OpenUCT :: Browsing by Author "Abiodun, Babatunde Joseph"

Browsing by Author "Abiodun, Babatunde Joseph"

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Open Access
Capability of CORDEX RCMs in simulating extreme rainfall events over South africa
(2014) Abba Omar, Sabina; Abiodun, Babatunde Joseph
In South Africa, extreme rainfall events often lead to widespread destruction, damage infrastructure, displace communities, strain water management and even destroy lives. Past studies have shown that reliable predictions of extreme rainfall events from regional climate models (RCMs) could help reduce the impact of these events. The present study evaluates the ability of nine RCMs in simulating extreme rainfall events over South Africa, focusing on the Western Cape (WC) and east coast (EC) areas. This study defines an extreme rainfall over a location as rainfall that is equal to or above the 95th percentile of the rainfall distribution at that location, and defines widespread extreme rainfall events (WEREs) over an area as events during which more than 50 of the grid-points in the area experience extreme rainfall. The 95th percentile threshold values were calculated over 11 years (1998-2008) of South Africa’s daily rainfall data from the nine RCMs (CCLM, REMO, PRECIS, CRCM5, ARPEGE, REGCM3, WRF, RACMO and RCA35), which participated in the Coordinated Regional Climate Downscaling Experiment (CORDEX) and used ERA-Interim (ERAINT) as their boundary forcing. The simulations were compared to two observation datasets (TRMM and GPCP), and to ERAINT rainfall data to understand whether these RCMs improve on the results from ERAINT. A self organizing map (SOM) was used to characterize WEREs identified in all the datasets into archetypal groups, and ERAINT data is used to describe the underlying circulations for each archetypal rainfall pattern. The number of WEREs mapped to each rainfall pattern for each dataset allows us to get an idea of whether certain RCMs are more likely to simulate certain rainfall patterns.
Open Access
Conceptualising and quantifying the nonlinear, chaotic climate: implications for climate model experimental design
(2015) Conradie, Willem Stefaan; Abiodun, Babatunde Joseph; Daron, Joseph D; Hauser, Tristan P
Uncertainty in climate system initial conditions (ICs) is known to limit the predictability of future atmospheric states. On weather time scales (i.e. hours to days), the separation between two atmospheric model trajectories, initially "indistinguishable" (compared to unavoidable uncertainties) from one another, diverges exponentially-on-average over time, so that the "memory" of model ICs is eventually lost. In other words, there is a theoretical limit in the lead time for skilful weather forecasts. However, the influence of perturbations to climate system model ICs - particularly in more slowly evolving climate system components (e.g., the oceans and ice sheets) - on the evolution of model "climates" on longer time scales is less well understood. Hence, in order to better understand the role of IC uncertainty in climate predictability, particularly in the context of climate change, it is necessary to develop approaches for investigating and quantifying - at various spatial and temporal scales - the nature of the influence of ICs on the evolution of climate system trajectories. To this end, this study explores different conceptualisations and competing definitions of climate and the climate system, focussing on the role of ICs. The influence of ICs on climate quantifications, using probability distributions, is subsequently investigated in a climate model experiments using a low-resolution version of the Community Climate System Model version 4 (CCSM4). The model experiment consists of 11 different 50-member ensemble simulations with constant forcing, and three 50-member ensemble simulations under a climate change scenario with transient forcing. By analysing the output at global and regional scales, at least three distinct levels of IC influence are detected: (a) microscopic influence; (b) interannual-scale influence; and (c) intercentennial-scale influence. Distinct patterns of interannual-scale IC influence appear to be attributable to aperiodic and quasi-periodic variability in the model. It is found that, over some spatial domains, significant (p < 0.01) differences in atmospheric variable "climatologies", taken from 60-year distributions of model trajectories, occur due to IC differences of a similar order to round-off error. In addition, climate distributions constructed using different approaches are found to differ significantly. There is some evidence that ensemble distributions of multidecadal temperature response to transient forcing conditions can be influenced by ICs. The implications for quantifying and conceptualising climate are considered in the context of the experimental results. It is concluded that IC ensemble experiments can play a valuable role in better understanding climate variability and change, as well as allowing for superior quantification of model climates.
