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Volume 18, issue 19 | Copyright

Special issue: Results of the project "Dynamics–aerosol–chemistry–cloud...

Atmos. Chem. Phys., 18, 14271-14295, 2018
https://doi.org/10.5194/acp-18-14271-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Oct 2018

Research article | 09 Oct 2018

Aerosol liquid water content in the moist southern West African monsoon layer and its radiative impact

Konrad Deetz1, Heike Vogel1, Sophie Haslett2, Peter Knippertz1, Hugh Coe2, and Bernhard Vogel1 Konrad Deetz et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 2National Centre for Atmospheric Science, and School of Earth and Environmental Sciences, University of Manchester, Manchester, UK

Abstract. 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 diurnal cycle in SWA, the analysis is separated into three characteristic phases: (a) the Atlantic inflow progression phase (15:00–02:00UTC), when winds from the Gulf of Guinea accelerate in the less turbulent evening and nighttime boundary layer, (b) the moist morning phase (03:00–08:00UTC), when the passage of the Atlantic inflow front leads to overall cool and moist conditions over land, and (c) the daytime drying phase (09:00–15:00UTC), in which the Atlantic inflow front reestablishes with the inland heating initiated after sunrise. This diurnal cycle also impacts, via relative humidity, the aerosol liquid water content. We analyzed the impact of relative humidity and clouds on the aerosol liquid water content. As shown by other studies, 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 in 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 20Wm−2, while longwave effects appear to be insignificant, especially during nighttime. The estimated relationships between total column aerosol liquid water and radiation are −305±39Wg−1 (shortwave in-cloud), −114±42Wg−1 (shortwave off-cloud) and about −10Wg−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.

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Water uptake can significantly increase the size and therefore alters the optical properties of aerosols. Our model study reveals that the high moisture and aerosol burden in the southern West African monsoon layer makes it favorable to quantify properties that determine the aerosol liquid water content and its impact on the aerosol optical depth and radiative transfer. Especially in moist tropical environments the relative humidity impact on AOD has to be considered in atmospheric models.
Water uptake can significantly increase the size and therefore alters the optical properties of...
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