Cloud and aerosol radiative effects as key players for anthropogenic
changes in atmospheric dynamics over southernWest Africa

Deetz, Konrad; Vogel, H.; Knippertz, Peter; Adler, Bianca; Taylor, Jonathan; Coe, Hugh; Bower, Keith; Haslett, Sophie L.; Flynn, Michael J.; Dorsey, J. R.; Crawford, Ian; Kottmeier, Christoph; Vogel, Bernhard

doi:10.5194/acp-2018-186

Cited by 3 publications

(8 citation statements)

References 9 publications

(17 reference statements)

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“…LLC form in the monsoon flow, where meteorological phenomena and geographical features combine. Schrage and Fink (2012); Schuster et al (2013); Adler et al (2017) and Deetz et al (2018) suggested that orographic and gravity waves lifting, horizontal cold air advection, shear-driven turbulence associated with the nocturnal low level jet (NLLJ) could all play a role in the LLC formation.…”

Section: Introductionmentioning

confidence: 99%

“…The continuous horizontal advection of cold air from the Guinean coast is maintained by the monsoon flow, but continental southern West Africa is also impacted by the sea-breeze. During daytime, the turbulence in CBL over land decelerates the south-westerly flow, which leads to a convergence zone along a line parallel to the coast (Knippertz et al, 2017;Adler et al, 2017;Deetz et al, 2018) between the cold maritime air to the south and the warmer air in the CBL to the north. Grams et al (2010) investigated such a quite stationary sea-breeze front during the day along the Mauritanian coast, which propagates farther inland at the end of the afternoon when the turbulence collapses.…”

Section: Introductionmentioning

confidence: 99%

“…In numerical simulations, the convergence zone has been shown to move slowly several tens of kilometers inland during the day. In the late afternoon, when turbulence vanishes in the CBL, the flow accelerates and the convergence zone moves farther and faster northwards over land (Adler et al, 2017;Deetz et al, 2018). This phenomenon is called the Gulf of Guinea Maritime Inflow (MI) in Adler et al (2019).…”

Section: Introductionmentioning

confidence: 99%

“…Haslett et al (2019a) show that during the monsoon season, aerosol particles in the region are likely to grow significantly because of the high moisture in the air, meaning that climate effects from increasing pollution will be enhanced. Deetz et al (2018) performed highly resolved process study simulations for 2-3 July 2016 with COSMO-ART to assess the aerosol direct and indirect effect on meteorological conditions over southern West Africa. They find that MI and stratus-to-cumulus transition are highly susceptible to the aerosol direct effect, leading to a spatial shift of the MI front and a temporal shift of the stratus-to-cumulus transition with changes in the aerosol amount.…”

Section: Introductionmentioning

confidence: 99%

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

Lohou

Kalthoff

Adler

et al. 2019

Preprint

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

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Lohou

Kalthoff

Adler

et al. 2019

Preprint

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“…These data show a clear diurnal cycle in the RH, with a humid boundary layer during the night and early morning becoming drier towards noon and into the evening. These features are characteristic of the Gulf of Guinea maritime inflow (Adler et al, 2017;Deetz et al, 2018b), a coastal air mass that propagates inland in the evening, bringing cool air to the southern West African region. The increase of RH in the evening is mainly caused by cooling due to this cold air advection (Adler et al, in press; Babić et al, in press).…”

mentioning

confidence: 99%

The radiative impact of out-of-cloud aerosol hygroscopic growth during the summer monsoon in southernWest Africa

Haslett¹,

Taylor²,

Deetz³

et al. 2018

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<p><strong>Abstract.</strong> Water in the atmosphere exists as both vapour and liquid water contained in particles. At high humidities, more water vapour condenses onto particles and causes them to swell, sometimes up to several times their original size. This significant change in size and chemical composition is termed hygroscopic growth and alters a particle's optical properties. Even in unsaturated conditions, this can change the aerosol direct effect, for example by increasing the extinction of incoming sunlight. This can have an impact on a region's energy balance and affect visibility. Here, aerosol and relative humidity measurements collected from aircraft and radiosondes during the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) campaign were used to estimate the effect of highly humid layers of air on aerosol optical properties during the monsoon season in southern West Africa. The effects of hygroscopic growth in this region are of particular interest due to the regular occurrence of high humidity and the high levels of pollution in the region. The Zdanovskii, Stokes and Robinson (ZSR) mixing rule is used to estimate the hygroscopic growth of particles under different conditions based on chemical composition. These results are used to estimate the aerosol optical depth (AOD) for 63 relative humidity profiles. A static aerosol profile was assumed. Therefore, these results show the extent of the AOD frequency distribution that can be explained by humidity alone, rather than predicting actual AOD values. The median AOD in the region from these calculations was 0.46, which compares to a median of 0.36 measured by sun photometers. The shape of the AOD frequency distribution was largely comparable to that of the sun photometer measurements, demonstrating that relative humidity is able to account for a large part of the region's AOD variability. Humid layers are found to have the most significant impact on AOD when they reach relative humidities greater than 98&#8201;%, which can result in a wet AOD up to seven times larger than the dry AOD. Unsaturated humid layers were found to reach these high levels of relative humidity in 37&#8201;% of observed cases. Aerosol concentrations in southern West Africa are projected to increase substantially in the coming years; results presented here show that the presence of highly humid layers in the region is likely to enhance the consequent effect on AOD significantly.</p>

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

Zouzoua¹,

Lohou²,

Assamoi³

et al. 2020

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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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Cloud and aerosol radiative effects as key players for anthropogenic changes in atmospheric dynamics over southernWest Africa

Cited by 3 publications

References 9 publications

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

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

The radiative impact of out-of-cloud aerosol hygroscopic growth during the summer monsoon in southernWest Africa

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

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