Browsing by Author "Abiodun, Babatunde J"

Now showing 1 - 7 of 7
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    Open Access
    CAM-EULAG: a non-hydrostatic atmospheric climate model with grid stretching
    (2011) Abiodun, Babatunde J; Gutowski, William J; Abatan, Abayomi A; Prusa, Joseph M
    This study evaluates the capability of a non-hydrostatic global climate model with grid stretching (CEU) that uses NCAR Community Atmospheric Model (CAM) physics and EULAG dynamics. We compare CEU rainfall with that produced by CAM using finite volume dynamics (CFV). Both models simulated climate from 1996 to 2000, using the same parameterization schemes.
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    Potential impacts of climate change on hydrological extremes in the Incomati River Basin
    (2021) Mogebisa, Tlakale; Abiodun, Babatunde J
    Climate change has been shown to influence extreme rainfall and flooding events over many river basins, yet there is a dearth of information on how to mitigate future risks and vulnerabilities in the Incomati River Basin (IRB), a basin known for extreme devastating flood events. This thesis investigates the potential impacts of climate change on extreme hydrological events that induce flood in the Incomati River Basin (IRB). A series of climate and hydrological simulation datasets were analysed for the study. The climate simulation datasets were acquired from the Global Meteorological Forcing Dataset (GMFD) and the CO-ordinated Regional Downscaling Experiment (CORDEX), but the hydrological simulation datasets were generated with the latest version of the Soil and Water Assessment Tool (called SWAT+), using GMFD and CORDEX as the climate forcing data. The CORDEX dataset was biased-corrected with GMFD, using the Quantile Delta Mapping (QDM) method. The SWAT+ was calibrated and evaluated over the basin to investigate the role of objective functions in SWAT+ calibration, four sensitivity experiments were performed using four objective functions (hereafter, 1-NSE or RMSE, 1-R 2 and PBIAS). To study the influence of the bias correction of CORDEX on hydrological simulations, the SWAT+ simulations were performed using the original and biased-corrected CORDEX datasets as the climate forcing. The impacts of climate change on the mean hydroclimate variables and on characteristics of extreme hydrological events (i.e. the intensity and frequency of extreme precipitation and streamflow events) were examined at four global warming levels (i.e. GWL1.5, GWL2.0, GWL2.5, GWL3.0) under the RCP8.5 future climate scenario. The results of the study show that SWAT+ gives realistic simulations of hydrological processes in the basin, although with notable biases in the simulated streamflow. The SWAT+ calibration over the basin is sensitive to the choice of objective function for the calibration. The calibration converges faster with 1-NSE or RMSE than with R2 or PBIAS. The performance of SWAT+ in simulating the streamflow over the basin depends on the statistical metrics used in the evaluation, while the NSE of the model SWAT+ simulation is poor (i.e. NSE ≈ -0.08) over all the stations, the PBIAS is very good (i.e. PBIAS ≈ 13.7%) at some stations. The bias correction of CORDEX datasets substantially reduces errors in the climate datasets and improves the quality of SWAT+ simulations over the basin. Moreover, it also reduces the level of uncertainty in the simulations. With global warming, a future increase in temperature is projected over the basin, but a decrease in annual precipitation is indicated over most part of the basin except at the south-west tip of the basin (i.e. around Nooitgedacht Dam), where precipitation is projected to increase. The changes in hydrological extreme events generally follows the precipitation pattern, in that, while less intense and less frequent extreme precipitation and streamflow events are projected over most parts of the basin, more intense and more frequent precipitation and streamflow are indicated in the vicinity of the dam. However, the projection also suggests that an increase in extreme precipitation and streamflow activities surrounding this water body could induce extreme streamflow events downstream of the basin. The results of this thesis have applications in mitigating the impacts of climate change on extreme hydrological events in the basin.
