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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Jun 2018

Research article | 07 Jun 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

A 12-Year Long Global Record of Optical Depth of Absorbing Aerosols above the Clouds Derived from OMI/OMACA Algorithm

Hiren Jethva1,2, Omar Torres2, and Changwoo Ahn3 Hiren Jethva et al.
  • 1Universities Space Research Association, 7178 Columbia Gateway Drive, Columbia, MD 21046, USA
  • 2NASA Goddard Space Flight Center, Earth Science Division, Code 614, Greenbelt, MD 20771, USA
  • 3Science Systems and Applications, Inc., 10210 Greenbelt Rd, Lanham, MD 20706 USA

Abstract. Aerosol-cloud interaction continues to be one of the leading uncertain components of the climate models, primarily due to the lack of adequate knowledge of the complex microphysical and radiative processes of the aerosol-cloud system. Situations when the light-absorbing aerosols such as carbonaceous particles and windblown dust overlay low-level cloud decks are commonly found in several regions of the world. Contrary to the known cooling effects of these aerosols in cloud-free scenario over darker surfaces, an overlapping situation of the absorbing aerosols over the cloud can lead to a significant level of atmospheric absorption exerting a positive radiative forcing (warming) at the top-of-atmosphere. We contribute to this topic by introducing a new global product of the above-cloud aerosol optical depth (ACAOD) of absorbing aerosols retrieved from the near-UV observations made by the Ozone Monitoring Instrument (OMI) onboard NASA's Aura platform. Physically based on an unambiguous color ratio effect in the near-UV caused by the aerosol absorption above the cloud, the OMACA (OMI Above-Cloud Aerosols) algorithm simultaneously retrieves the optical depths of aerosols and clouds under a prescribed state of the atmosphere. The OMACA algorithm shares many similarities with the two-channel cloud-free OMAERUV algorithm, including the use of AIRS carbon monoxide for the aerosol type identification, CALIOP-based aerosol layer height dataset, and OMI-based surface albedo database. We present the algorithm architecture, inversion procedure, retrieval quality flags, initial validation results, and results from a 12-year long OMI record (2005–2016) including global climatology of the frequency of occurrence, ACAOD, and aerosol-corrected cloud optical depth. A comparative analysis of the coincident and collocated OMACA-retrieved ACAOD and equivalent accurate measurements from the HSRL-2 lidar for the ORACLES phase I operation (August-September 2016) revealed a good agreement (R=0.77, RMSE=0.10). The long-term OMACA record reveals several important regions of the world, including Southeastern Atlantic Ocean, southern Indian Ocean, South-East Asia, tropical Atlantic Ocean off the coast of western Africa, and northern Arabian sea where the carbonaceous aerosols from the seasonal biomass burning and mineral dust originated over the continents are found to overlie low-level cloud decks with moderate (0.30.8 in the proximity to the sources). No significant long-term trend in the frequency of occurrence of aerosols above the clouds and ACAOD is noticed when OMI observations that are free from the row anomaly throughout the operation are considered. If not accounted, the effects of aerosol absorption above the clouds introduce low bias in the retrieval of cloud optical depth with a profound impact at increasing ACAOD and cloud brightness. The OMACA aerosol product from OMI presented in this paper offers a crucial missing piece of information of the aerosol loading above cloud that will help us to quantify the radiative effects of clouds when overlaid with aerosols and its resultant impact on cloud properties and climate.

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Hiren Jethva et al.
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Hiren Jethva et al.
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Publications Copernicus
Short summary
We introduce a new global satellite product of aerosol amounts lofted above the clouds from near-UV observations of Aura/OMI. The global decadal record derived from the product has revealed unprecedented quantitative information of light-absorbing aerosols above the cloud over several oceanic and continental regions of the world. The new dataset characterizing the properties of aerosol-cloud overlap will help quantify their radiative effects and representation in climate models.
We introduce a new global satellite product of aerosol amounts lofted above the clouds from...