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https://doi.org/10.5194/acp-2020-602
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-602
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  01 Jul 2020

01 Jul 2020

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This preprint is currently under review for the journal ACP.

Breakup of nocturnal low-level stratiform clouds during southern West African Monsoon Season

Maurin Zouzoua1, Fabienne Lohou2, Paul Assamoi1, Marie Lothon2, Véronique Yoboue1, Cheikh Dione3, Norbert Kalthoff4, Bianca Adler4, Karmen Babić4, and Xabier Pedruzo-Bagazgoitia5 Maurin Zouzoua et al.
  • 1Laboratoire de Physique de l'atmosphère et de Mécanique des fluides, Université Félix Houphouët Boigny, Abidjan, Côte d'Ivoire
  • 2Laboratoire d’Aérologie, Université de Toulouse, CNRS, UPS, Toulouse, France
  • 3African Centre of Meteorological Applications for Development, Niamey, Niger
  • 4Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 5Meterology and Air Quality Group, Wageningen University and Research, Wageningen, the Netherlands

Abstract. Within the framework of the DACCIWA (Dynamics-Aerosol-Chemistry-Cloud-Interactions over West Africa) project, and based on a field experiment conducted in June and July 2016, we analyse the daytime breakup of the continental low-level stratiform clouds in southern West Africa. We use the observational data gathered during twenty-two precipitation-free occurrences at Savè supersite, in Benin. Our analysis, which starts since the stratiform cloud formation usually at night, focuses on the role played by the coupling between the cloud and the surface in the transition towards shallow convective clouds. It is based on several diagnostics, including Richardson number and various cloud macrophysical properties. The distance between lifting condensation level and cloud base height is used as a criterion of coupling. We also make an attempt to estimate the most predominant terms of the liquid water path budget on early morning.

When the nocturnal low-level stratiform cloud forms, it is decoupled from the surface, except in one case. On early morning, the cloud is found coupled with the surface in nine cases and is remained decoupled in the thirteen other cases. The coupling, which occurs within the four hours after the cloud formation, is accompanied with a cloud base lowering and near-neutral thermal stability in the subcloud layer. Further, at the initial stage of the transition, the stratiform cloud base is slightly cooler, wetter and more homogeneous in the coupled cases. The moisture jump at cloud top is found usually around 2 g kg−1, and the temperature jump within 1–5 K, which is significantly smaller than typical marine stratocumulus, and explained by the monsoon flow environment within which the stratiform cloud develops. No significant difference of liquid water path budget terms was found between the coupled and decoupled cases. In agreement with previous numerical studies, we found that the stratiform cloud maintenance before the sunrise results from the interplay between the predominant radiative cooling, and, the entrainment and large scale subsidence at its top.

Three transition scenarios were observed, depending on the state of the coupling at the initial stage. In the coupled cases, the low-level stratiform cloud remains coupled until its break up. In five of the decoupled cases, the cloud couples with the surface as the LCL is rising. In the eight remaining cases, the stratiform cloud remains decoupled from the surface all along its life cycle. In case of coupling during the transition, the stratiform cloud base lifts with the growing convective boundary layer roughly between 06:30 and 08:00 UTC. The cloud deck breakup occurring at 11:00 UTC or later leads to the formation of shallow convective clouds. When the decoupling subsists, shallow cumulus clouds form below the stratiform cloud deck between 06:30 and 09:00 UTC. The breakup time in this scenario has a stronger variability, and occurs before 11:00 UTC in most of the cases. Thus we argue that the coupling with the surface during the daytime hours has a crucial role in the low-level stratiform cloud maintenance and in its transition towards shallow convective clouds.

Maurin Zouzoua et al.

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Maurin Zouzoua et al.

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Short summary
Based on a field experiment conducted in June–July 2016, we analyzed the daytime breakup of continental low-level stratiform clouds over southern West Africa in order to provide a complementary guidance for model evaluation during Monsoon season. The cloud exhibits weaker temperature and moisture jumps at the top as compared to marine stratiform clouds. The coupling with the surface during the daytime hours has a crucial role in its maintenance and transition towards shallow convective clouds.
Based on a field experiment conducted in June–July 2016, we analyzed the daytime breakup of...
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