Open Access
Future changes in extreme rainfall events and African easterly waves over West Africa
(2016) Egbebiyi, Temitope Samuel; Abiodun, Babatunde Joseph
This study examines the relationship between African Easterly Waves (AEWs) and extreme rainfall events over West Africa, and investigates how climate change could alter this relationship in the future. Satellite observations, reanalysis data, and regional climate model (RCA4) simulations (forced with eight global climate simulations) were analysed for the study. The study used the 95th percentile of daily rainfall as a threshold to identify extreme rainfall events, and applied spectral analysis to extract 3-5 days and 6-9 days AEWs from 700hPa meridional wind component over West Africa. The capability of RCA4 to reproduce the rainfall climatology, extreme rainfall events, the characteristics of AEWs and the contribution of AEWs to extreme rainfall events over the region during the past climate (1971-2005) was examined and quantified using statistical analysis. The future changes (2031-2065) in these parameters were projected for the RCP4.5 and RCP8.5 climate-change scenarios. The results of the study show that RCA4 gives a realistic simulation of the West African climate, including the annual rainfall pattern, the structure of AEWs, and the characteristics of the African Easterly Jet that feeds AEWs. The bias in the simulated threshold of extreme rainfall is within the uncertainty of the observed values. The model also captures the link between the structure of AEWs and the rainfall pattern over West Africa, and shows that the percentage contribution of AEWs to extreme rainfall events over the region ranges from 20 to 60%, as depicted by reanalysis data. For the RCP4.5 and RCP8.5 scenarios, the RCA4 ensemble mean projects a future increase in annual rainfall and in the frequency and intensity of extreme rainfall events over the sub-continent, but the increase is generally higher for the RCP8.5 scenario. It also projects a decrease in the frequency of rain days, no changes in the structure of the AEWs, and an increase in the variance of the waves. However, the simulations from the ensemble mean shows no substantial changes in the contribution of AEWs to the extreme rainfall events, suggesting that the increase in the frequency and intensity of the extreme rainfall events may not be attributable to the changes in AEWs. The study's application is in understanding and mitigating the future impact of climate extremes over West Africa.
Open Access
Improving integrated wildfire management in the Fynbos Biome of South Africa using information on synoptic-scale atmospheric features that promote wildfires
(2015) Harrison, Dean Charles; Abiodun, Babatunde Joseph; Anderson, Pippin
Wildfire, an essential element for the Fynbos Biome of South Africa, can be a threat to property and human life if it is not well managed. Despite many studies on the dynamics and management of wildfire, the role of the atmosphere in inducing regional circulations that promote widespread wildfire is not well known. This dissertation studies the characteristics of wildfire in the Fynbos Biome, identifies synoptic-scale atmospheric features that produce favourable conditions for the wildfire, and examines possibility of using the features as indicators for wildfire occurrence. Ten years (2003 - 2012) of fire data from the MODIS "active-fires" datasets were analysed over the study domain. Daily Fire Danger Index (FDI) was calculated over Southern Africa for this period using maximum temperature (Tmax), minimum relative humidity (RHmin), and maximum wind speed (Wmax) data from the Climate Forecasting System Reanalysis datasets (CFSR) at a 0.5°x0.5° horizontal resolution. The Self Organising Maps (SOMs) technique was used to classify the FDI (anomaly) patterns on the fire days, and the atmospheric dynamics associated with each pattern were studied.
