Numerical simulations of aerosol radiative effects and their impact on clouds and atmospheric dynamics over southern West Africa

Abstract. Southern West Africa (SWA) is undergoing rapid and significant socioeconomic changes associated with a massive increase in air pollution. Still, the impact of atmospheric pollutants, in particular that of aerosol particles, on weather and climate in this region is virtually unexplored. In this study, the regional-scale model framework COSMO-ART is applied to SWA for a summer monsoon process study on 2–3 July 2016 to assess the aerosol direct and indirect effect on clouds and atmospheric dynamics. The modeling study is supported by observational data obtained during the extensive field campaign of the project DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) in June–July 2016. As indicated in previous studies, a coastal front is observed that develops during daytime and propagates inland in the evening (Atlantic inflow). Increasing the aerosol amount in COSMO-ART leads to reduced propagation velocities with frontal displacements of 10–30 km and a weakening of the nocturnal low-level jet. This is related to a subtle balance of processes related to the decrease in near-surface heating: (1) flow deceleration due to reduced land–sea temperature contrast and thus local pressure gradient, (2) reduced turbulence favoring frontal advance inland and (3) delayed stratus-to-cumulus transition of 1–2 h via a later onset of the convective boundary layer. The spatial shift of the Atlantic inflow and the temporal shift of the stratus-to-cumulus transition are synergized in a new conceptual model. We hypothesize a negative feedback of the stratus-to-cumulus transition on the Atlantic inflow with increased aerosol. The results exhibit radiation as the key player governing the aerosol affects on SWA atmospheric dynamics via the aerosol direct effect and the Twomey effect, whereas impacts on precipitation are small.

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

10.5194/acp-2018-186 ◽

2018 ◽

Author(s):

Konrad Deetz ◽

Heike Vogel ◽

Peter Knippertz ◽

Bianca Adler ◽

Jonathan Taylor ◽

...

Keyword(s):

West Africa ◽

Regional Scale ◽

Atmospheric Dynamics ◽

Atmospheric Pollutants ◽

Scale Model ◽

Model Framework ◽

Local Pressure ◽

Temporal Shift ◽

Near Surface ◽

The Impact

Abstract. Southern West Africa (SWA) undergoes rapid and significant socioeconomic changes associated with a massive increase in air pollution. Still, the impact of atmospheric pollutants, in particular that of aerosol particles, on weather and climate in this region is virtually unexplored. In this study, the regional-scale model framework COSMO-ART is applied to SWA for a summer monsoon process study on 2–3 July 2016 to assess the aerosol direct and indirect effect on clouds and the atmospheric dynamics. The modeling study is supported by observational data obtained during the extensive field campaign of the project DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) in June–July 2016. As indicated in previous studies, a coastal front is observed that develops during daytime and propagates inland in the evening (Atlantic Inflow). Increasing the aerosol amount in COSMO-ART leads to reduced propagation velocities with frontal displacements of 10–30 km and a weakening of the nocturnal low-level jet. This is related to a subtle balance of processes related to the decrease in near-surface heating: (1) flow deceleration due to reduced land-sea temperature contrast and thus local pressure gradient, (2) reduced turbulence favoring frontal advance inland and (3) delayed stratus-to-cumulus transition of 1–2 h via a later onset of the convective boundary layer. The spatial shift of the Atlantic Inflow and the temporal shift of the stratus-to-cumulus transition are synergized in a new conceptual model. We hypothesize a negative feedback of the stratus-to-cumulus transition on the Atlantic Inflow with increased aerosol. The results exhibit radiation as the key player governing the aerosol affects on SWA atmospheric dynamics via the aerosol direct effect and the Twomey effect, whereas impacts on precipitation are small.

