Satellite inference of water vapour and above-cloud aerosol combined effect on radiative budget and cloud-top processes in the southeastern Atlantic Ocean

摘要

Aerosols have a direct effect on the Earth's radiative budget andcan also affect cloud development and lifetime, and the aerosols aboveclouds (AAC) are particularly associated with high uncertainties in globalclimate models. Therefore, it is a prerequisite to improve the descriptionand understanding of these situations. During the austral winter, largeloadings of biomass burning aerosols originating from fires in the southernAfrican subcontinent are lifted and transported westwards, across thesoutheastern Atlantic Ocean. The negligible wet scavenging of these absorbingaerosols leads to a near-persistent smoke layer above one of the largeststratocumulus cloud decks on the planet. Therefore, the southeastern Atlanticregion is a very important area for studying the impact of above-cloudabsorbing aerosols, their radiative forcing and their possible effects onclouds. In this study we aim to analyse and quantify the effect of smoke loadings oncloud properties using a synergy of different remote sensing techniques fromA-Train retrievals (methods based on the passive instruments POLDER andMODIS and the operational method of the spaceborne lidar CALIOP), collocatedwith ERA-Interim re-analysis meteorological profiles. To analyse thepossible mechanisms of AAC effects on cloud properties, we developed ahigh and low aerosol loading approach, which consists in evaluating the change inradiative quantities (i.e. cloud-top cooling, heating rate verticalprofiles) and cloud properties with the smoke loading. During this analysis,we account for the variation in the meteorological conditions over oursample area by selecting the months associated with one meteorological regime(June–August). The results show that the region we focus on is primarily under theenergetic influence of absorbing aerosols, leading to a significant positiveshortwave direct effect at the top of the atmosphere. For larger loads ofAACs, clouds are optically thicker, with an increase in liquid water path of20 g m−2 and lower cloud-top altitudes by 100 m. These results do notcontradict the semi-direct effect of above-cloud aerosols, explored inprevious studies. Furthermore, we observe a strong covariance between theaerosol and the water vapour loadings, which has to be accounted for. Adetailed analysis of the heating rate profiles shows that within the smokelayer, the absorbing aerosols are 90 % responsible for warming the ambientair by approximately 5.7 K d−1. The accompanying water vapour, however, hasa longwave effect at distance on the cloud top, reducing its cooling byapproximately 4.7 K d−1 (equivalent to 7 %). We infer that this decreasedcloud-top cooling in particular, in addition with the higher humidity abovethe clouds, might modify the cloud-top entrainment rate and its effect,leading to thicker clouds. Therefore, smoke (the combination of aerosol andwater vapour) events would have the potential to modify and probablyreinforce the underlaying cloud cover.