Open Access
The influence of horizontal resolution and boundary forcing in simulating hurricanes over the South Atlantic Ocean using WRF
(2017) Bluff, Gemma Kendall Pelton; Abiodun, Babatunde Joseph
A hurricane is a threat to socio-economic activities in coastal communities bordering the South Atlantic Ocean (SAO). Hurricanes rarely form over this region and as such these communities are not prepared for them. Previous studies have suggested that anthropogenic warming may lead to more frequent hurricanes over the region and have demonstrated the capability of the Weather Research and Forecasting model (WRF) in capturing the impacts of the warming on hurricanes. However, none of the studies have investigated how the model's horizontal resolution and boundary forcing could alter the characteristics of simulated hurricanes. The present study used WRF to perform a series of experiments to simulate two hurricanes (Hurricane Catarina and Hurricane Anita) over the SAO at three horizontal resolutions (3.3 km, 10 km, and 30 km), using two reanalysis datasets (ERA-Interim (hereafter ERAINT) and NCEP CFSR (hereafter CFSR)) as the boundary forcing data. The performances of the reanalysis and WRF are compared with observational data from the International Best Track and Archive for Climate Stewardship. The results show that both reanalyses datasets give a good representation of the two hurricanes, but they grossly underestimate the intensity thereof. CFSR gives a better representation than that of ERAINT. However, both reanalyses also suggest that the South Atlantic Convergence Zone may be the moisture belt for hurricane formation over the SAO. WRF gives a credible simulation of the hurricanes. In simulating Hurricane Catarina, WRF performs best at a 10-km resolution; but in reproducing Hurricane Anita, the model performs best at a 3.3 km resolution. For both cases, the model performs better when forced with ERAINT than with CFSR. Hence, the study shows that increasing the resolution of the model may not necessarily improve the simulated hurricane over the SAO, and that, the quality of the simulated hurricane depends on the dataset that provides the boundary forcing. The results of the study have improved the understanding of hurricane characteristics in the SAO, and have shown the potentials of WRF to forecast and project future events as well as for downscaling the potential impacts of future climate change on hurricanes over the SAO.
Open Access
Investigating the link between southern African droughts and global atmospheric teleconnections using regional climate models
(2015) Meque, Arlindo Oliva; Abiodun, Babatunde Joseph; Hewitson, Bruce
Drought is one of the natural hazards that threaten the economy of many nations, especially in Southern Africa, where many socio-economic activities depend on rain-fed agriculture. This study evaluates the capability of Regional Climate Models (RCMs) in simulating the Southern African droughts. It uses the Standardized Precipitation-Evapotranspiration Index (SPEI, computed using rainfall and temperature data) to identify 3-month droughts over Southern Africa, and compares the observed and simulated drought patterns. The observation data are from the Climate Research Unit (CRU), while the simulation data are from 10 RCMs (ARPEGE, CCLM, HIRHAM, RACMO, REMO, PRECIS, RegCM3, RCA, WRF, and CRCM) that participated in the Regional Climate Downscaling Experiment (CORDEX) project. The study also categorizes drought patterns over Southern Africa, examines the persistence and transition of these patterns, and investigates the roles of atmospheric teleconnections on the drought patterns. The results show that the drought patterns can occur in any season, but they have preference for seasons. Some droughts patterns may persist up to three seasons, while others are transient. Only about 20% of the droughts patterns are induced solely by El Niño Southern Oscillation (ENSO), other drought patterns are caused by complex interactions among the atmospheric teleconnections. The study also reveals that the Southern Africa drought pattern is generally shifting from a wet condition to a dry condition, and that the shifting can only be captured with a drought monitoring index that accounts for temperature influence on drought. Only few CORDEX RCMs simulate the Southern African droughts as observed. In this regard, the ARPEGE model shows the best simulation. The best performance may be because the stretching capability of ARPEGE helps the model to eliminate boundary condition problems, which are present in other RCMs. In ARPEGE simulations, the stretching capability would allow a better interaction between large and small scale features, and may lead to a better representation of the rain producing systems in Southern Africa. The results of the study may be applied to improve monitoring and prediction of regionally-extensive drought over Southern Africa, and to reduce the socio-economic impacts of drought in the region.
