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Discussion papers
https://doi.org/10.5194/acp-2019-537
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-537
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Jun 2019

Research article | 07 Jun 2019

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

What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?

Karmen Babić1, Norbert Kalthoff1, Bianca Adler1, Julian F. Quinting1, Fabienne Lohou2, Cheikh Dione3, and Marie Lothon2 Karmen Babić et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 2Laboratoire d’Aérologie, Université de Toulouse, CNRS, UPS, Toulouse, France
  • 3African Center for Meteorological Applications for Development, Niamey, Niger

Abstract. Nocturnal low-level stratus clouds (LLC) are frequently observed in the atmospheric boundary layer (ABL) over southern West Africa (SWA) during the summer monsoon season. Considering the effect these clouds have on the surface energy and radiation budgets as well as on the diurnal cycle of the ABL, they are undoubtedly important for the regional climate. However, an adequate representation of LLC in the state–of–the–art weather and climate models is still a challenge, which is largely due to the lack of high-quality observations in this region. In several recent studies, a unique and comprehensive data set collected in summer 2016 during the DACCIWA (Dynamics-Aerosol-Cloud-Chemistry Interactions in West Africa) ground-based field campaign was used for the first observational analyses of the parameters and physical processes relevant for the LLC formation over SWA. However, occasionally stratus-free nights occur during the monsoon season as well. Using observations and ERA5 reanalysis, we investigate differences in the boundary layer conditions during 6 stratus-free and 20 stratus nights observed during the DACCIWA campaign. Our results suggest that the interplay between three major mechanisms is crucial for the formation of LLC during the monsoon season: (i) the onset time and strength of the nocturnal low-level jet (NLLJ), (ii) horizontal cold-air advection and (iii) background moisture level. Namely, weaker or later onset of NLLJ leads to reduced contribution from horizontal cold-air advection. This in turn results in a weaker cooling and thus saturation is not reached. Such deviation in the dynamics of NLLJ is related to the arrival of cold air mass propagating northwards from the coast called Gulf of Guinea maritime inflow. Additionally, stratus-free nights occur when the intrusions of dry air masses, originating from e.g. central or south Africa, reduce the background moisture over the large parts of SWA. Based on the backward trajectories analysis, another possible reason for clear nights is descending of air originating from drier levels above the marine boundary layer.

Karmen Babić et al.
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Data sets

DACCIWA field campaign, Savè super-site, UPS instrumentation S. Derrien, Y. Bezombes, B. Bret, O. Gabella, C. Jarnot, P. Medina, E. Piques, C. Delon, C. Dione, B. Campistron, P. Durand, C. Jambert, F. Lohou, M. Lothon, F. Pacifico, F., and Y. Meyerfeld https://doi.org/10.6096/dacciwa.1618

DACCIWA field campaign, Savè super-site, Cloud and precipitation J. Handwerker, S. Scheer, and T. Gamer https://doi.org/10.6096/dacciwa.1686

DACCIWA field campaign, Savè super-site, Surface measurements M. Kohler, N. Kalthoff, J. Seringer, and S. Kraut https://doi.org/10.6096/dacciwa.1690

Radiosonde - All sites M. Maranan and A. Fink https://doi.org/10.6096/baobab-dacciwa.1656

DACCIWA field campaign, Savè super-site, Thermodynamic data sets A. Wieser, B. Adler, and B. Deny https://doi.org/10.6096/dacciwa.1659

Karmen Babić et al.
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Short summary
This study investigates differences in atmospheric conditions between nights with and without low-level stratus clouds (LLC) over southern West Africa. We use high-quality observations collected during 2016 summer monsoon season and ERA5 reanalysis data set. Our results show that the formation of LLC depends on the interplay between the onset time and strength of the nocturnal low-level jet, horizontal cold-air advection and the overall moisture level in the whole region.
This study investigates differences in atmospheric conditions between nights with and without...
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