scholarly journals A meteorological and chemical overview of the DACCIWA field campaign in West Africa in June–July 2016

2017 ◽  
Author(s):  
Peter Knippertz ◽  
Andreas H. Fink ◽  
Adrien Deroubaix ◽  
Eleanor Morris ◽  
Flore Tocquer ◽  
...  
Keyword(s):  
West Africa ◽  
Biomass Burning ◽  
Urban Pollution ◽  
Monsoon Onset ◽  
Boreal Summer ◽  
Study Region ◽  
Field Campaign ◽  
Low Level ◽  
Onset Phase ◽  
Pollution Plumes

Abstract. In June and July 2016 the Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) project organised a major international field campaign in southern West Africa (SWA) including measurements from three inland ground supersites, urban sites in Cotonou and Abidjan, radiosondes and three research aircraft. A significant range of different weather situations was encountered during this period, including the monsoon onset. The purpose of this paper is to characterise the large-scale setting for the campaign as well as synoptic and mesoscale weather systems affecting the study region in the light of existing conceptual ideas, mainly using objective and subjective identification algorithms based on (re-) analysis and satellite products. In addition, it is shown how the described synoptic variations influence the atmospheric composition over SWA through advection of mineral-dust, biomass-burning and urban-pollution plumes. The boreal summer of 2016 was characterised by Pacific La Niña, Atlantic El Niño and warm eastern Mediterranean conditions, whose competing influences on precipitation led to an overall average rainy season. During the relatively dusty pre-onset Phase 1 (1–21 June 2016), three westward propagating coherent cyclonic vortices between 4 and 13° N modulated winds and rainfall in the Guinea coastal area. The monsoon onset occurred in connection with a marked extratropical trough and cold surge over northern Africa, leading to a breakdown of the Saharan heat low and African easterly jet and a suppression of rainfall. During this period, quasi-stationary low-level vortices associated with the trough transformed into more tropical, propagating disturbances resembling an African easterly wave (AEW). To the east of this system, moist southerlies penetrated deep into the continent. The post-onset Phase 2 (22 June–20 July 2016) was characterised by a significant increase of low-level cloudiness, unusually dry conditions and strong northeastward dispersion of urban pollution plumes in SWA as well as rainfall modulation by westward propagating AEWs in the Sahel. Around 12–14 July 2016 an interesting and so-far undocumented cyclonic-anticyclonic vortex couplet crossed SWA. The anticyclonic centre had its origin in the southern hemisphere and transported unusually dry air filled with aged aerosol into the region. During Phase 3 (21–26 July 2016), a similar vortex couplet slightly farther north created enhanced westerly moisture transports into SWA and extraordinarily wet conditions, accompanied by a deep penetration of the biomass-burning plume from central Africa. Finally, a return to more undisturbed monsoon conditions took place during Phase 4 (27–31 July 2016). The in-depth synoptic analysis reveals that several significant weather systems during the DACCIWA campaign cannot be attributed unequivocally to any of the tropical waves and disturbances described in the literature, and thus deserve further study.

scholarly journals A meteorological and chemical overview of the DACCIWA field campaign in West Africa in June–July 2016

2017 ◽  
Vol 17 (17) ◽  
pp. 10893-10918 ◽  
Author(s):  
Peter Knippertz ◽  
Andreas H. Fink ◽  
Adrien Deroubaix ◽  
Eleanor Morris ◽  
Flore Tocquer ◽  
...  
Keyword(s):  
West Africa ◽  
Biomass Burning ◽  
Urban Pollution ◽  
Monsoon Onset ◽  
Boreal Summer ◽  
Deep Penetration ◽  
Study Region ◽  
Field Campaign ◽  
Low Level ◽  
Onset Phase