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    Simulating the Characteristics and Influences of the Botswana High over Southern Africa using the Model for Prediction Across Scales (MPAS)
    (2022) Maoyi, Molulaqhooa Linda; Abiodun, Babatunde J
    The Botswana High is a prominent mid-tropospheric system that modulates rainfall over subtropical Southern Africa, but the capability of a Global Climate Model (GCM) to reproduce the characteristics and influences of this system on drought remains unknown. Furthermore, the summer variability of the Botswana High has been linked to the El Niño Southern Oscillation (ENSO). However, it remains unknown whether the high's variability is a direct response to ENSO. To that end, this thesis examines the capability of a GCM with quasiuniform resolution (Model Prediction Across Scales, hereafter MPAS) in simulating the characteristics and influences of the Botswana High on drought modes over the subcontinent as well as the influence of ENSO on the high. To simulate the characteristics of the Botswana High and its influence on drought modes, the MPAS model is applied to simulate the global climate at 240km quasi-uniform resolution over the globe for the study period 1980-2010. The model results are validated against gridded observation dataset (Climate Research Unit, CRU), satellite dataset (Global Precipitation Climatology Project, GPCP), and reanalysis datasets (Climate Forecast System Reanalysis, CFSR; the National Oceanic and Atmospheric Administration, NOAA; and ERA-Interim reanalysis 5, ERA5). To investigate the response of the Botswana High to ENSO, this thesis carried out two MPAS model experiments. The first model experiment used observed SSTs everywhere during the study period, while the second experiment used observed SSTs everywhere except over the Pacific Ocean, where monthly climatological SSTs are imposed. The results of this thesis show that MPAS replicates all the essential features in the climatology of climate variables (e.g. temperature, rainfall, 500 hPa geopotential height and vertical motion) over Southern Africa, reproduces the spatial and temporal variation of the Botswana High, and captures the influence of the Botswana High on droughts and deep convections over the subcontinent. In all the datasets (CRU, ERA5, 20C and MPAS), the most dominant five Drought Modes (hereafter DM1-DM5) over Southern Africa jointly explain more than 60% of the interannual variability in the 3-month summer droughts for SPEI and for SPI. ERA5 and MPAS agree that the Botswana High influences the interannual variability of DM1; however, the influence is strong in ERA5 (r = -0.85) and moderate in MPAS (r = -0.42). In addition to that, wet years (+ve SPEI and SPI) are characterized by a weak Botswana High and drought years (-ve SPEI and SPI) by a strong Botswana High. In addition to that, the wet and dry years correspond to the -ve and +ve phases of El Niño Southern Oscillation (ENSO), respectively. Given this, the results of this thesis suggest that the Botswana High might be a conduit pipe through which ENSO signals influence DM1 over the region. Investigation into the impact of ENSO on the Botswana High reveals that the absence of ENSO forcing reduces the amplitude of the Botswana High variability, but the signal of the variability remains. While ENSO enhances the strength of the Botswana High, it does not aid the formation of the High. The result of the thesis has application in the improvement and application of MPAS for drought early warning systems over Southern Africa.
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    Simulating the Characteristics of Tropical Cyclones over the South-West Indian Ocean: Sensitivity to Model Horizontal Resolution
    (2022) Donkin, Paige Tayla; Abiodun, Babatunde J
    Tropical cyclones (TCs) are the deadliest and most economically devastating of all natural hazards. Those that occur in the South-West Indian Ocean (SWIO) often strike vulnerable nations and can have serious socio-economic impacts, yet there is a dearth of information on how well these systems are simulated by contemporary climate models. This study investigates the performance of four global climate models and one variable-resolution climate model in simulating the characteristics of SWIO TC activity through both a climatological and individual event analysis, and investigates the influence of horizontal resolution on the simulated TC characteristics. The climatological analysis makes use of a multi-model, multi-resolution set of simulations from four modelling groups included in the CMIP6 High Res MIP project, covering the period 1980-2009. The individual event analysis focuses on the simulation of two highly destructive TC events which occurred in the SWIO in early 2019 – namely TC Idai and TC Kenneth. The simulation of individual events makes use of the atmospheric Model for Prediction Across Scales (MPAS-A), a non-hydrostatic variable-resolution model which is employed at multiple resolutions ranging from 60km–3km. In both analyses, systems were identified using a novel resolution dependent tracking algorithm that