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Regional characterisation of soil properties by combining soil science and hyperspectral and thermal remote sensing: A technical overview of lab and field remote sensing methods

10.5194/egusphere-egu21-4818 ◽

2021 ◽

Author(s):

Richard Mommertz ◽

Lars Konen ◽

Martin Schodlok

Keyword(s):

Remote Sensing ◽

Soil Properties ◽

Regional Scale ◽

Arable Land ◽

Soil Mapping ◽

Soil Science ◽

Near Surface ◽

Soil Parameter ◽

The Impact ◽

Parameter Retrieval

Soil is one of the world&#8217;s most important natural resources for human livelihood as it provides food and clean water. Therefore, its preservation is of huge importance. For this purpose, a proficient regional database on soil properties is needed. The project &#8220;ReCharBo&#8221; (Regional Characterisation of Soil Properties) has the objective to combine remote sensing, geophysical and pedological methods to determine soil characteristics on a regional scale. Its aim is to characterise soils non-invasive, time and cost efficient and with a minimal number of soil samples to calibrate the measurements. Konen et al. (2021) give detailed information on the research concept and first field results in a presentation in the session &#8220;SSS10.3 Digital Soil Mapping and Assessment&#8221;. Hyperspectral remote sensing is a powerful and well known technique to characterise near surface soil properties. Depending on the sensor technology and the data quality, a wide variety of soil properties can be derived with remotely sensed data (Chabrillat et al. 2019, Stenberg et al. 2010). The project aims to investigate the effects of up and downscaling, namely which detail of information is preserved on a regional scale and how a change in scales affects the analysis algorithms and the possibility to retrieve valid soil parameter information. Thus, e.g. laboratory and field spectroscopy are applied to gain information of samples and fieldspots, respectively. Various UAV-based sensors, e.g. thermal & hyperspectral sensors, are applied to study soil properties of arable land in different study areas at field scale. Finally, airborne (helicopter) hyperspectral data will cover the regional scale. Additionally forthcoming spaceborne hyperspectral satellite data (e.g. Prisma, EnMAP, Sentinel-CHIME) are a promising outlook to gain detailed regional soil information. In this context it will be discussed how the multisensor data acquisition is best managed to optimise soil parameter retrieval. Sensor specific properties regarding time and date of acquisition as well as weather/atmospheric conditions are outlined. The presentation addresses and discusses the impact of a multisensor and multiscale remote sensing data collection regarding the results on soil parameter retrieval.&#160;ReferencesChabrillat, S., Ben-Dor, E. Cierniewski, J., Gomez, C., Schmid, T. & van Wesemael, B. (2019): Imaging Spectroscopy for Soil Mapping and Monitoring. Surveys in Geophysics 40:361&#8211;399. https://doi.org/10.1007/s10712-019-09524-0Stenberg, B., Viscarra Rossel, R. A., Mounem Mouazen, A. & Wetterlind, J. (2010): Visible and Near Infrared Spectroscopy in Soil Science. In: Donald L. Sparks (editor): Advances in Agronomy. Vol. 107. Academic Press:163-215. http://dx.doi.org/10.1016/S0065-2113(10)07005-7

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A bias-corrected CMIP5 dataset for Africa using the CDF-t method – a contribution to agricultural impact studies

Earth System Dynamics ◽

10.5194/esd-9-313-2018 ◽

2018 ◽

Vol 9 (1) ◽

pp. 313-338 ◽

Cited By ~ 18

Author(s):

Adjoua Moise Famien ◽

Serge Janicot ◽

Abe Delfin Ochou ◽

Mathieu Vrac ◽

Dimitri Defrance ◽

...

Keyword(s):

