A 12-year long global record of optical depth of absorbing aerosols above the clouds derived from the OMI/OMACA algorithm
Authors: Jethva Hiren , Torres Omar , Ahn Changwoo 
Published: October 2018

Abstract. Aerosol–cloud interaction continues to be one of the leading
uncertain components of climate models, primarily due to the lack of
adequate knowledge of the complex microphysical and radiative processes of
the aerosol–cloud system. Situations when 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 the atmosphere. We contribute to this
topic by introducing a new global product of 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 aerosol type
identification, CALIOP-based aerosol layer height dataset, and an 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 OMACA-retrieved ACAOD, collocated with
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, 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
(0.3 < ACAOD < 0.5, away from the sources) to higher
levels of ACAOD (> 0.8 in the proximity to the sources), including the southeastern Atlantic Ocean,
southern Indian Ocean, Southeast Asia, the tropical Atlantic Ocean off the coast
of western Africa, and northern Arabian sea. 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 for,
the effects of aerosol absorption above the clouds introduce low bias in the
retrieval of cloud optical depth with a profound impact on increasing ACAOD
and cloud brightness. The OMACA aerosol product from OMI presented in this
paper offers a crucial missing piece of information from the aerosol loading
above cloud that will help us to quantify the radiative effects of clouds
when overlaid with aerosols and their resultant impact on cloud properties and
climate.