Breakup of nocturnal low-level stratiform clouds during the southern West African monsoon season
Authors: Zouzoua Maurin , Lohou Fabienne , Assamoi Paul , Lothon Marie , Yoboue Véronique , Dione Cheikh , Kalthoff Norbert , Adler Bianca , Babić Karmen , Pedruzo-Bagazgoitia Xabier , Derrien Solène 
Published: February 2021

Abstract. Within the framework of the DACCIWA
(Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) project
and based on a field experiment conducted in June and July 2016, we analyze
the daytime breakup of continental low-level stratiform clouds in southern
West Africa. We use the observational data gathered during 22
precipitation-free occurrences at Savè, Benin. Our analysis, which
starts from the stratiform cloud formation usually at night, focuses on
the role played by the coupling between cloud and surface in the transition
towards shallow convective clouds during daytime. It is based on several
diagnostics, including the Richardson number and various cloud macrophysical
properties. The distance between the cloud base height and lifting
condensation level is used as a criterion of coupling. We also make an
attempt to estimate the most predominant terms of the liquid water path
budget in the early morning. When the nocturnal low-level stratiform cloud forms, it is decoupled from
the surface except in one case. In the early morning, the cloud is found
coupled with the surface in 9 cases and remains decoupled in the 13
other cases. The coupling, which occurs within the 4 h after cloud
formation, is accompanied by 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 coupled cases. The moisture jump at the cloud top is usually
found to be lower than 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 in which the stratiform cloud
develops over West Africa. No significant difference in liquid water path
budget terms was found between coupled and decoupled cases. In agreement
with previous numerical studies, we found that the stratiform cloud
maintenance before sunrise results from the interplay between the
predominant radiative cooling, entrainment and large-scale subsidence at its top. Three transition scenarios were observed depending on the state of coupling
at the initial stage. In coupled cases, the low-level stratiform cloud remains
coupled until its breakup. In five of the decoupled cases, the cloud couples
with the surface as the lifting condensation level rises. In the eight
remaining cases, the stratiform cloud remains hypothetically decoupled from
the surface throughout its life cycle since the height of its base remains
separated from the condensation level. In cases 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 cases. Thus, we argue that the coupling with the surface during
daytime hours has a crucial role in the low-level stratiform cloud
maintenance and its transition towards shallow convective clouds.