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Discussion papers
https://doi.org/10.5194/acp-2018-775
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-775
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Sep 2018

Research article | 05 Sep 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes

Bianca Adler1, Karmen Babić1, Norbert Kalthoff1, Fabienne Lohou2, Marie Lothon2, Cheikh Dione2, Xabier Pedruzo-Bagazgoitia3, and Hendrik Andersen1 Bianca Adler et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, German
  • 2Laboratoire d’Aérologie, Université de Toulouse, CNRS, UPS, Toulouse France
  • 3Meteorology and Air Quality Group, Wageningen University and Research, Wageningen, the Netherlands

Abstract. During the West African summer Monsoon season, extended nocturnal stratiform low-level clouds (LLC) frequently form in the atmospheric boundary layer over southern West Africa and persist long into the following day affecting the regional climate. A unique data set was gathered within the framework of the Dynamics-Aerosol-Chemistry-Cloud-Interactions in West Africa (DACCIWA) project, which allows, for the first time, for an observational analysis of the processes and parameters decisive for LLC formation. In this study, in situ and remote sensing measurements from radiosondes, ceilometer, cloud radar and energy balance stations from a measurement site near Savè in Benin are analyzed amongst others for 11 nights. The aim is to study LLC characteristics, the intra-night variability of boundary layer conditions and physical processes relevant for LLC formation, as well as to assess the importance of these processes. Typical nocturnal phases are identified and mean profiles are calculated for the individual phases revealing pronounced differences: a stable surface inversion, which forms after sunset, is eroded by differential horizontal cold air advection with the Gulf of Guinea maritime inflow, a cool air mass propagating northwards from the coast in the late afternoon and the evening, and shear-generated turbulence related to a nocturnal low-level jet. The analysis of the contributions to the relative humidity changes before the LLC formation reveals that cooling in the atmospheric boundary layer is decisive to reach saturation, while moisture changes play a minor role. We quantify the heat budget terms and find that about 50% of the cooling prior to the LLC formation is caused by horizontal cold air advection, roughly 20% by radiative flux divergence and about 22% by sensible heat flux divergence in the presence of a low-level jet. The outcomes of this study contribute to the development of a conceptual model on LLC formation, maintenance and dissolution over southern West Africa.

Bianca Adler et al.
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This study deals with nocturnal stratiform low-level clouds which frequently from in the atmospheric boundary layer over southern West Africa. We use observational data from 11 nights to characterize the clouds and intra-night variability of boundary layers conditions as well as to assess the physical processes relevant for cloud formation. We find that cooling is more important than moistening in order to reach saturation and that horizontal advection of cool maritime air is decisive.
This study deals with nocturnal stratiform low-level clouds which frequently from in the...
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