Abstract
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 <45∘ 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.
Highlights
The Orbiting Carbon Observatory-2’s (OCO-2) primary mission is to retrieve atmospheric CO2 concentration (XCO2) using reflected sunlight (Crisp, 2015; Crisp et al, 2004; Eldering et al, 2017)
These soundings are rich in cloud information (Richardson and Stephens, 2018) and here we present the OCO2CLD-LIDAR-AUX product, which exploits these unused OCO-2 data in concert with collocated measurements from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite
We compare the statistics of OCO-2 minus Moderate Resolution Imaging Spectroradiometer (MODIS) τ retrievals in Fig. 4 for the full sample (a, b) and subsets split according to their SZA and radiance ratio (c, d)
Summary
The Orbiting Carbon Observatory-2’s (OCO-2) primary mission is to retrieve atmospheric CO2 concentration (XCO2) using reflected sunlight (Crisp, 2015; Crisp et al, 2004; Eldering et al, 2017). One model study suggests that a global cooling of −8.0 ± 0.1 W m−2 could be achieved primarily by brightening these clouds through increasing cloud condensation nuclei (Latham et al, 2008) This is approximately the heating that would result from a quadrupling of atmospheric CO2, and such large potential radiative changes make understanding their processes very desirable. This paper describes the retrieval algorithm, data sources and modelling techniques, describes and validates outputs against other satellite products and summarizes and maps the retrieved cloud properties It is organized as follows: Sect. 2 discusses the structure of the targeted clouds and the history and principle of their retrieval, Sect. 3 describes the OCO2 mission, instrumentation and orbit, Sect. 4 describes the retrieval, Sect. 5 explores the data with comparison to its priors, MODIS and CALIPSO, Sect. 6 reports and maps the full dataset retrieval statistics and Sect. 7 concludes
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