Open Access
Local variation and regional transport of tropospheric ozone over Cape Town
(2014) Nzotungicimpaye, Claude-Michel; Abiodun, Babatunde Joseph
[Fix subscripts in abstract.] The attractive image of Cape Town is threatened by periods of poor air quality occurring most often between April and September, during episodes of brown haze. When this haze occurs, it appears in the morning as a layer of concentrated pollution, likely to be associated with photochemical pollutants such as tropospheric ozone (O3) and some of its precursors. Previous studies have identified local emission sources and meteorological conditions associated with both the air pollution and the brown haze in Cape Town. However, due to the transport of air pollutants, emissions from remote sources may also contribute to air pollution levels in Cape Town. This dissertation investigates the local variation and the regional-scale transport of atmospheric pollution over Cape Town, with a focus on O3 pollution. The study analyses O3 observations from local air quality stations and uses two atmospheric chemistry-transport models to simulate the photochemical pollution over southern Africa.
Open Access
Model evaluation for seasonal forecasting over southern Africa
(2011) Browne, Nana Ama Kum; Abiodun, Babatunde Joseph
This study contributes to a broader effort of institutions toward improving seasonal forecasts over southern Africa. The primary objective is to understand where global models show shortcomings in their simulations, and how this impacts on their seasonal forecast skill. It is proposed that the skill of a model in simulating natural climate variability is an appropriate metric for a model's potential skill in seasonal forecasting. Thus the study investigates the performance of two global models in simulating the regional processes in relation to the processes variability, and how this is related to their forecast skill.
Open Access
Modeling the potential impacts of vegetation change on the future climate of Southern Africa
(2015) Naik, Myra; Abiodun, Babatunde Joseph
Many studies have projected a future warming over Southern Africa without including the influence of on - going vegetation changes in the region. This study investigates how the vegetation changes may alter the projected warming. For the study, two regional climate models (RegCM and WRF) were applied to simulate the present day (1970 - 2005) and the future (2030 - 2065; IPCC RCP 4.5) climate, with and without vegetation change. The study considers two scenarios of vegetation change: the first accounts for the potential impacts of natural bush encroachment and commercial forestation in the eastern part of South Africa, while the second accounts for the expansion of grass cover along the western region of the Grassland Biome in South Africa. The result s of this study agree with that of previous studies in that elevated greenhouse emissions will induce warming over Southern Africa in the future , but further indicate that the ongoing vegetation change s may considerably alter the magnitude of the warming. Forestation may enhance the warming over the forested area and induce cooling elsewhere. It may also produce wet conditions locally and induce dry conditions over other area within the region. In contrast, the expansion of grass cover may reduce the projected warming over the grass covered area and induce warming elsewhere. It may also induce dry conditions locally and produce wet conditions over other areas in the sub-continent. Both vegetation change scenarios ( i.e. forestation and expansion of grass cover ) alter the projected future climate changes through their influence s on local surface albedo ; while forestation decreases the surface albedo, the expansion of grass cover increase s it . However, the changes in rainfall and temperature from the vegetation changes could enhance the frequency drought over some areas and reduce it over other areas with in Southern Africa. This study, therefore, suggests that the vegetation changes may produce unexpected impacts on future climate. It also suggests that before using vegetation changes to mitigate climate change in Southern Africa, the biogeochemical benefits (i.e. carbon sequestration) should be carefully weighed against biogeophysical effects (i.e. changes in albedo).
Open Access
Rainfall variability over southern Africa
(2014) Driver, Penny Meredith; Reason, Chris; Abiodun, Babatunde Joseph
Southern Africa is subject to high inter annual rainfall variability and the factors influencing southern African rainfall are not fully understood. The variability has been linked with various sea surface temperature anomalies (SSTAs) in each of the three major ocean basins as well as variability in the strength and position of regional atmospheric features. One of the atmospheric factors that may play a substantial role in southern African rainfall variability is the Botswana high, a high pressure cell which exists at the 500hPa level and is centred over central Namibia and western Botswana during austral summer. 20th Century reanalysis data is used to further investigate this feature and analysis reveals an association between the strength of the Botswana high and ENSO. Further analysis indicates that a connection between the Botswana high and rainfall over southern Africa not only exists during ENSO years, but is also apparent during neutral years that display ENSO-like characteristics in the Botswana high. This result may assist in producing better rainfall forecasts for non-ENSO years. The frequency of dry days over southern Africa during austral summer is investigated using GPCP observational data. Correlation analysis is generally in agreement with previous studies and showed that dry day frequency(DDF) over the Limpopo and North East Zambia regions is correlated with ENSO, while DDF over coastal northern Angola and central South Africa is correlated with SSTs in the Indian and Atlantic Oceans. The possible role played by DDF during JFM 1998 and JFM 2010 is investigated and results indicate that the distribution of DDF over southern Africa was notably different during these two seasons and may have contributed to the unexpected rainfall experienced over southern Africa.