West Africa ◽

Air Temperature ◽

Climate Model ◽

Global Climate Model ◽

Cumulative Distribution ◽

Shortwave Radiation ◽

Climate Change Scenarios ◽

Reference Dataset ◽

Near Surface ◽

The Impact

Abstract. The objective of this paper is to present a new dataset of bias-corrected CMIP5 global climate model (GCM) daily data over Africa. This dataset was obtained using the cumulative distribution function transform (CDF-t) method, a method that has been applied to several regions and contexts but never to Africa. Here CDF-t has been applied over the period 1950–2099 combining Historical runs and climate change scenarios for six variables: precipitation, mean near-surface air temperature, near-surface maximum air temperature, near-surface minimum air temperature, surface downwelling shortwave radiation, and wind speed, which are critical variables for agricultural purposes. WFDEI has been used as the reference dataset to correct the GCMs. Evaluation of the results over West Africa has been carried out on a list of priority user-based metrics that were discussed and selected with stakeholders. It includes simulated yield using a crop model simulating maize growth. These bias-corrected GCM data have been compared with another available dataset of bias-corrected GCMs using WATCH Forcing Data as the reference dataset. The impact of WFD, WFDEI, and also EWEMBI reference datasets has been also examined in detail. It is shown that CDF-t is very effective at removing the biases and reducing the high inter-GCM scattering. Differences with other bias-corrected GCM data are mainly due to the differences among the reference datasets. This is particularly true for surface downwelling shortwave radiation, which has a significant impact in terms of simulated maize yields. Projections of future yields over West Africa are quite different, depending on the bias-correction method used. However all these projections show a similar relative decreasing trend over the 21st century.

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Aerosol distribution in the northern Gulf of Guinea: local anthropogenic sources, long-range transport and the role of coastal shallow circulations

10.5194/acp-2018-346 ◽

2018 ◽

Author(s):

Cyrille Flamant ◽

Adrien Deroubaix ◽

Patrick Chazette ◽

Joel Brito ◽

Marco Gaetani ◽

...

Keyword(s):

Boundary Layer ◽

West Africa ◽

Long Range ◽

Regional Scale ◽

Lower Troposphere ◽

Atmospheric Pollutants ◽

Anthropogenic Sources ◽

Long Range Transport ◽

Aerosol Distribution ◽

Range Transport

Abstract. The complex vertical distribution of aerosols over coastal southern West Africa (SWA) is investigated using airborne observations and numerical simulations. Observations were gathered on 2 July 2016 offshore of Ghana and Togo, during the field phase of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa project. The aerosol loading in the lower troposphere includes emissions from coastal cities (Accra, Lomé, Cotonou and Lagos) as well as biomass burning aerosol and dust associated with long-range transport from Central Africa and the Sahara, respectively. Our results indicate that the aerosol distribution is impacted by subsidence associated with zonal and meridional regional scale overturning circulations associated with the land-sea surface temperature contrast and orography over Ghana and Togo. Numerical tracer release experiments highlight the dominance of aged emissions from Accra on the observed pollution plume loadings over the ocean. The contribution of aged emission from Lomé and Cotonou is also evident above the marine boundary layer. Lagos emissions do not play a role for the area west of Cotonou. The tracer plume does not extend very far south over the ocean (i.e. less than 100 km from Accra), mostly because emissions are transported northeastward near the surface over land and westward above the marine atmospheric boundary layer. The latter is possible due to interactions between the monsoon flow, complex terrain and land-sea breeze systems, which support the vertical mixing of the urban pollution. This work sheds light on the complex – and to date undocumented – mechanisms by which coastal shallow circulations distribute atmospheric pollutants over the densely populated SWA region.

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How might climate change affect river flows across West Africa?

Climatic Change ◽

10.1007/s10584-021-03256-0 ◽

2021 ◽

Vol 169 (3-4) ◽

Author(s):

Ponnambalam Rameshwaran ◽

Victoria A. Bell ◽

Helen N. Davies ◽

Alison L. Kay

Keyword(s):