Abstract. In June and July 2016 the Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) project organised a major international field campaign in southern West Africa (SWA) including measurements from three inland ground supersites, urban sites in Cotonou and Abidjan, radiosondes, and three research aircraft. A significant range of different weather situations were encountered during this period, including the monsoon onset. The purpose of this paper is to characterise the large-scale setting for the campaign as well as synoptic and mesoscale weather systems affecting the study region in the light of existing conceptual ideas, mainly using objective and subjective identification algorithms based on (re-)analysis and satellite products. In addition, it is shown how the described synoptic variations influence the atmospheric composition over SWA through advection of mineral dust, biomass burning and urban pollution plumes.The boreal summer of 2016 was characterised by Pacific La Niña, Atlantic El Niño and warm eastern Mediterranean conditions, whose competing influences on precipitation led to an overall average rainy season. During the relatively dusty pre-onset Phase 1 (1–21 June 2016), three westward-propagating coherent cyclonic vortices between 4 and 13° N modulated winds and rainfall in the Guinea coastal area. The monsoon onset occurred in connection with a marked extratropical trough and cold surge over northern Africa, leading to a breakdown of the Saharan heat low and African easterly jet and a suppression of rainfall. During this period, quasi-stationary low-level vortices associated with the trough transformed into more tropical, propagating disturbances resembling an African easterly wave (AEW). To the east of this system, moist southerlies penetrated deep into the continent. The post-onset Phase 2 (22 June–20 July 2016) was characterised by a significant increase in low-level cloudiness, unusually dry conditions and strong northeastward dispersion of urban pollution plumes in SWA as well as rainfall modulation by westward-propagating AEWs in the Sahel. Around 12–14 July 2016 an interesting and so-far undocumented cyclonic–anticyclonic vortex couplet crossed SWA. The anticyclonic centre had its origin in the Southern Hemisphere and transported unusually dry air filled with aged aerosol into the region. During Phase 3 (21–26 July 2016), a similar vortex couplet slightly farther north created enhanced westerly moisture transports into SWA and extraordinarily wet conditions, accompanied by a deep penetration of the biomass burning plume from central Africa. Finally, a return to more undisturbed monsoon conditions took place during Phase 4 (27–31 July 2016). The in-depth synoptic analysis reveals that several significant weather systems during the DACCIWA campaign cannot be attributed unequivocally to any of the tropical waves and disturbances described in the literature and thus deserve further study.

scholarly journals Low-level stratiform clouds and dynamical features observed within the southern West African monsoon

2019 ◽  
Vol 19 (13) ◽  
pp. 8979-8997 ◽  
Author(s):  
Cheikh Dione ◽  
Fabienne Lohou ◽  
Marie Lothon ◽  
Bianca Adler ◽  
Karmen Babić ◽  
...  
Keyword(s):  
West Africa ◽  
Boreal Summer ◽  
Time Range ◽  
Atmospheric Conditions ◽  
Low Level ◽  
Stratus Clouds ◽  
Stratiform Clouds ◽  
Monsoon Flow ◽  
Low Level Jet ◽  
Core Height

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. Numerical climate and weather models need finer description and knowledge of cloud macrophysical characteristics and of the dynamical and thermodynamical structures occupying the lowest troposphere, in order to be properly evaluated in this region. 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 stratiform low-level cloud macrophysical properties are analyzed using in situ and remote sensing measurements continuously collected from 20 June to 30 July at Savè, Benin, roughly 180 km 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 an ultra-high-frequency (UHF) 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 northward from the coast of the Gulf of Guinea. The results show that these dynamical structures are very regularly observed during the entire 41 d documented period. Monsoon flow is observed every day during our study period. The so-called “maritime inflow” and the nocturnal low-level jet are also systematic features in this area. According to synoptic atmospheric conditions, the maritime inflow reaches Savè around 18:00–19:00 UTC 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 clouds form between 22:00 and 06:00 UTC at an elevation close to the nocturnal low-level jet core height. The cloud base height, 310±30 m 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±100 m a.g.l., is typically found above the jet core. The nocturnal low-level jet, low-level stratiform clouds, monsoon flow, and maritime inflow reveal significant day-to-day and intra-seasonal variability during the summer given the importance of the different monsoon phases and synoptic atmospheric conditions. Distributions of strength, depth, onset time, breakup time, etc. are quantified here. These results contribute to satisfy the main goals of DACCIWA and allow a conceptual model of the dynamical structures in the lowest troposphere over the southern part of West Africa.

scholarly journals Aerosol influences on low-level clouds in the West African monsoon

2019 ◽  
Author(s):  
Jonathan W. Taylor ◽  
Sophie L. Haslett ◽  
Keith Bower ◽  
Michael Flynn ◽  
Ian Crawford ◽  
...  
Keyword(s):  
West Africa ◽  
Southern Hemisphere ◽  
Biomass Burning ◽  
West African Monsoon ◽  
Cloud Microphysics ◽  
West African ◽  
The West ◽  
African Monsoon ◽  
Low Level ◽  
Local Emissions

