Interactive comment on “ Aerosol liquid water content in the moist southern West African monsoon layer and its radiative impact ” by

Water uptake can significantly increase the size and therefore alters the optical properties of aerosols. In this study, the regional-scale model framework COSMO-ART is applied to Southern West Africa (SWA) for a summer monsoon process study on 2–3 and 6–7 July 2016. The high moisture and aerosol burden in the monsoon layer makes SWA favorable to quantify properties that determine the aerosol liquid water content and its impact on radiative transfer. Given the marked 5 diurnal cycle in SWA, the analysis is separated into three characteristic phases: (a) Atlantic Inflow progression phase (152 UTC), when winds from the Gulf of Guinea accelerate in the less turbulent evening and nighttime boundary layer, (b) Moist morning phase (3-8 UTC), when the passage of the Atlantic Inflow front leads to overall cool and moist conditions over land and (c) Daytime drying phase (9-15 UTC), in which the Atlantic Inflow front re-establishes with the inland heating initiated after sunrise. This diurnal cycle imprints, via the relative humidity, also the aerosol liquid water content. We 10 analyzed the impact of relative humidity and clouds on the aerosol liquid water content. As shown by other studies, the accumulation mode particles are the dominant contributor of aerosol liquid water. We find aerosol growth factors of 2 (4) for submicron (coarse) mode particles, leading to a substantial increase of mean aerosol optical depth from 0.2 to 0.7. Considering the aerosol liquid water content leads to a decrease in shortwave radiation of about 20 W m−2, while longwave effects appear to be insignificant, especially during nighttime. The estimated relationships between total column aerosol liquid wa15 ter and radiation are -305±39 W g−1 (shortwave in-cloud), -114±42 W g−1 (shortwave off-cloud) and about -10 W g−1 (longwave). The results highlight the need to consider the relative humidity dependency of aerosol optical depth in atmospheric models, particularly in moist tropical environments, where their effect on radiation can be very large. List of acronyms used in this study.Acronym DescriptionACC Accumulation modeADE Aerosol Direct EffectAGL Above Ground LayerAI Atlantic InflowAIE Aerosol Indirect EffectAIT Aitken modeALWC Aerosol Liquid Water ContentAOD Aerosol Optical 20 DepthASL Above Sea LevelCDNC Cloud Droplet Number ConcentrationCOARSE Coarse modeCOSMO-ART Consortium for Small-scale Modeling Aerosol and Reactive Trace gasesDACCIWA Dynamics-aerosol-chemistry-cloud interactions in West AfricaDWD Deutscher Wetterdienst (German Weather Service)ECDF Empirical Cumulative Distribution FunctionGF Growth FactorGRAALS General Radiative Algorithm Adapted to Linear-type Solutions radiation schemeHaChi Haze in China campaignICA In-Cloud AreaICON Icosahedral Nonhydrostatic ModelNo-ALWC Model realization neglecting ALWC in the 25 radiation calculationOCA Off-Cloud AreaPBL Planetary Boundary LayerPOA Primary Organic AerosolReference Reference