includes wind speed, vorticity, and warm core thresholds. All model simulations were validated against the Reunion Specialised Meteorological Centre (RSMC) best track observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis dataset (ERA5). The results of the CMIP6 analysis indicate that all models produce realistic representations of TCs over the SWIO, but underestimate TC intensity, and feature a westward shift bias in the spatial distributions of TC activity. Increasing model horizontal resolution enhances the quality of the simulated TC frequency and intensity, with higher-resolution models showing more realistic storm counts and an increased ability to represent the full range of observed TC intensities. However, increasing horizontal resolution does not improve the spatial distribution of cyclogenesis locations or TC tracks, nor the ability of the models to capture the long-term trends in TC activity. Similarly, the results of the MPAS-A analysis indicate that MPAS-A produces realistic TC tracks, but shows considerable southward track and landfall biases, and severely underestimates the intensities of the systems. A local increase in horizontal resolution over the SWIO improves all aspects of the TC simulation, including the TC track, intensity, wind-pressure relationship, and vertical structure. However, the level of improvement differs based on the physics parameterization suite employed in the model, the characteristic of interest, and the event in question. The present study illustrates the extent to which an increase in model horizontal resolution can improve simulated TC characteristics and offers insights on how to improve future projections of TC activity over the SWIO region.
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    Simulating the Dynamics and Influences of the West African Westerly Jet
    (2022) Makinde, Akintunde Israel; Abiodun, Babatunde J
    The West African westerly jet (WAWJ) is a key rainfall producing system over West Africa. While past studies have examined the dynamics of the WAWJ and its influences on Sahelian rainfall, there is no information on how well the jet is simulated by contemporary climate models. This thesis examines the capability of the Coupled Model Intercomparison Project version 6 models (CMIP6) and the Atmospheric component of Model Prediction Across Scale (MPAS) in simulating the WAWJ, its moisture transport over West Africa, and its influence on Sahelian precipitation. Three types of climate dataset (observation, reanalysis, and simulations) were analysed in the thesis. The observation dataset (from the Climate Research Unit or ‘CRU') and the reanalysis dataset (from the European Centre for Medium-Range Weather Forecast Atmospheric Reanalysis, or ‘ERA5') were used to evaluate 26 CMIP6 models (for 35 years: 1980–2014) and MPAS (for 30 years: 1985–2014). To investigate the sensitivity of the simulated WAWJ to model resolution, two additional simulations were performed with MPAS, focusing on WAWJ strong years (1989, 1994, and 1999) and weak years (1986, 1990, and 2000). This thesis defined the WAWJ as a low-level westerly jet (with average maximum speed of 5 m s-1 ) at 925 hPa over the eastern Atlantic Ocean and over the West African coast and used standard statistical metrics to quantify the capability of the models in simulating the characteristics and influences of the WAWJ. The majority of the CMIP6 models capture the temporal and spatial structures of the WAWJ and agree with ERA5 that the jet attains its maximum speed in August. However, most simulated jets form earlier and are stronger than the observed jet. While most of the CMIP6 models capture the link between the jet and temperature distribution over West Africa, they struggle to reproduce the relationship between the jet and precipitation distribution over the sub-continent, especially over the Sahel. Most models failed to replicate the increase in the moisture transport (i.e., the eastward and north-eastward transports) associated with a stronger WAWJ, as in ERA5. Some models capture the increased moisture transport but do not translate it to increased precipitation over the Sahel. MPAS performs well in simulating various features in temperature, precipitation, and wind fields over West Africa, but with wet and warm biases over the region. It also simulates the WAWJ but the simulated jet forms too early and is too strong. In addition, the position and dynamics of the simulated jet differ from the observed jet because the model fails to capture the local pressure gradient force that induces the WAWJ over the Atlantic Ocean. The model underestimates the relationship between the WAWJ and Sahelian precipitation because it limits WAWJ moisture transports to the southern part of Sahel, in contrast to the observation. The sensitivity simulations show that increasing the horizontal resolution of the model does not improve the MPAS simulation of the WAWJ or the WAWJ moisture transport to the Sahel. The results of the study have application in improving the climate models for seasonal predictions and future projections over West Africa, especially over the Sahel.
