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Discussion papers
https://doi.org/10.5194/acp-2019-566
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-566
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Jul 2019

Research article | 05 Jul 2019

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

Conceptual model of diurnal cycle of stratiform low-level clouds over southern West Africa

Fabienne Lohou1, Norbert Kalthoff2, Bianca Adler2, Karmen Babić2, Cheikh Dione1, Marie Lothon1, Xabier Pedruzo-Bagazgoitia3, and Maurin Zouzoua4 Fabienne Lohou et al.
  • 1Laboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, France
  • 2Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Germany
  • 3Wageningen University and Research, The Netherlands
  • 4Université Felix Houphouët Boigny, Ivory Coast

Abstract. DACCIWA (Dynamics Aerosol Chemistry Cloud Interactions in West Africa) project and the associated ground-based field experiment, which took place during the summer 2016, provided a comprehensive dataset on the low-level stratiform clouds (LLC) which develop almost every night over southern West Africa. The LLC, inaccurately represented in the climate and weather forecasts, form in the monsoon flow during the night and break up the day after, affecting considerably the radiation budget. The DACCIWA field experiment dataset supports several published studies which give an overview of the measurements during the campaign, analyze the dynamical features in which the LLC develop, and quantify the processes involved in the LLC formation. Based on the main results of these studies and new analyses, we propose in this paper a conceptual model of the diurnal cycle of the LLC over southern West Africa. Four main phases compose the diurnal cycle of the LLC. The stable and the jet phases are the two steps during which the relative humidity increases, due to the cooling of the air, until the air is saturated and the LLC form. The horizontal advection of cold air from the Guinean coast by the maritime inflow and the nocturnal low level jet (NLLJ) represents 50 % of the total cooling. The remaining half is mainly due to divergence of net radiation and turbulence flux. The third step of the LLC diurnal cycle is the stratus phase which starts during the night and lasts until the onset of buoyancy driven turbulence on the following day. During the stratus phase, interactions between the LLC and NLLJ imply a modification of the wind speed vertical profile in the cloud layer, and a mixing of the subcloud layer by shear-driven turbulence below the NLLJ core. The breakup of the LLC occurs during the convective phase and can follow three different scenarios which depend on the intensity of the shear-driven turbulence observed during the night. The breakup time has a considerable impact on the energy balance of the Earth's surface and, consequently, on the depth of the convective boundary layer, which could vary by a factor of two from day-to-day.

Fabienne Lohou et al.
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Fabienne Lohou et al.
Data sets

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

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

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

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

Fabienne Lohou et al.
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