West African Climate Systems - Batch 4

Effects of Pre-Monsoon Biomass Burning Aerosols on Rainfall Characteristics over West Africa

2023-05-01T00:00:00Z

Effects of Pre-Monsoon Biomass Burning Aerosols on Rainfall Characteristics over West Africa Njie, Teeda This study investigate the effects of pre-monsoon biomass burning aerosols (BBA) on rainfall characteristics over West Africa. The specific objectives aimed to be achieved are; estimating the distribution of pre-monsoon biomass burning aerosols (BBA) over the study area; analyzing rainfall characteristics over the study area; evaluating the capability of the regional climate model (WRF-Chem) on capturing BBA effect on monsoon rainfall and determining the influence of BBA on rainfall characteristics and cloud formation. AERONET Aerosol Optical Depth (AOD) and Angstrom Exponent (AE) data were used to estimate the temporal distribution of AOD and AE and classification of aerosol types over the five selected AERONET sites namely; Agoufou, Banizoumbou, Dakar, IER_Cinzana and Ilorin. Rainfall data from ERA5 for the period of 1998-2021 was used to evaluate rainfall characteristics such as distribution, variability, normal, wet and dry (drought) condition and rainfall trend over the five selected sites. Simulations were also run using WRF-Chem model to evaluate the capability of the model in capturing BBA and to investigate the effect of BBA on rainfall and cloud formation. The study found out that AOD peaks in March-June in all the sites except Ilorin were AOD peaks in January. The maximum values of AE were in December-January for all the sites except Ilorin were maximum AE value was in August. This shows the presences of fine mode aerosols. It has been found that desert dust aerosol was the dominant aerosol in all the sites throughout the study period. The normal and wet climatic condition were dominant for both annual and seasonal rainfall in all the sites during the study period. High rainfall variability throughout the study period and all the seasons with no trend for annual and negative trend for MAM and JJA season. This means that it is easier to use mean to predict rainfall performance for the annual rainfall but difficult the seasonal rainfall performance in the study area. Lastly, the study found out that the WRF-Chem model overestimated the rainfall characteristics and the effect of BBA radiation can increase or decrease rainfall amount depending on the period/season over West Africa. The WRF-Chem model also underestimate the values of OLR. But the effect BBA radiative has increased the rate of convective cloud formation 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

Impacts of Aerosols on Precipitation over West Africa

2023-07-01T00:00:00Z

Impacts of Aerosols on Precipitation over West Africa Pouye, Modou The aim of this study is to investigate the interaction between West African aerosols and convective, stratiform and shallow clouds, which affect precipitation quality and amount. Gridded satellite observation precipitation datasets Tropical Rainfall Measuring Mission (TRMM), in-situ precipitation data, reanalysis aerosol data ECMWF Atmospheric Composition Reanalysis 4 (EAC4) and rainwater samples were analysed to study the high aerosol concentration impacts on precipitation amount, frequency and quality over the Sahel and Guinea region. Mann-kendal Test was used to verify the trend in time series data. Sperman correlation was used to identify statistical links between aerosols and precipitation. To investigate the quantitative effects of aerosols on rain frequency over West Africa, the precipitation data was categorized into three groups: light (r < 2.5 mm/h), moderate (2.5 mm/h ≤ r < 10 mm/h), and heavy rain (10 mm/h ≤ r < 50 mm/h). Rain events from 12 stations located across West Africa were classified into two groups, based on aerosol concentration percentile. The first group contained precipitation data with the 10th percentile aerosol concentration considered as clean condition. While the second group contained data with the 90th percentile aerosol concentration considered as polluted condition. Each group was then further divided based on the percentile of aerosol events to obtain the rainfall frequency percentage. Precipitation reduction is observed during the seasons of December - January - February (DJF) and June - July - August (JJA) when black carbon and organic matter concentrations increased over West Africa. Additionally, results suggest that precipitation enhancement is observed during March - April - May (MAM) and September - October - November (SON) seasons when sulphate concentration increased. This may suggest that the West African atmospheric black carbon, organic matter and sulphate variation affect the amount of seasonal rainfall. However, the increase in sea salt particles during SON, when there is higher rainfall, could be related to the role of sea salt particles as cloud condensation nuclei, which could influence the amount and timing of rainfall in the region. Results show that dust is the dominant aerosol over all stations used in this study. Organic matter is the second most significant aerosol in this region, which contributes to atmospheric pollution. A comparison between the Clean-case (C-case) and Polluted-case (P-case) shows that aerosol concentration variations affect precipitation class frequency over West Africa. Results suggest that aerosol pollution mostly decreases light rain and increases moderate and heavy rain over West Africa. Chemical laboratory rainwater quality analysis illustrates that Ouagadougou experienced acid rain during the rainy season of 2022. It indicates that there may be sources of sulphur emissions in these areas that are contributing to the elevated sulphate levels in the rainwater. The rainwater sample from Dakar showed sodium levels above the threshold, which could be an indication of potential sources of sodium contamination in the area. It indicates also that the levels of nitrate in the rainwater samples were within the normal range in most cities, but rainwater in some cities shows that the levels of nitrate in the rainwater samples were below the nitrate thresholds of 1.5 and 5 mg/L. 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

Influence of Climate Change on Hygrological Drought in the Volta River Basin, West Africa