Climate Change ◽

West Africa ◽

Regional Scale ◽

Weather Data ◽

Eastern Region ◽

Water Shortages ◽

River Flows ◽

Considerable Uncertainty ◽

Modelling Framework ◽

The Impact

AbstractWest Africa and its semi-arid Sahelian region are one of the world’s most vulnerable regions to climate change with a history of extreme climate variability. There is still considerable uncertainty as to how projected climate change will affect precipitation at local and regional scales and the consequent impact on river flows and water resources across West Africa. Here, we aim to address this uncertainty by configuring a regional-scale hydrological model to West Africa. The model (hydrological modelling framework for West Africa—HMF-WA) simulates spatially consistent river flows on a 0.1° × 0.1° grid (approximately 10 km × 10 km) continuously across the whole domain and includes estimates of anthropogenic water use, wetland inundation, and local hydrological features such as endorheic regions. Regional-scale hydrological simulations driven by observed weather data are assessed against observed flows before undertaking an analysis of the impact of projected future climate scenarios from the CMIP5 on river flows up to the end of the twenty-first century. The results indicate that projected future changes in river flows are highly spatially variable across West Africa, particularly across the Sahelian region where the predicted changes are more pronounced. The study shows that median peak flows are projected to decrease by 23% in the west (e.g. Senegal) and increase by 80% in the eastern region (e.g. Chad) by the 2050s. The projected reductions in river flows in western Sahel lead to future droughts and water shortages more likely, while in the eastern Sahel, projected increases lead to future frequent floods.

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Can the implementation of Low Impact Development reduce basin runoff?

10.5194/hess-2021-32 ◽

2021 ◽

Author(s):

Xinxin Sui ◽

Frans van de Ven

Keyword(s):

Urban Development ◽

Development Strategy ◽

Regional Scale ◽

Low Impact Development ◽

Green Roofs ◽

Distributed Model ◽

Model Framework ◽

Permeable Pavements ◽

Peak Runoff ◽

The Impact

Abstract. Low impact development (LID) was promoted as an alternative to conventional urban drainage methods. The effects of LID at site or urban scales have been widely evaluated. This project aims to investigate the impact of LID implementation on basin runoff at regional scale in a half urbanized catchment; especially the overlap of urban and rural sub-flows at peak times is concerned. A SUPERFLEX conceptual model framework was adapted as a semi-distributed model to simulate the rainfall-runoff relationship in the catchment for San Antonio, Texas as a case study. Scenario analyses of both urban development and LID implementation were conducted. Results show that (1) the infill urban development strategy benefits more from runoff control than the sprawl urban development strategy; (2) in non-flood season permeable pavements, bioretention cells, and vegetated swales decrease peak runoff forcefully and permeable pavements, bioretention cells, and green roofs are good at runoff volume retention; (3) contrary to the general opinion about the peak reduction effect of LID, for partly urbanized, partly rural basins and extremely wet conditions, the implementation of LID practices delays urban peak runoff and may cause stacking of rural and urban sub-flows, leading to larger basin peaks.

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Direct radiative effect of the Russian wildfires and their impact on air temperature and atmospheric dynamics during August 2010

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-15829-2013 ◽

2013 ◽

Vol 13 (6) ◽

pp. 15829-15866

Author(s):

J. C. Péré ◽

B. Bessagnet ◽

M. Mallet ◽

F. Waquet ◽

I. Chiapello ◽

...

Keyword(s):

