http://197.159.135.214/jspui/handle/123456789/978 2026-04-23T15:09:25Z 2026-04-23T15:09:25Z Dajuma, Alima http://197.159.135.214/jspui/handle/123456789/1058 2026-02-13T10:49:50Z 2019-09-01T00:00:00Z

Impact of Natural and Anthropogenic Aerosols on Clouds and Radiative Properties over West Africa Dajuma, Alima Using a high-resolution regional climate model COSMO-ART, the impact of natural and anthropogenic aerosols loads on radiative and cloud optical properties over West Africa is assessed through sensitivity experiments. Based on a monthly climatology, model simulations compare satisfactory with wind fields from reanalysis data, cloud observations, daily average particulate matter (PM), temperature, relative humidity and satellite retrieved CO mixing ratio. However, COSMO-ART shows a slight overestimation of spatial distribution of CO mixing ratio compared to satellite observation. Regarding the atmospheric composition, COSMO-ART shows good representation with the observation of aerosol mass concentration in upwind marine and urban outflow, whereas it underestimates the observed aerosols in the regional background. Evaluating two emission inventories datasets; simulation conducted with DACCIWA emissions performed better than with EDGAR emissions for the chemistry. However, aerosols compounds are mostly underestimated by COSMO-ART except for NO3 using EDGAR inventory. Focusing on a case study of 02 July 2016, using CO as an indicator for biomass burning plume, individual mixing events south of the coast of Côte d’Ivoire due to midlevel convective clouds injecting parts of the biomass burning plume into the boundary layer were identified. Idealized tracer experiments suggested that about 20% of the CO mass from the 2–4km layer are mixed below 1km within two days over the Gulf of Guinea. Quantifying the impact of biomass burning aerosols on cloud and radiation, it is found that 27% increase of cloud droplets number concentration (CDNC) over the Gulf of Guinea and 7.5% over the whole simulated domain spatially averaged. A reduction of 50 W m-2 of surface shortwave (SW) radiation and 20 W m-2 respectively over the Gulf of Guinea (3–4ºN; 9 ºW –1ºE) and over the study domain (3–11ºN; 9 ºW –1ºE) spatially averaged for all sky conditions. The maximum increase of surface concentration for 05- 06 July 2016 case study due to biomass burning aerosols is 154 μg m-3 for CO, 27 μg m-3 for O3 and 6–7 μg m-3 for PM which agrees with WRF-CHEM simulation. Same investigation was done to quantify the impact of local source anthropogenic aerosols on clouds and radiation. 90% of the CDNC are from anthropogenic aerosols and a shift of median clouds radius from 12 to 10 μm as result of anthropogenic aerosols. Regarding the surface SW, a decrease of up to 60 W m-2 is simulated over the study domain as a result of anthropogenic aerosol. A reduction of monsoon wind speed of about 1.2 m s-1 occurred as a result of anthropogenic aerosols. The impact of future anthropogenic emissions increase is examined under two RCP 2.6 8.5 scenarios. All pollutants are projected to increase more or less, over the whole West African region except for BC which shows some decrease in Nigeria part of Ghana and Niger. Looking at local scale, coastal cities (Abidjan and Accra) is predicted to exceed the World Health Organization (WHO) with respect to NOx and O3. In conclusion, the results of this study underscore the need to investigate the impact of aerosols (anthropogenic, biomass burning) on the cloud properties and radiative budget on a longer time scales and a need for air quality monitoring over the region especially over the coastal cities. A Thesis submitted to the West African Science Service Centre on Climate Change and Adapted Land Use and the Federal University of Technology, Akure, Nigeria, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Degree in West African Climate Systems

2019-09-01T00:00:00Z Quenum, Gandome Mayeul Leger Davy http://197.159.135.214/jspui/handle/123456789/1057 2026-02-13T10:29:46Z 2019-09-01T00:00:00Z