Abstract. Low-level clouds (LLC) cover a wide area of southern West Africa (SWA) during the summer monsoon months, and have an important cooling effect on the regional climate. Previous studies of these clouds have focused on modelling and remote sensing via satellite. We present the first comprehensive set of regional, in situ measurements of cloud microphysics, taken during June – July 2016, as part of the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in West Africa) campaign, assessing spatial and temporal variation in the properties of these clouds. LLC developed overnight and mean cloud cover peaked a few hundred kilometres inland around 10:00 local solar time (LST), before clouds began to dissipate and convection intensified in the afternoon. Additional sea breeze clouds developed near the coast in the late morning, reaching a maximum extent around 12:00 LST. Regional variation in LLC cover was largely determined by the modulation of the cool maritime inflow by the local orography, with peaks on the upwind side of hills and minima on the leeward sides. In the broad-scale cloud field, no lasting impacts related to anthropogenic aerosol were observed downwind of major population centres. The boundary layer cloud drop number concentration (CDNC) was locally variable inland, ranging from 200 to 840 cm−3 (10th and 90th percentiles at standard temperature and pressure), but showed no systematic regional variations. Enhancements were seen in pollution plumes from the coastal cities, but were not statistically significant across the region. The majority of accumulation mode aerosols, and therefore cloud condensation nuclei, were from ubiquitous biomass burning smoke transported from the southern hemisphere. Consequently, all clouds measured (inland and offshore) had significantly higher CDNC and lower effective radius than clouds over the remote south Atlantic from literature. A parcel model sensitivity analysis showed that doubling or halving local emissions only changed the calculated CDNC by 13–22 %, as the high background meant local emissions were a small fraction of total aerosol. As the population of SWA grows, local emissions are expected to rise. Biomass burning smoke transported from the southern hemisphere is likely to dampen any effect of these increased local emissions on cloud-aerosol interactions. An integrative analysis between local pollution and Central African biomass burning emissions must be considered when predicting anthropogenic impacts on the regional cloud field during the West African monsoon.

scholarly journals The life cycle of nocturnal low-level clouds over southern West Africa analysed using high-resolution simulations

2016 ◽  
Author(s):  
Bianca Adler ◽  
Norbert Kalthoff ◽  
Leonhard Gantner
Keyword(s):  
Life Cycle ◽  
High Resolution ◽  
West Africa ◽  
Single Case ◽  
Monsoon Season ◽  
Cloud Formation ◽  
Field Campaign ◽  
Low Level ◽  
Low Level Jet ◽  
Spatio Temporal

Abstract. We performed a high-resolution numerical simulation to study the life cycle of extensive low-level clouds which frequently form over southern West Africa during the monsoon season. This study was made in preparation for a field campaign in 2016 within the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) project and focuses on an area around the city of Save in southern Benin. Nocturnal low-level clouds evolve a few hundred metres above the ground around the same level as a distinct low-level jet. Several processes are found to determine the spatio-temporal evolution of these clouds including (i) significant cooling of the nocturnal atmosphere due to horizontal advection with the south-westerly monsoon flow during the first half of the night, (ii) vertical cold air advection due to gravity waves leading to clouds in the wave crests and (iii) enhanced convergence and upward motion upstream of existing clouds that trigger new clouds. The latter is caused by an upward shift of the low-level jet in cloudy areas leading to horizontal convergence in the lower part and to horizontal divergence in the upper part of the cloud layer. Although this single case study hardly allows for a generalisation of the processes found, the results added to the optimisation of the measurements strategy for the field campaign and the observations will be used to the test the hypotheses for cloud formation resulting from this study.

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

2018 ◽  
Author(s):  
Cheikh Dione ◽  
Fabienne Lohou ◽  
Marie Lothon ◽  
Bianca Adler ◽  
Karmen Babić ◽  
...  
Keyword(s):  
West Africa ◽  
Onset Time ◽  
Boreal Summer ◽  
Cloud Base ◽  
Time Range ◽  
Low Level ◽  
Stratus Clouds ◽  
Monsoon Flow ◽  
Low Level Jet ◽  
Core Height

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 180 km 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 18:00–19:00 UTC 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 22:00 UTC and 06:00 UTC at an elevation close to the nocturnal low level jet core height. The cloud base height, 310 ± 30 m 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 ± 100 m a.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.

scholarly journals First measurements of reactive α-dicarbonyl concentrations on PM<sub>2.5</sub> aerosol over the Boreal forest in Finland during HUMPPA-COPEC 2010 – source apportionment and links to aerosol aging

2012 ◽  
Vol 12 (14) ◽  
pp. 6145-6155 ◽  
Author(s):  
C. J. Kampf ◽  
A. L. Corrigan ◽  
A. M. Johnson ◽  
W. Song ◽  
P. Keronen ◽  
...  
Keyword(s):  
Boreal Forest ◽  
Biomass Burning ◽  
Sampling Site ◽  
Urban Pollution ◽  
Composition Analysis ◽  
Oh Radicals ◽  
Particle Phase ◽  
Aerosol Aging ◽  
The City ◽  
Pollution Plumes