Open Access
Simulating the characteristics of droughts in Southern Africa
(2014) Ujeneza, Eva Liliane; Abiodun, Babatunde Joseph
Drought is widely considered as one of the most devastating natural disasters in the world. In particular, drought is a big threat in Southern Africa because the economy of most of the population in the region is based on rain-fed agriculture. Previous studies have projected that global warming may enhance the frequency and intensity of droughts over Southern Africa in the future. However, the credibility of this projection depends on the ability of the global and regional climate models (GCMs and RCMs) in simulating the characteristics of drought. This thesis presents the characteristics of the Southern African droughts and evaluates the capability of global and regional climate models in simulating these characteristics. The thesis used a multi-scaled standardized drought index (called standardized precipitation evapo-transpiration index, SPEI) in characterizing droughts at 3- and 12-month scales over Southern Africa. The spatial patterns of the droughts are identified using the principal component analysis (PCA) on the SPEI, while the temporal characteristics of the drought patterns are studied using wavelet analysis. The relationship between each drought pattern and global SSTs (and climate indices) is quantified using correlation analysis and wavelet coherence analysis. The study uses correlation analysis to quantify the capability of the models in simulating the drought patterns.
Open Access
Simulating the characteristics of tropical cyclones over the South West Indian Ocean using an adaptive Stretched-Grid Global Climate Model
(2015) Maoyi, Molulaqhooa Linda; Abiodun, Babatunde Joseph; Jackson-Veitch, Jennifer
Tropical Cyclones are one of the most devastating natural phenomena. Previous attempts to simulate Tropical Cyclones (TCs) over the South West Indian Ocean (SWIO) have used Global Circulation Models (GCMs) with uniform grid. This study examines the capability of a GCM with adaptive grid stretching (CAM-EULAG, hereafter CEU) in simulating the characteristics of TCs over the SWIO. In the study, the CEU model is applied with a fine horizontal grid resolution (0.5°x0.5°) over the SWIO and coarser resolution (1°x1° - 2°x2.25°) over the rest of the globe. The model simulation is performed for 11 years (1999-2010) and validated against the Joint Typhoon Warning Centre (JTWC) best track observation, Global Precipitation Climatology Project (GPCP) satellite data, and ERA-Interim (ERAINT) reanalysis data. The study also considers the impact of El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and the South Atlantic Subtropical Dipole (SASD) on TC counts over the SWIO.
