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Discussion papers
https://doi.org/10.5194/acp-2018-1149
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1149
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Dec 2018

Research article | 05 Dec 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Low Level Cloud and Dynamical Features within the Southern West African Monsoon

Cheikh Dione1, Fabienne Lohou1, Marie Lothon1, Bianca Adler2, Karmen Babić2, Norbert Kalthoff2, Xabier Pedruzo-Bagazgoitia3, Yannick Bezombes1, and Omar Gabella1 Cheikh Dione et al.
  • 1Laboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, France
  • 2Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Germany
  • 3Wageningen University and Research, The Netherlands

Abstract. During the Boreal summer, the monsoon season that takes place in West Africa is accompanied by low stratus clouds over land, that stretch from the Guinean coast several hundred kilometers inland. These clouds form during the night and dissipate during the following day. Inherently linked with the diurnal cycle of monsoon flow, those clouds still remain poorly documented and understood.Moreover, numerical climate and weather models lack fine quantitative documentation of cloud macrophysical characteristics and the dynamical and thermodynamical structures occupying the lowest troposphere. The Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) field experiment, which took place in summer 2016, addresses this knowledge gap. Low level atmospheric dynamics and low-level cloud macrophysical properties are analyzed using in-situ and remote sensing continuous measurements collected from 20 June to 30 July at Savè, Benin, roughly 180km from the coast. The macrophysical characteristics of the stratus clouds are deduced from a ceilometer, an infrared cloud camera and cloud radar. Onset times, evolution, dissipation times, base heights and thickness are evaluated. The Data from a UHF (Ultra High Frequency) wind profiler, a microwave radiometer and an energy balance station are used to quantify the occurrence and characteristics of the monsoon flow, the nocturnal low-level jet and the cold air mass inflow propagating northwards from the coast of the Gulf of Guinea. The results show that these dynamical structures are very regularly observed during the entire 41-day documented period. Monsoon flow is observed 100% of the time. The so-called maritime inflow and the nocturnal low level jet are also systematic features in this area. According to monsoon flow conditions, the maritime inflow reaches Savè around 18:00–19:00UTC on average: this timing is correlated with the strength of the monsoon flow. This time of arrival is close to the time range of the nocturnal low level jet settlement. As a result, these phenomena are difficult to distinguish at the Savè site. The low level jet occurs every night, except during rain events, and is associated 65% of the time with low stratus clouds. Stratus cloud form between 22:00UTC and 06:00UTC at an elevation close to the nocturnal low level jet core height. The cloud base height, 310±30m above ground level (a.g.l.) is rather stationary during the night and remains below the jet core height. The cloud top height, at 640±100ma.g.l., is typically found above the jet core. The nocturnal low level jet, low level clouds, monsoon flow and maritime inflow reveal significant day-to-day variability during the summer. Distributions of strength, depth, onset time, break up time, etc. are quantified here.

Cheikh Dione et al.
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Low atmospheric dynamics and low-level cloud (LLC) macrophysical propreties are analyzed using in-situ and remote sensing data collected from 20 June to 30 July at Savè, Benin during DACCIWA field campaign in 2016. We find that the low level jet (LLJ), LLCs, monsoon flow and maritime inflow reveal a day-to-day variability. LLCs form at the same level than the jet core height. The cloud base height is stationary at night and remains below the jet. The cloud top height is found above the jet.
Low atmospheric dynamics and low-level cloud (LLC) macrophysical propreties are analyzed using...
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