Three-wavelength approach for aerosol-cloud discrimination in the SAGE III/ISS aerosol extinction dataset
Authors: Bhatta Surendra , Pandit Amit K. , Loughman Robert P. , Vernier Jean-Paul 
Published: March 2023

The tropical upper troposphere and lower stratosphere (UTLS) region is
dominated by aerosols and clouds affecting Earth’s radiation budget
and climate. Thus, satellites’ continuous monitoring and
identification of these layers is crucial for quantifying their
radiative impact. However, distinguishing between aerosols and clouds
is challenging, especially under the perturbed UTLS conditions during
post-volcanic eruptions and wildfire events. Aerosol-cloud
discrimination is primarily based on their disparate
wavelength-dependent scattering and absorption properties. In this
study, we use aerosol extinction observations in the tropical
(15°N-15°S) UTLS from June 2017 to February 2021, available from the
latest generation of the Stratospheric Aerosol and Gas Experiment
(SAGE) instrument-SAGE III onboard the International Space Station
(ISS) to study aerosols and clouds. During this period, the SAGE
III/ISS provided better coverage over the tropics at additional
wavelength channels (relative to previous SAGE missions) and witnessed
several volcanic and wildfire events that perturbed the tropical UTLS.
We explore the advantage of having an extinction coefficient at an
additional wavelength channel (1550 nm) from the SAGE III/ISS
in aerosol-cloud discrimination using a method based on thresholds of
two extinction coefficient ratios, R1 (520 nm/1020 nm) and R2 (1020 nm/1550 nm). This
method was proposed earlier by Kent et
al. [Appl.
Opt. 36, 8639
(1997)APOPAI0003-693510.1364/AO.36.008639] for the SAGE
III-Meteor-3M but was never tested for the tropical region under
volcanically perturbed conditions. We call this method the Extinction
Color Ratio (ECR) method. The ECR method is applied to the SAGE
III/ISS aerosol extinction data to obtain cloud-filtered aerosol
extinction coefficients, cloud-top altitude, and seasonal cloud
occurrence frequency during the entire study period. Cloud-filtered
aerosol extinction coefficient obtained using the ECR method revealed
the presence of enhanced aerosols in the UTLS following volcanic
eruptions and wildfire events consistent with the Ozone Mapping and
Profiler Suite (OMPS) and space-borne lidar-Cloud-Aerosol Lidar with
Orthogonal Polarization (CALIOP). The cloud-top altitude obtained from
the SAGE III/ISS is within 1 km of the nearly co-located
observations from OMPS and CALIOP. In general, the seasonal mean
cloud-top altitude from the SAGE III/ISS events peaks during the
December, January, and February months, with sunset events showing
higher cloud tops than the sunrise events, indicating the seasonal and
diurnal variation of the tropical convection. The seasonal altitude
distribution of cloud occurrence frequency obtained from the SAGE
III/ISS also agrees well with CALIOP observations within 10%. We show
that the ECR method is a simple approach that relies on thresholds
independent of the sampling period, providing cloud-filtered aerosol
extinction coefficients uniformly for climate studies irrespective of
the UTLS conditions. However, since the predecessor of SAGE III did
not include a 1550 nm channel, the usefulness of this approach
is limited to short-term climate studies after 2017.