Satellite inference of water vapour and above-cloud aerosol combined effect on radiative budget and cloud-top processes in the southeastern Atlantic Ocean
Authors: Deaconu Lucia T. , Ferlay Nicolas , Waquet Fabien , Peers Fanny , Thieuleux François , Goloub Philippe 
Published: September 2019

Abstract. Aerosols have a direct effect on the Earth's radiative budget and
can also affect cloud development and lifetime, and the aerosols above
clouds (AAC) are particularly associated with high uncertainties in global
climate models. Therefore, it is a prerequisite to improve the description
and understanding of these situations. During the austral winter, large
loadings of biomass burning aerosols originating from fires in the southern
African subcontinent are lifted and transported westwards, across the
southeastern Atlantic Ocean. The negligible wet scavenging of these absorbing
aerosols leads to a near-persistent smoke layer above one of the largest
stratocumulus cloud decks on the planet. Therefore, the southeastern Atlantic
region is a very important area for studying the impact of above-cloud
absorbing aerosols, their radiative forcing and their possible effects on
clouds. In this study we aim to analyse and quantify the effect of smoke loadings on
cloud properties using a synergy of different remote sensing techniques from
A-Train retrievals (methods based on the passive instruments POLDER and
MODIS and the operational method of the spaceborne lidar CALIOP), collocated
with ERA-Interim re-analysis meteorological profiles. To analyse the
possible mechanisms of AAC effects on cloud properties, we developed a
high and low aerosol loading approach, which consists in evaluating the change in
radiative quantities (i.e. cloud-top cooling, heating rate vertical
profiles) and cloud properties with the smoke loading. During this analysis,
we account for the variation in the meteorological conditions over our
sample area by selecting the months associated with one meteorological regime
(June–August). The results show that the region we focus on is primarily under the
energetic influence of absorbing aerosols, leading to a significant positive
shortwave direct effect at the top of the atmosphere. For larger loads of
AACs, clouds are optically thicker, with an increase in liquid water path of
20 g m−2 and lower cloud-top altitudes by 100 m. These results do not
contradict the semi-direct effect of above-cloud aerosols, explored in
previous studies. Furthermore, we observe a strong covariance between the
aerosol and the water vapour loadings, which has to be accounted for. A
detailed analysis of the heating rate profiles shows that within the smoke
layer, the absorbing aerosols are 90 % responsible for warming the ambient
air by approximately 5.7 K d−1. The accompanying water vapour, however, has
a longwave effect at distance on the cloud top, reducing its cooling by
approximately 4.7 K d−1 (equivalent to 7 %). We infer that this decreased
cloud-top cooling in particular, in addition with the higher humidity above
the clouds, might modify the cloud-top entrainment rate and its effect,
leading to thicker clouds. Therefore, smoke (the combination of aerosol and
water vapour) events would have the potential to modify and probably
reinforce the underlaying cloud cover.