A new measurement approach for validating satellite-based above-cloud aerosol optical depth
Abstract. The retrieval of aerosol parameters from passive
satellite instruments in cloudy scenes is challenging, partly because clouds
and cloud-related processes may significantly modify aerosol optical depth
(AOD) and particle size, a problem that is further compounded by 3D
radiative processes. Recent advances in retrieval algorithms such as the
“color ratio” method, which utilizes the measurements at a shorter
(470 nm)
and a longer (860 nm) wavelength, have demonstrated the simultaneous
derivation of AOD and cloud optical depth (COD) for scenes in which absorbing
aerosols are found to overlay low-level cloud decks. This study shows
simultaneous retrievals of above-cloud aerosol optical depth (ACAOD) and
aerosol-corrected cloud optical depth (COD) from airborne measurements of
cloud-reflected and sky radiances using the color ratio method. These
airborne measurements were taken over marine stratocumulus clouds with
NASA's Cloud Absorption Radiometer (CAR) during the SAFARI 2000 field campaign
offshore of Namibia. The ACAOD is partitioned between the AOD below-aircraft
(AOD_cloudtop) and above-aircraft AOD (AOD_sky). The results show good agreement between AOD_sky and
sun-photometer measurements of the above-aircraft AOD. The results also show
that the use of aircraft-based sun-photometer measurements to validate
satellite retrievals of the ACAOD is complicated by the lack of information
on AOD below aircraft. Specifically, the CAR-retrieved AOD_cloudtop captures this “missing” aerosol layer caught between the aircraft
and cloud top, which is required to quantify above-cloud aerosol loading and
effectively validate satellite retrievals. In addition, the study finds a
strong anticorrelation between the AOD_cloudtop and COD for
cases in which COD   10 and a weaker anticorrelation for COD  >  10, which may be associated with the uncertainties in the color
ratio method at lower AODs and CODs. The influence of 3D radiative effects
on the retrievals is examined, and the results show that at cloud troughs, 3D
effects increase retrieved ACAOD by about 3 %–11 % and retrieved COD by
about 25 %. The results show that the color ratio method has little
sensitivity to 3D effects at overcast stratocumulus cloud decks. These
results demonstrate a novel airborne measurement approach for assessing
satellite retrievals of aerosols above clouds, thereby filling a major gap
in global aerosol observations.