO-2 tau difference is 0.8[-3.8,6.9], and r(e) is -0.3 [-2.8,2.1] mu m. The tau difference is due to optically thick and horizontally heterogeneous cloud scenes. As well as an improved passive P-top retrieval, this a priori information will allow for a purely OCO-2-based Bayesian retrieval of cloud droplet number concentration (N-d). Finally, our cloud flagging procedure may also be useful for future partial-column above-cloud CO2 abundance retrievals.