What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?

Babić, Karmen; Kalthoff, Norbert; Adler, Bianca; Quinting, Julian; Lohou, Fabienne; Dione, Cheikh

doi:10.5194/acp-19-13489-2019

Cited by 11 publications

(11 citation statements)

References 25 publications

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“…Schrage and Fink (2012) and Schuster et al (2013) suggested a dominant role played by the NLLJ on the LLSC formation over southern West Africa, because it intensifies the cold air advection and generates shear-driven turbulent mixing. The NLLJ typically forms over land at the end of the day when daytime buoyancy-driven turbulence ceases and the Coriolis force predominates, accelerating the wind towards low pressure (Blackadar, 1957). Although the NLLJ is observed almost every night in West Africa during the monsoon season (Parker et al, 2005;Lothon et al, 2008), this formation mechanism may not be applicable at these low latitudes where the Coriolis force is weak.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2020

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Abstract. The DACCIWA (Dynamics Aerosol Chemistry Cloud Interactions in West Africa) project and the associated ground-based field experiment, which took place during summer 2016, provided a comprehensive dataset on the low-level stratiform clouds (LLSCs), which develop almost every night over southern West Africa. The LLSCs, inaccurately represented in climate and weather forecasts, form in the monsoon flow during the night and break up during the following morning or afternoon, affecting considerably the radiation budget. Several published studies give an overview of the measurements during the campaign, analyse the dynamical features in which the LLSCs develop, and quantify the processes involved in the LLSC 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 LLSCs over southern West Africa. Four main phases compose the diurnal cycle of the LLSC. The stable and the jet phases are the two steps during which the relative humidity increases, due to cooling of the air, until the air is saturated and the LLSCs form. Horizontal advection of cold air from the Guinean coast by the maritime inflow and the nocturnal low-level jet (NLLJ) represents 50 % of the local total cooling. The remaining half is mainly due to divergence of net radiation and turbulence flux. The third step of the LLSC diurnal cycle is the stratus phase, which starts during the night and lasts until the onset of surface-buoyancy-driven turbulence on the following day. During the stratus phase, interactions between the LLSCs and the NLLJ lead to a modification of the wind speed vertical profile in the cloud layer, and a mixing of the sub-cloud layer by shear-driven turbulence below the NLLJ core. The breakup of the LLSC occurs during the convective phase and follows three different scenarios which depend on the intensity of the turbulence observed during the night in the sub-cloud layer. 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 2 from day-to-day.

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Section: Introductionmentioning

confidence: 99%

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

et al. 2020

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“…The Sc-Cu transition in other climatological regions was the subject of several studies, most of which were performed over the ocean (e.g., Bretherton et al, 1999;Duynkerke et al, 2004;Sandu and Stevens, 2011;de Roode et al, 2016;Mohrmann et al, 2019;Sarkar et al, 2019) and a few over land (e.g., Price, 1999;Ghonima et al, 2016). In these studies, the stratocumulus is initially coupled with the surface, with convective turbulence produced by the cloud-top radiative cooling.…”

Section: Reviewmentioning

confidence: 99%

Breakup of nocturnal low-level stratiform clouds during the southern West African monsoon season

et al. 2021

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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.

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“…The main process behind this cooling is the horizontal advection of cooler air from Guinea coast, due to the combination of a maritime inflow (MI) (Adler et al, 2017;Deetz et al, 2018) and the NLLJ (Schrage and Fink, 2012;Dione et al, 2019). The onset time and the strength of the NLLJ, as well as the level of background humidity in the ABL, are crucial for the LLSC formation (Babić et al, 2019b). Indeed, from two cases study, Babić et al (2019b) showed that weaker and later NLLJ onset leads to a reduced cooling, so that the saturation within the ABL may not be reached.…”

Section: Sate Of Artmentioning

confidence: 99%

“…The onset time and the strength of the NLLJ, as well as the level of background humidity in the ABL, are crucial for the LLSC formation (Babić et al, 2019b). Indeed, from two cases study, Babić et al (2019b) showed that weaker and later NLLJ onset leads to a reduced cooling, so that the saturation within the ABL may not be reached. The formation of the LLSC marks the end of the jet phase and the beginning of the stratus phase.…”

Section: Sate Of Artmentioning

confidence: 99%

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

Zouzoua¹,

Lohou²,

Assamoi³

et al. 2020

Preprint

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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.

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What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?

Cited by 11 publications

References 25 publications

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

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

Breakup of nocturnal low-level stratiform clouds during the southern West African monsoon season

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

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