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    The response of southern African vegetation to droughts in past and future climates
    (2018) Lawal, Shakirudeen Abimbola; Abiodun, Babatunde J
    Drought and climate change pose a threat to southern African vegetation. This study examines the response of southern African vegetation to drought in both past and future climates. Multiyear and multi-simulation datasets from three dynamic global vegetation models (DGVMs), namely, Community Land Model version 4 (CLM4), Community Land Model version 4 with Variable Infiltration Capacity hydrology (CLM4VIC), and Organising Carbon and Hydrology in Dynamic Ecosystems designed by Laboratoire des Sciences du Climat et de l’Environnement (ORCHIDEE-LSCE). These three DGVMs and the Community Earth System Model (CESM) were analyzed for the study. The DGVM simulations were forced with the reanalysis climate dataset from the National Centers for Environmental Prediction (NCEP) and the Climatic Research Unit - NCEP (CRUNCEP). The simulated climate results were evaluated with observation datasets from the Climatic Research Unit (CRU), while the simulated vegetation index (i.e. Normalized Difference Vegetation Index, NDVI) were evaluated with NDVI data from the Global Inventory Modelling and Mapping Studies (GIMMS). Meteorological droughts were analyzed at different timescales (1- to 18-month timescales), using two drought indexes: the Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI). The responses of vegetation to drought were quantified by means of Pearson Correlation Analysis. The DGVMs were applied to study the sensitivity of vegetation to fire, while the CESM was used to project impact of climate change on the characteristics of southern African vegetation in the future (up to the year 2100) under the 8.5 Representative Concentration Pathway (RCP8.5) scenario, focusing on impacts at 1.5oC and 2.0oC global warming levels (GWLs). Analysis of the observed data shows that the spatial distribution of vegetation across southern Africa is more influenced by the rainfall distribution than by the temperature distribution. The observed correlation between drought index and vegetation index is higher than 0.8 over southeastern part of the region at 3-month drought timescale, and there is no difference between the spatial distribution of the correlation between the SPEI and the vegetation index, and between the SPI and the vegetation index. The three DGVMs failed to capture the response of vegetation to drought; however, the CLM4 shows the best performance while ORCHIDEELSCE fared the worst of the three. The CLM4 simulation show that fire strongly influences growth of vegetation over the summer rainfall region but it has weak influence over vegetation in the western arid zone. The CESM strongly captures the spatial patterns of precipitation and the vegetation index across southern Africa, but it overestimates the magnitudes of the vegetation index across the region, except in Namibia and Angola. The CESM also underestimates the correlation between drought indexes with vegetation, and the timescales at which the vegetation respond to droughts. The CESM projects an increase in the drought intensity as a result of an increased temperature across southern African biomes. However the increase in drought intensity is more pronounced with the SPEI than with the SPI. CESM also projects a future decrease in the vegetation index (i.e. NDVI) in the region except in the dry savanna biome. The impacts of 1.5oC GWLs on the vegetation fluxes vary throughout southern Africa, and the magnitudes of changes in the vegetation fluxes are affected by a further increase in global warming over the region. While there is a good agreement among the CESM simulations on the projected changes in vegetation fluxes across the biomes, the uncertainty in the projections is higher with 1.5oC than with 2.0oC GWL. The results of the study can be applied to mitigate the impacts of climate variability and change on southern African vegetation. Specific mitigation efforts that could be applied to reduce the impacts of droughts and climate change are watershed management, improved vegetation management, impact monitoring, environmental awareness, and remote sensing tools.
