A new Orbiting Carbon Observatory 2 cloud flagging method and rapid retrieval of marine boundary layer cloud properties

Abstract. 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 ( Ptop ). 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 ∘ . Firstly, a multi-layer perceptron network was trained to identify liquid clouds over the ocean with sufficient optical depth ( τ>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 τ , effective radius ( re ) and Ptop 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 Ptop 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 τ difference is 0.8 [ - 3.8 , 6.9 ] , and re is - 0.3 [ - 2.8 , 2.1 ] µ m . The τ 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 for 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.