Marine liquid cloud geometric thickness retrieved from OCO-2's oxygen A-band spectrometer
Abstract. This paper introduces the OCO2CLD-LIDAR-AUX product, which uses the Cloud-Aerosol Lidar and Infrared
Pathfinder Satellite Observation (CALIPSO) lidar and the Orbiting Carbon
Observatory-2 (OCO-2) hyperspectral A-band spectrometer. CALIPSO provides a
prior cloud top pressure ( Ptop ) for an OCO-2-based retrieval of
cloud optical depth, Ptop and cloud geometric thickness expressed
in hPa. Measurements are of single-layer liquid clouds over oceans from
September 2014 to December 2016 when collocated data are available. Retrieval
performance is best for solar zenith angles  ∘ and when
the cloud phase classification, which also uses OCO-2's weak CO2 
band, is more confident. The highest quality optical depth retrievals agree
with those from the Moderate Resolution Imaging Spectroradiometer (MODIS)
with discrepancies smaller than the MODIS-reported uncertainty. Retrieved
thicknesses are consistent with a substantially subadiabatic structure over
marine stratocumulus regions, in which extinction is weighted towards the
cloud top. Cloud top pressure in these clouds shows a 4 hPa bias compared
with CALIPSO which we attribute mainly to the assumed vertical structure of
cloud extinction after showing little sensitivity to the presence of
CALIPSO-identified aerosol layers or assumed cloud droplet effective radius.
This is the first case of success in obtaining internal cloud structure from
hyperspectral A-band measurements and exploits otherwise unused OCO-2 data.
This retrieval approach should provide additional constraints on
satellite-based estimates of cloud droplet number concentration from visible
imagery, which rely on parameterization of the cloud thickness.