Extreme Climate Events Prediction over Westafrica using a Coupled Atmosphere-Hydrology Model System and Climate Indices Quenum, Gandome Mayeul Leger Davy Rising temperature is one of the direct indicators of global climate change. To investigate how the rising global temperature will affect the spatial pattern of rainfall and consequent flood and drought in West Africa, precipitation and potential evapotranspiration variables from ten Global Climate Models (GCMs) under the RCP8.5 scenario were downscaled by the Rossby Centre regional atmospheric model (RCA4) from the Coordinated Regional Climate Downscaling Experiment (CORDEX) and analysed at four specific global warming levels (GWLs) (i.e., 1.5℃, 2.0℃, 2.5℃, and 3.0℃) above the pre-industrial level. This study utilized four indices: the standardized precipitation evapotranspiration index (SPEI), the precipitation concentration index (PCI), the precipitation concentration degree (PCD), and the precipitation concentration period (PCP) to explore the spatio-temporal variations in the characteristics of precipitation concentrations. Additionally, studying the impact of the four GWLs on consecutive dry days (CDD), consecutive wet days (CWD), and frequency of the intense rainfall events led to a better understanding of the spatiotemporal pattern of extreme precipitation. The onset of rainfall comes one month earlier in the Gulf of Guinea compared to the historical period, with increasing rainfall intensity in the whole study domain. To encourage adaptation to the various changes in climate in general, and particularly in respect of rainfall, the study proposes two adaptation methods that can be implemented at the local (country) level, as well as some mitigation and adaptation strategies at the regional level. More practically, to analyze flood events which became more frequent since 2000 in West Africa, this research improve on previous analysis by designing an experimental work using the coupled atmosphere-hydrology modeling system WRF-Hydro over Ouémé-river basin in Benin for the period 2008-2010. Such a coupled model allows exploring the contribution of atmospheric components into the flood event, and its ability to simulate and predict accurate streamflow. The potential of WRF-Hydro to correctly simulating streamflow in the Ouémé-river basin is assessed by forcing the model with operational analysis dataset from the ECMWF. Atmospheric and land surface processes are resolved at a spatial resolution of 5 km. The additional surface and subsurface water flow routing are computed at a resolution 1:10. Key parameters of the hydrological module of WRFHydro were calibrated offline and tested online with the coupled WRF/WRF-Hydro. As a result, WRF-Hydro was able to simulate the discharge in Ouémé river on offline and fully-coupled modes with a Kling-Gupta Efficiency (KGE) of 0.70 and 0.76 respectively. In fully-coupled modes, the model captures the flood event that occurred in 2010 in the catchments of interest. The uncertainty of atmospheric modeling on coupled results is assessed with the stochastic kinetic-energy backscatter scheme (SKEBS) by generating an ensemble of 10 members for three rainy seasons. It shows that the coupled model performance in terms of KGE ranges form 0.14-0.79 and 0.13- 0.75 at Savè and Bétérou respectively. This ability in realistically reproducing observed discharge in the Ouémé-river basin demonstrates the potential of the coupled WRFHydro modeling system for flood forecasting applications. A Thesis submitted to the West African Science Service Centre on Climate Change and Adapted Land Use and the Federal University of Technology, Akure, Nigeria, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Degree in West African Climate Systems

2019-09-01T00:00:00Z Kumi, Naomi http://197.159.135.214/jspui/handle/123456789/1056 2026-02-13T10:21:34Z 2019-09-01T00:00:00Z

Improving Seasonal Rainfall Prediction over West Africa using Dynamic Climate Models Kumi, Naomi Reliable prediction of seasonal rainfall is crucial for decision-making in various socio-economic sectors in West Africa, but obtaining reliable forecasts poses a big challenge to weather forecasters across the region, because their seasonal forecasts are mostly based on empirical models. While several recent studies are suggesting that the use of dynamic climates models may be a solution to the challenge, there is a dearth of information on how well these models simulate parameters like rainfall onset date (ROD), rainfall cessation date (RCD) and length of rainy season (LRS) over West Africa. The present study evaluates the performance of both global and regional climate models (GCMs and RCMs) in simulating these parameters over the study domain in the past and present climate. These datasets are from the China Meteorological Administration (CMA) Sub-seasonal to Seasonal (S2S) prediction, the UK Met Office Unified Model (MetUM), and 8 of RCMs that participated in the Coordinated Regional Climate Downscaling Experiment (CORDEX). The study also examines how a further modification of the Betts-Miller Janjic (BMJ) convective scheme in the Weather Research and Forecasting (WRF) model can improve the prediction of seasonal rainfall over West Africa. This study further assesses the potential impacts of 1.5°C and 2°C global warming levels (GWL15 and GWL20) on ROD, RCD and LRS in West Africa. Using common definitions within the sub-region, the simulated RODs, RCDs and LRS are compared with observation from satellite datasets, and the models’ capability to reproduce the inter-annual variability of these parameters over the climatological zones in the sub-continent is statistically quantified. The impacts of GWL15 and GWL20 on each parameter were also quantified and compared. The outcomes of the study show that all the models have some biases in their simulations although they do produce convincing results. The CMA model realistically simulates the observed spatial pattern and the interannual variability of RODs in the study area, as well as the observed seasonal movement of the West African Monsoon (WAM) and its associated rainfall patterns. The MetUM also reproduces the latitudinal progression of the observed RODs, RCDs and LRS over West Africa suitably, but performs poorly in simulating their inter-annual variability, even though there is improvement in the simulations of the new versions. It was also found that the CORDEX RCM ensemble correctly replicated and captured the essential features in the observed RODs, RCDs and LRS in the historical climate, and the RCM spread also enclosed the observed values. Most of the selected convection schemes reliably simulate the observed spatial distribution of RODs, RCDs, and LRS in the study area but overestimated the average monthly rainfall over the entire West African region. A new version of the BMJ scheme outperforms the default scheme in the sub-continent. The study project the western and eastern Sahel as hot-spots for a delayed ROD and reduced LRS in the 1.5°C and 2°C warmer climate under the Representative Concentration Pathway (RCP4.5 and RCP8.5) scenarios. The results of this study will be beneficial for agricultural and water resources planning decision-making and in reducing the impacts of global warming over West Africa. A Thesis submitted to the West African Science Service Centre on Climate Change and Adapted Land Use and the Federal University of Technology, Akure, Nigeria, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Degree in West African Climate Systems

2019-09-01T00:00:00Z