Optical Properties ◽

Eastern Europe ◽

Air Temperature ◽

Regional Scale ◽

Air Entrainment ◽

Atmospheric Dynamics ◽

Aerosol Layer ◽

Satellite Measurements ◽

Near Surface ◽

Aerosol Plume

Abstract. The present study aims at investigating the shortwave aerosol direct radiative forcing (ADRF) and its feedback on air temperature and atmospheric dynamics during a major fire event that occurred in Russia during August 2010. The methodology is based on an off-line coupling between the CHIMERE chemistry-transport and the Weather Research and Forecasting (WRF) models. First, simulations for the period 5–12 August 2010 have been evaluated by using AERONET and satellite measurements of the POLarization and Directionality of the Earth's Reflectance (POLDER) and the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) sensors. During this period, elevated POLDER AOT are found over a large part of Eastern Europe with values above 2 (at 550 nm) in the aerosol plume. According to CALIOP observations, particles remain confined within the first five kilometres of the atmospheric layer. Comparisons with satellite measurements show the ability of CHIMERE to reproduce the regional and vertical distribution of aerosols during their transport from the source region. Over Moscow, AERONET measurements indicate an important increase of AOT (340 nm) from 0.7 on 5 August to 2–4 between 6 and 10 August when the aerosol plume is advected over the city. Particles are mainly observed in the fine size mode (radius in the range 0.2–0.4 μm) and are characterized by elevated SSA (0.95–0.96 between 440 and 1020 nm). Also, comparisons of simulations with AERONET measurements show that aerosol physical-optical properties (size distribution, AOT, SSA) have been well simulated over Moscow in term of intensity and/or spectral dependence. Secondly, modelled aerosol optical properties have been used as input in the radiative transfer code of WRF to evaluate their direct radiative impact. Simulations indicate a significant reduction of solar radiation at the ground (up to 80–150 W m−2 in diurnal-averaged) over a large part of Eastern Europe due to the presence of the aerosol plume. This ADRF causes an important reduction of the near-surface air temperature between 0.2 and 2.6°C at a regional scale. Moscow has been also affected by the aerosol plume, especially between 6 and 10 August. During this period, aerosol causes a significant reduction of surface shortwave radiation (up to 70–84 W m−2 in diurnal-averaged) with a moderate part (20–30%) due to solar absorption within the aerosol layer. The resulting feedbacks lead to a cooling of the air up to 1.6°C at the surface and 0.1°C at an altitude of 1500–2000 m (in diurnal-averaged), that contribute to stabilize the atmospheric boundary layer (ABL). Indeed, a reduction of the ABL height of 13 to 65% have been simulated during daytime in presence of aerosols. This decrease is the result of a lower air entrainment as the vertical wind speed in the ABL is shown to be reduced by 5 to 80% (at midday) when the feedback of the ADRF is taken into account. In turn, CHIMERE simulations driven by the WRF meteorological fields including this ADRF feedback result in a large increase in the modeled near-surface PM10 concentrations (up to 99%) due to their lower vertical dilution in the ABL, which tend to reduce model biases with the ground PM10 values observed over Moscow during this specific period.

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Conceptual and numerical modeling approach of the Guarani Aquifer System

Hydrology and Earth System Sciences ◽

10.5194/hess-17-295-2013 ◽

2013 ◽

Vol 17 (1) ◽

pp. 295-314 ◽

Cited By ~ 20

Author(s):

L. Rodríguez ◽

L. Vives ◽

A. Gomez

Keyword(s):

Hydraulic Conductivity ◽

Water Balance ◽

Water Budget ◽

Flow System ◽

Regional Scale ◽

Early Years ◽

Scale Model ◽

Aquifer System ◽

Guarani Aquifer ◽

The Impact

Abstract. In large aquifers, relevant for their considerable size, regional groundwater modeling remains challenging given geologic complexity and data scarcity in space and time. Yet, it may be conjectured that regional scale groundwater flow models can help in understanding the flow system functioning and the relative magnitude of water budget components, which are important for aquifer management. The Guaraní Aquifer System is the largest transboundary aquifer in South America. It contains an enormous volume of water; however, it is not well known, being difficult to assess the impact of exploitation currently used to supply over 25 million inhabitants. This is a sensitive issue because the aquifer is shared by four countries. Moreover, an integrated groundwater model, and therefore a global water balance, were not available. In this work, a transient regional scale model for the entire aquifer based upon five simplified, equally plausible conceptual models represented by different hydraulic conductivity parametrizations is used to analyze the flow system and water balance components. Combining an increasing number of hydraulic conductivity zones and an appropriate set of boundary conditions, the hypothesis of a continuous sedimentary unit yielded errors within the calibration target in a regional sense. The magnitude of the water budget terms resulted very similar for all parametrizations. Recharge and stream/aquifer fluxes were the dominant components representing, on average, 84.2% of total inflows and 61.4% of total outflows, respectively. However, leakage was small compared to stream discharges of main rivers. For instance, the simulated average leakage for the Uruguay River was 8 m3 s−1 while the observed absolute minimum discharge was 382 m3 s−1. Streams located in heavily pumped regions switched from a gaining condition in early years to a losing condition over time. Water is discharged through the aquifer boundaries, except at the eastern boundary. On average, pumping represented 16.2% of inflows while aquifer storage experienced a small overall increment. The model water balance indicates that the current rate of groundwater withdrawals does not exceed the rate of recharge in a regional sense.