2023-10-01T00:00:00Z

Influence of Climate Change on Hygrological Drought in the Volta River Basin, West Africa Odoom, Peter Rock Ebo Aerosols’ presence in the atmosphere is known to influence health and weather-related matters. This study investigates the influence of climate change on hydrological drought in the Volta River Basin. The specific objectives were to assess; the potential impacts of climate change on meteorological drought over the basin, the ability of the SWATplus model in simulating the water balance of the basin, and the impact of future climate on hydrological droughts. The datasets used were the observation data (GMFD and CRU) and projected climate dataset (CMIP6 and NEX-GDDP), a digital elevation model, land use and the FAO digital soil map (2003). The Standardized Precipitation-Evapotranspiration Index at 12- and 24-month scales were used to characterise meteorological drought and the Standardized Streamflow Index (SSFI) for hydrological drought at a 12-month scale. The principal component analysis (PCA) and the wavelet analysis were utilised to assess the spatiotemporal patterns of drought using SPEI computed from GMFD. PCA was also performed on the SPEI of the CMIP6 and NEX-GDDP to determine the spatiotemporal patterns of droughts. The SWATplus was calibrated and evaluated using streamflow records at some selected stations. The calibrated model was employed to assess the future climate change impacts using the ACCESS-CM2 output. The SSFI were then computed using the simulated streamflow output as input data. Results showed that NEX-GDDP model captured the climate of VRB accurately as compared to CMIP6. GMFD and CRU perform reasonably well in the stations evaluated. Four drought modes (DM1 – north, DM2 – south, DM3 – east, and DM4 – west) obtained from 12- and 24-month SPEI explained 85 % and 87 % of variance in the VRB. The wavelet analysis reveals cycles with periodicities ranging from 1–16 years in all DMs which corresponded to periods of drought and wetness. Most CMIP6 and NEX-GDDP models were able to capture the spatial patterns of DM1 andDM2. The comparison of the CMIP6 and NEX-GDDP model's ability suggests that bias correction can either improve or reduce the models’ performance in reproducing the drought modes. Some NEX-GDDP models performed better than the CMIP6 counterpart. Climate change assessment in the VRB suggests an increment in temperature (1–4 °C) and a decrease of 0–2.5 mm/year2 in precipitation. Most models projected wetter conditions under SSP5-8.5 in the Near term (2021–2050) and Far term (2081–2100) while more precipitation is expected under SSP2-4.5 in the MF (2051–2080). The calibration of the SWATplus model revealed- good performance in Nawuni, Sabari and Saboba with NSE scores of 0.7, 0.68 and 0.81, R2 of 0.72, 0.69 and 0.91, and Pbias (PBIAS) of -9.1, -1.9 and -18 respectively. Bamboi had a poor NSE (0.101) but good PBIAS (22.7) and R2 (0.52). The validation statistics were satisfactory for all stations. Projected streamflow show significant increase in the future in line with projected precipitation. Projections indicate reduced drought events and intensities under certain scenarios. Consequently, the VRB is expected to face increased flood risks due to projected increasing streamflow, posing significant threats to agriculture, infrastructure, and human well-being. More efforts should prioritize flood risk management in the VRB to address these challenges. 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

Effects of Land-Use Change and Aerosol Reguiative Properties on the West African Climate System

2023-05-01T00:00:00Z

Effects of Land-Use Change and Aerosol Reguiative Properties on the West African Climate System Olubukumni, Awoleye Peace West Africa due to its climatological and geographical conditions is an essential domain for the characterization of atmospheric radiative forcing across the globe. This study investigated the interaction between aerosols and solar radiation over West African climatic zones (Sahara, Sahel, Savannah, and the coast of Guinea) between December 2005 and January 2006. The study also analyzed the dynamics of various aerosol sources and types and their contribution to the forcing effect observed, reporting variations in high load of anthropogenic and dust aerosols in the climatic zones. The study also identifies the distinguishing characteristics of the aerosol radiative property parameters, including the fact that high AOD causes high atmospheric absorption and low surface net radiation fluxes, when the Angstrom exponent is low, a greater amount of radiation is absorbed at the top of the atmosphere, and the radiative properties vary with changing aerosol concentration and distribution in the atmosphere. Additionally, an increase in anthropogenic aerosols leads to an increase in SSA. The study included the regional climate model RegCM 4.7.1 to conduct a sensitivity study, identifying the optimal cumulus convective precipitation scheme, planetary boundary layer (PBL), and land-surface scheme. The resulting simulation demonstrated a strong correlation with observed precipitation patterns and well-simulated temperatures. This study also aimed to explore how radiative mechanisms linked to seasonal dynamics in natural vegetation and abrupt land cover changes caused by human activities can influence the West African climate system. In taller vegetation, such as forests and savannas, albedo increased during the growing period, leading to a decline in net shortwave radiation, while in shorter vegetation, such as grasslands and shrublands, albedo decreased, resulting in an increase in net shortwave radiation. However, the cooling and warming impacts of albedo during the growing period in taller and shorter vegetation, respectively, were significantly outweighed by ET cooling, which had a greater impact on LST seasonality in West Africa. Land cover changes caused an average increase in albedo, with the highest surge observed in the dry period. The albedo changes resulting from all land cover changes combined generated a regional instantaneous shortwave surface radiative forcing of -0.03W/m2. Despite this small regional forcing, the study showed that land cover changes from forest to cropland and savanna to grassland conversions had a more significant contribution to LST cooling than other changes in class. However, these conversion classes displayed the largest observed LST warming of up to 1.2oK and 0.4oK, respectively. 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