Abstract. The first dataset for summertime boreal forest concentrations of two atmospherically relevant α-dicarbonyl compounds, glyoxal (Gly) and methylglyoxal (Mgly) on PM2.5 aerosol was obtained during the HUMPPA-COPEC-2010 field measurement intensive in Hyytiälä, Finland. Anthropogenic influences over the course of the campaign were identified using trace gas signatures and aerosol particle chemical composition analysis. The data evaluation allowed the identification of different events such as urban pollution plumes, biomass burning and sawmill emissions as sources of high Gly and Mgly concentrations. Mean aerosol concentrations during periods of biogenic influence were 0.81 ng m−3 for Gly and 0.31 ng m−3 for Mgly. Mgly was generally less abundant in PM2.5, probably due to its shorter photolysis lifetime and less effective partitioning into the particle phase due to its smaller effective Henry's Law constant compared to Gly. This is in contrast with previous urban studies which show significantly more Mgly than Gly. Peak concentrations for Gly coincided with nearby sources, e.g. high VOC emissions from nearby sawmills, urban pollution plumes from the city of Tampere located 50 km southwest of the sampling site and biomass burning emissions from wildfires. Calculated ratios of Gly in PM2.5 and total organic matter in PM1 aerosols indicate higher values in less aged aerosols. Irreversible processing of Gly in the particle phase, e.g. via oxidation by OH radicals, organo sulfate or imidazole formation are processes currently discussed in the literature which could likely explain these findings.

scholarly journals Nocturnal low-level clouds over southern West Africa analysed using high-resolution simulations

2017 ◽  
Vol 17 (2) ◽  
pp. 899-910 ◽  
Author(s):  
Bianca Adler ◽  
Norbert Kalthoff ◽  
Leonhard Gantner
Keyword(s):  
High Resolution ◽  
West Africa ◽  
Single Case ◽  
Monsoon Season ◽  
Cloud Formation ◽  
Field Campaign ◽  
Low Level ◽  
Monsoon Flow ◽  
Low Level Jet ◽  
Spatio Temporal

Abstract. We performed a high-resolution numerical simulation to study the development of extensive low-level clouds that frequently form over southern West Africa during the monsoon season. This study was made in preparation for a field campaign in 2016 within the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) project and focuses on an area around the city of Savè in southern Benin. Nocturnal low-level clouds evolve a few hundred metres above the ground around the same level as a distinct low-level jet. Several processes are found to determine the spatio-temporal evolution of these clouds including (i) significant cooling of the nocturnal atmosphere caused by horizontal advection with the south-westerly monsoon flow during the first half of the night, (ii) vertical cold air advection due to gravity waves leading to clouds in the wave crests and (iii) enhanced convergence and upward motion upstream of existing clouds that trigger new clouds. The latter is caused by an upward shift of the low-level jet in cloudy areas leading to horizontal convergence in the lower part and to horizontal divergence in the upper part of the cloud layer. Although this single case study hardly allows for a generalisation of the processes found, the results added to the optimisation of the measurements strategy for the field campaign and the observations will be used to test the hypotheses for cloud formation resulting from this study.

scholarly journals First measurements of reactive α-dicarbonyl concentrations on PM<sub>2.5</sub> aerosol over the boreal forest in Finland during HUMPPA-COPEC 2010 – source apportionment and links to aerosol aging

2012 ◽  
Vol 12 (1) ◽  
pp. 723-751
Author(s):  
C. J. Kampf ◽  
A. L. Corrigan ◽  
A. M. Johnson ◽  
W. Song ◽  
P. Keronen ◽  
...  
Keyword(s):  
Boreal Forest ◽  
Biomass Burning ◽  
Sampling Site ◽  
Urban Pollution ◽  
Composition Analysis ◽  
Oh Radicals ◽  
Particle Phase ◽  
Aerosol Aging ◽  
The City ◽  
Pollution Plumes

Abstract. The first dataset for summertime boreal forest concentrations of two atmospherically relevant α-dicarbonyl compounds, glyoxal (Gly) and methylglyoxal (Mgly) on PM2.5 aerosol was obtained during the HUMPPA-COPEC-2010 field measurement intensive in Hyytiälä, Finland. Identification of anthropogenic influences over the course of the campaign, using trace gas signatures and aerosol particle chemical composition analysis, allowed the identification of different events such as urban pollution plumes, biomass burning and sawmill emissions as sources of high Gly and Mgly concentrations. Mean aerosol concentrations during periods of biogenic influence were 0.81 ng m−3 for Gly and 0.31 ng m−3 for Mgly. Mgly was generally less abundant in PM2.5, probably due to its shorter photolysis lifetime and less effective partitioning into the particle phase due to its smaller effective Henry's Law constant compared to Gly. This is in contrast with previous urban studies which show significantly more Mgly than Gly. Peak concentrations for Gly coincided with nearby sources, e.g. high VOC emissions from nearby sawmills, urban pollution plumes from the city of Tampere located 50 km southwest of the sampling site and biomass burning emissions from wildfires. Calculated ratios of Gly in PM2.5 and total organic matter in PM1 aerosols indicate higher values in less aged aerosols. Irreversible processing of Gly in the particle phase, e.g. via oxidation by OH radicals, organo sulfate or imidazole formation are processes currently discussed in the literature which could likely explain these findings.

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