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

Abstract

<jats:p>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. </jats:p>

Sensitivity of low-level clouds and precipitation to anthropogenic aerosol emission in southern West Africa: a DACCIWA case study

“…In Savè, there is a clear diurnal cycle of LLC because they form at low altitude (few hundred meters) during the night, get elevated by the increase of the PBL height until noon, and break up between 12:00 and 18:00 UTC (Dione et al, 2019;Zouzoua et al, 2020). We note that in the morning, the clouds are always present, mostly with CBH below 500 m amsl.…”

“…From 12:00 to 18:00 UTC, the variability (shown by the standard deviation, vertical bars in Fig. 7) of CBH increases, because LLC break up and dissipate around this time (Dione et al, 2019;Zouzoua et al, 2020). From 18:00 to 00:00 UTC, less LLC are observed.…”

“…During the months of June and July, the maritime air from the Atlantic reaches Savè (Benin), one of the DACCIWA supersites about 185 km north of the coast, between 16:00 and 21:00 UTC associated with the apparition of the nocturnal low-level jet and the LLC (Dione et al, 2019;Zouzoua et al, 2020). Between 22:00 UTC and 06:00 UTC, the LLC layer continues it 2 https://doi.org/10.5194/acp-2020-1306 Preprint.…”

On the interactions between clouds, radiation, turbulence and vegetation in the atmospheric boundary layer

“…The cloud cover is calculated as the percentage of cloud base height measurements below 1000 m Zouzoua, 2019). The values are averaged over 19 days during the campaign to prevent too high variability by single point observations of individual days.…”

“…The same pattern for cloud cover is shown by observations, although one hour in advance. Note, however, that the observations of cc are averaged over 19 days timeseries selected due to a cloud onset before 4 00 UTC (Zouzoua, 2019). The variability between the days considered in the average also explains the cc values below 1 before 6 00 UTC and after 8 30 UTC.…”