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    Understanding the characteristics of droughts over Eastern Africa in past and future climates
    (2020) Nguvava, Mariam Melikizedek; Abiodun, Babatunde J
    Drought poses a threat to socio-economic activities across eastern Africa and its river basins. While there are indications that global warming may continue to enhance evaporation and intensify droughts at all scales, most drought projections over eastern Africa are based on rainfall alone and are limited to meteorological droughts. The present study combines rainfall and Potential Evapotranspiration (PET) to examine the characteristics of meteorological and hydrological droughts in present and future climates at the regional and river basin scales. To accomplish that we have applied five objectives; i) Study the temporal and spatial characteristics of eastern Africa droughts modes, ii) Investigate how some atmospheric teleconnections influence the characteristics of the Africa droughts modes, iii) Examine the influence of 1.5°C and 2°C global warming levels on drought modes in eastern Africa under two future climate scenarios, RCP 4.5 and RCP8.5 iv) Assess how increases in global warming will influence drought characteristics over eastern African river basins. v) Examine the potential impacts of climate change and land use change on water availability in the Rufiji River basin (RRB), Tanzania, with an emphasis of hydrological droughts in this basin. Different types of datasets, including gridded and station observation datasets, regional climate model simulations (CORDEX: Coordinated Regional Climate Downscaling Experiment) and hydrological simulations (SWAT: Soil and Water Assessment Tool), were analysed for the study. The meteorological drought were characterised using two indices (i.e. Standardized Precipitation Evapotranspiration Index, SPEI; Standardized Precipitation Index, SPI) at 3- and 12-month scales, while the hydrological droughts were characterised using four indices (i.e. soil water index, SWI; Surface Runoff Index, RFI; Water Yield Index, WYI; and Stream Flow index, SFI). The study combined principal component analysis (PCA) with wavelet analysis to identify the spatio-temporal structure of four dominant drought modes over the region. It also used wavelet coherence to quantify the influence of four atmospheric teleconnections (i.e. El Niño Southern Oscillation, ENSO; Indian Ocean Dipole, IOD; Tropical Atlantic Dipole Index, TADI; and Quasi-Biennial Oscillation, QBO) on the drought modes. The study also projects the characteristics of future droughts over eastern Africa and its major river basins at different global warming levels (GWLs). Series of hydrological simulations were used to assess the sensitivity of future droughts to four land use change scenarios (i.e. increase in forestry, shrubs, cropland and agriculture) over the Rufiji River Basin (RRB), a prominent river basin in eastern Africa. Although eastern Africa have been documented with several drought studies, the application of a combination of PCA, Wavelet analysis, wavelet coherence and Self Organizing Maps provides more comprehensive representation of droughts in the region using SPEI/SPI derived from both models and observations The results of the study show that the four drought modes, which have their core areas over different parts of eastern Africa, account for more than 45% of drought variability in the region. All the drought modes are strongly coupled with either ENSO or IOD indices (or both); but, in addition, one of the modes is also strongly coupled with the TADI. CORDEX models give a realistic simulation of the relevant climate variables for calculating drought indices over eastern Africa and the river basins. However, the ensemble mean struggles to reproduce the spatial distribution and frequency of drought intensity in the region. The CORDEX simulations project no changes in the spatial structure of the drought modes but suggest an increase in SPEI drought intensity and frequency over the hotspots of the drought modes and elsewhere in the region. The magnitude of the increase, which varies over the drought mode hotspots, increases with increasing GWLs. The projections also show that the increase in intensity and frequency of drought can be attributed more to increased PET than to reduced precipitation. In contrast to the SPEI projection, the SPI projection shows a weak change in intensity and frequency of droughts, and the magnitude of the increase does not vary with the GWLs. Over the river basins, the SPEI projections are more robust than the SPI projections. Over the RRB, the future projections of some hydrological drought indices (i.e. RFI and SFI) follow the change in the SPEI projections, while others (i.e. SWI and WYI) follow that of SPI. Among the four land use scenarios considered, only forestry and shrubs show a substantial change in the hydrological drought indices. The results of the study thus give valuable insight into the characteristics of future droughts in eastern Africa and provide a useful guide to the effectiveness of using land cover to reduce the severity of hydrological droughts over river basins in the region. However, resolution of CORDEX dataset (50km, i.e. 0.44deg) could be among the potential limitation as it is too low to capture the influence of local-scale processes (e.g. sea breeze, mountain induced circulations) on drought over the region.