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Regional-scale analysis of high-mountain multi-hazard and risk indicators in the Pamir (Tajikistan) with GRASS GIS

Natural Hazards and Earth System Science ◽

10.5194/nhess-13-2779-2013 ◽

2013 ◽

Vol 13 (11) ◽

pp. 2779-2796 ◽

Cited By ~ 34

Author(s):

F. E. Gruber ◽

M. Mergili

Keyword(s):

Spatial Scales ◽

Regional Scale ◽

Risk Indicators ◽

High Mountain ◽

Scale Analysis ◽

Model Framework ◽

Rock Face ◽

Grass Gis ◽

Hazard And Risk ◽

The Impact

Abstract. We present a model framework for the regional-scale analysis of high-mountain multi-hazard and -risk indicators, implemented with the open-source software package GRASS GIS. This framework is applied to a 98 300 km2 study area centred in the Pamir (Tajikistan). It includes (i) rock slides, (ii) ice avalanches, (iii) periglacial debris flows and (iv) lake outburst floods. First, a hazard indicator is assigned to each relevant object (steep rock face, glacier or periglacial slope, lake). This indicator depends on the susceptibility and on the possible event magnitude. Second, the possible travel distances, impact areas and, consequently, impact hazard indicators for all types of processes are computed using empirical relationships. The impact hazard indicators are finally superimposed with an exposure indicator derived from the type of land use, resulting in a raster map of risk indicators finally discretized at the community level. The analysis results are presented and discussed at different spatial scales. The major outcome of the study, a set of comprehensive regional-scale hazard and risk indication maps, shall represent an objective basis for the prioritization of target communities for further research and risk mitigation measures.

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Aerosol liquid water content in the moist southern West African monsoon layer and its radiative impact

10.5194/acp-2018-420 ◽

2018 ◽

Author(s):

Konrad Deetz ◽

Heike Vogel ◽

Sophie Haslett ◽

Peter Knippertz ◽

Hugh Coe ◽

...

Keyword(s):

Relative Humidity ◽

Water Content ◽

Aerosol Optical Depth ◽

Optical Depth ◽

Diurnal Cycle ◽

Liquid Water ◽

Liquid Water Content ◽

Scale Model ◽

Model Framework ◽

The Impact

Abstract. Water uptake can significantly increase the size and therefore the optical properties of aerosols. In this study, the regional-scale model framework COSMO-ART is applied to Southern West Africa (SWA) for a summer monsoon process study on 2–3 and 6–7 July 2016. The high moisture and aerosol burden in the monsoon layer makes SWA favorable to quantify properties that determine the aerosol liquid water content and its impact on radiative transfer. Given the marked diurnal cycle in SWA, the analysis is separated into three characteristic phases: (a) Atlantic Inflow progression phase (15–2 UTC), when winds from the Gulf of Guinea accelerate in the less turbulent evening and nighttime boundary layer, (b) Moist morning phase (3–8 UTC), when the passage of the Atlantic Inflow front leads to overall cool and moist conditions over land and (c) Daytime drying phase (9–15 UTC), in which the Atlantic Inflow front re-establishes with the inland heating initiated after sunrise. This diurnal cycle imprints, via the relative humidity, also the aerosol liquid water content. We analyzed the impact of relative humidity and clouds on the aerosol liquid water content. As shown by other studies, the accumulation mode particles are the dominant contributor of aerosol liquid water. We find aerosol growth factors of 2 (4) for submicron (coarse) mode particles, leading to a substantial increase of mean aerosol optical depth from 0.2 to 0.7. Considering the aerosol liquid water content leads to a decrease in shortwave radiation of about 20 W m−2, while longwave effects appear to be insignificant, especially during nighttime. The estimated relationships between total column aerosol liquid water and radiation are −305 ± 39 W g−1 (shortwave in-cloud), −114 ± 42 W g−1 (shortwave off-cloud) and about −10 W g−1 (longwave). The results highlight the need to consider the relative humidity dependency of aerosol optical depth in atmospheric models, particularly in moist tropical environments, where their effect on radiation can be very large.

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