Abstract. All-sky direct aerosol radiative effects (DARE) play a significant
yet still uncertain role in climate. This is partly due to poorly quantified
radiative properties of aerosol above clouds (AAC). We compute global
estimates of shortwave top-of-atmosphere DARE over opaque water clouds
(OWCs), DAREOWC, using observation-based aerosol and cloud radiative
properties from a combination of A-Train satellite sensors and a radiative
transfer model. There are three major differences between our
DAREOWC calculations and previous studies: (1) we use the
depolarization ratio method (DR) on CALIOP (Cloud–Aerosol Lidar with
Orthogonal Polarization) Level 1 measurements to compute the AAC frequencies
of occurrence and the AAC aerosol optical depths (AODs), thus introducing
fewer uncertainties compared to using the CALIOP standard product; (2) we
apply our calculations globally, instead of focusing exclusively on regional
AAC “hotspots” such as the southeast Atlantic; and (3) instead of the
traditional look-up table approach, we use a combination of satellite-based
sensors to obtain AAC intensive radiative properties. Our results agree with
previous findings on the dominant locations of AAC (south and northeast
Pacific, tropical and southeast Atlantic, northern Indian Ocean and northwest Pacific), the season of maximum occurrence and aerosol optical depths (a
majority in the 0.01–0.02 range and that can exceed 0.2 at 532 nm) across the globe. We find positive averages of global seasonal DAREOWC between
0.13 and 0.26 W m−2 (i.e., a warming effect on climate). Regional
seasonal DAREOWC values range from −0.06 W m−2 in the
Indian Ocean offshore from western Australia (in March–April–May) to
2.87 W m−2 in the southeast Atlantic (in
September–October–November). High positive values are usually paired with
high aerosol optical depths (>0.1) and low single scattering albedos (<0.94), representative of, for example, biomass burning aerosols. Because we use
different spatial domains, temporal periods, satellite sensors, detection
methods and/or associated uncertainties, the DAREOWC estimates in
this study are not directly comparable to previous peer-reviewed results.
Despite these differences, we emphasize that the DAREOWC estimates
derived in this study are generally higher than previously reported. The
primary reasons for our higher estimates are (i) the possible underestimate
of the number of dust-dominated AAC cases in our study; (ii) our use of Level
1 CALIOP products (instead of CALIOP Level 2 products in previous studies)
for the detection and quantification of AAC aerosol optical depths, which
leads to larger estimates of AOD above OWC; and (iii) our use of gridded
4∘×5∘ seasonal means of aerosol and cloud properties in our
DAREOWC calculations instead of simultaneously derived aerosol and
cloud properties from a combination of A-Train satellite sensors. Each of
these areas is explored in depth with detailed discussions that explain both
the rationale for our specific approach and the subsequent ramifications for our
DARE calculations.
Aerosol radiative effects with dual view AOD retrievals
