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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 17, 899-910, 2017
https://doi.org/10.5194/acp-17-899-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
20 Jan 2017
Nocturnal low-level clouds over southern West Africa analysed using high-resolution simulations
Bianca Adler, Norbert Kalthoff, and Leonhard Gantner Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
Abstract. We performed a high-resolution numerical simulation to study the development of extensive low-level clouds that frequently form over southern West Africa during the monsoon season. This study was made in preparation for a field campaign in 2016 within the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) project and focuses on an area around the city of Savè in southern Benin. Nocturnal low-level clouds evolve a few hundred metres above the ground around the same level as a distinct low-level jet. Several processes are found to determine the spatio-temporal evolution of these clouds including (i) significant cooling of the nocturnal atmosphere caused by horizontal advection with the south-westerly monsoon flow during the first half of the night, (ii) vertical cold air advection due to gravity waves leading to clouds in the wave crests and (iii) enhanced convergence and upward motion upstream of existing clouds that trigger new clouds. The latter is caused by an upward shift of the low-level jet in cloudy areas leading to horizontal convergence in the lower part and to horizontal divergence in the upper part of the cloud layer. Although this single case study hardly allows for a generalisation of the processes found, the results added to the optimisation of the measurements strategy for the field campaign and the observations will be used to test the hypotheses for cloud formation resulting from this study.

Citation: Adler, B., Kalthoff, N., and Gantner, L.: Nocturnal low-level clouds over southern West Africa analysed using high-resolution simulations, Atmos. Chem. Phys., 17, 899-910, https://doi.org/10.5194/acp-17-899-2017, 2017.
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Short summary
This study deals with the development of nocturnal low-level clouds, which frequently form over southern West Africa during the monsoon season. We performed numerical simulations with the COSMO model and identified processes possibly relevant to cloud formation. The study was made in preparation of a field campaign, which took place in summer 2016 within the DACCIWA project and the results contributed to the optimisation of the measurement strategy during the campaign.
This study deals with the development of nocturnal low-level clouds, which frequently form over...
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