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In this study, we provide a comprehensive analysis of aerosol interactionwith warm boundary layer clouds over the South-East Atlantic. We use aerosoland cloud parameters derived from MODIS observations, together withco-located CALIPSO estimates of the layer altitudes, to derive statisticalrelationships between aerosol concentration and cloud properties. TheCALIPSO products are used to differentiate between cases of mixedcloud-aerosol layers from cases where the aerosol is located well-above thecloud top. This technique allows us to obtain more reliable estimates ofthe aerosol indirect effect than from simple relationships based on verticallyintegrated measurements of aerosol and cloud properties. Indeed, it permits usto somewhat distinguish the effects of aerosol and meteorology on theclouds, although it is not possible to fully ascertain the relativecontribution of each on the derived statistics.  Consistently with the results from previous studies, our statistics clearlyshow that aerosol affects cloud microphysics, decreasing the Cloud DropletRadius (CDR). The same data indicate a concomitant strong decrease in cloudLiquid Water Path (LWP), which is inconsistent with the hypothesis ofaerosol inhibition of precipitation (Albrecht, 1989). We hypothesise thatthe observed reduction in LWP is the consequence of dry air entrainment atcloud top. The combined effect of CDR decrease and LWP decrease leads torather small sensitivity of the Cloud Optical Thickness (COT) to an increasein aerosol concentration. The analysis of MODIS-CALIPSO coincidencesalso evidences an aerosol enhancement of low cloud cover. Surprisingly, the CloudFraction (CLF) response to aerosol invigoration is much stronger when(absorbing) particles are located above cloud top than in cases of physicalinteraction. This result suggests a relevant aerosol radiative effect on lowcloud occurrence: absorbing particles above the cloud top may heat thecorresponding atmosphere layer, decrease the vertical temperature gradient,increase the low tropospheric stability and provide favourable conditions forlow cloud formation.  We also analyse the impact of anthropogenic aerosols on precipitation,through the statistical analysis of CDR-COT co-variations. A COT value of 10is found to be the threshold beyond which precipitation is mostly formed, inboth clean and polluted environments. For larger COT, polluted clouds showevidence of precipitation suppression.  Results suggest the presence of two competing mechanisms governing LWPresponse to aerosol invigoration: a drying effect due to aerosol enhancedentrainment of dry air at cloud top (predominant for optically thin clouds)and a moistening effect due to aerosol inhibition of precipitation(predominant for optically thick clouds)

Aerosol indirect effect on warm clouds over South-East Atlantic, from co-located MODIS and CALIPSO observations