Open Access
Transferability of regional climate models over different climatic domains
(2010) Gbobaniyi, Emiola Olabode; Hewitson, Bruce; Abiodun, Babatunde Joseph
In the continuing quest to improve climate model predictions to meet the increasing demand for knowledge on the regional effects of global climate change, it is pertinent to increase our understanding of how the underlying processes of climate are represented in the models we use to make these predictions. Concerted efforts in model evaluations and intercomparison have provided numerous insights into various model biases which plague current state-of-the-art regional climate models (RCMs). Model evaluation and assessment is crucial to model development and understanding how physical processes are represented in models is necessary for improving model parameterizations. This thesis explored model transferability as a new approach for systematic process-based intercomparison of RCMs. It investigated an untested transferability hypothesis which states that “for non-monsoon regions experiencing weak synoptic scale forcing, the height of the cloud base is correlated with the daytime surface fluxes”. An initial transferability experiment was conducted over Cabauw, the Netherlands (51.97°N, 4.93°E) to assess the models’ skill in resolving the diurnal and seasonal cycles and to investigate the simulated connections between surface and hydrometeorological variables over a non-monsoon station. The ability of models to resolve these cycles correctly is a good metric of their predictive capabilities. The data used for the study comprises three-hourly surface observations for the period October 2002 – December 2004 from the Coordinated Enhanced Observing Period (CEOP) measuring campaigns of the Global Energy and Water Cycle Experiment (GEWEX) and three-year simulations (2002 -2004) from five RCMs (CLM, GEMLAM, MRCC, RCA3 and RSM). In simulating seasonal and diurnal cycles of CBH and surface variables, the European models (CLM and RCA3) demonstrate a clear home advantage over the North American models (GEMLAM, MRCC and RSM). Principal component analysis revealed that the models couple the cloud base height with surface fluxes as in observations and that this coupling is not sensitive to changes in wind speed. This study found that summer daytime loadings gave the strongest couplings of variables. Three major processes were identified over Cabauw. First and most dominant is the surface energy process which couples sensible and latent heat with net radiation. The second process is thermodynamic, coupling temperature and surface moisture (specific humidity), and the third is a dynamic process which couples pressure and wind speed. A model intercomparison was then carried out across the six midlatitude domains to test the validity of the Cabauw findings. In observations, CBH is well coupled with the surface fluxes over Cabauw, Bondville, Lamont and BERMS, but coupled only with temperature over Lindenberg and Tongyu. All the models (except GEMLAM) simulated a good coupling with surface fluxes at all stations. In GEMLAM, there is no coupling between CBH and surface fluxes at any station. In less homogenous domains of the study, a very slight decrease in the strength of coupling is seen in most of the models, under strong large scale forcing. This would suggest that the coupling between cloud base height and surface fluxes in the models is possibly more influenced by radiative forcing than by synoptic controls. This second study confirmed the findings at Cabauw that the simulated cloud base is correlated with surface energy fluxes and the sign of the correlations in the models is as in observations. This finding is important for the modeling community as it establishes the fact that the models are actually simulating the direction of influence of surface fluxes and possibly, soil water variability, on cloud processes.
Open Access
The transport of pollutants over South Africa and atmospheric sulphur in Cape Town
(2013) Jenner, Samantha Louise; Abiodun, Babatunde Joseph; Pieterse, Edgar
Cape Town experiences unpleasant pollution (called the Brown Haze) in winter. Sulphur is a major constituent of this haze. To reduce sulphur pollution, and its negative impact on health in Cape Town, air quality management has focussed on identifying local sources and reducing the local emissions of sulphur. Meanwhile, the transport of sulphur pollutants from areas outside Cape Town can contribute to ambient sulphur concentrations. This work studies the transport of sulphur pollutants over South Africa and examines whether Cape Town is a net source of or sink for the pollutant. It shows the link between sulphur emissions on the Mpumalanga Highveld (the most polluted area in South Africa) and sulphur pollution in Cape Town. Two atmosphere chemistry-transport models (RegCM-Chem and WRF-Chem) were used for this study. The models were applied to simulate the atmospheric flow and chemistry transport over South Africa for two years (2001 and 2002), and the results were validated with observations within Cape Town. The models reproduced observed seasonal variability in atmospheric wind flow and sulphur concentrations over Cape Town. Results from both models show a transport of sulphur pollutants from the Mpumalanga Highveld to Cape Town. While the sulphur pollutants from the Mpumalanga Highveld are transported eastward (toward the Indian Ocean) at middle atmospheric levels, the pollutants are transported south-westward (towards Cape Town) at lower atmospheric levels. In addition, the pollutants are transported from the Mpumalanga Highveld to Cape Town, following the south coast of South Africa, in April. During an extreme sulphur pollution event in Cape Town, there is formation of either a col or a converging flow over the city. These features encourage the accumulation of sulphur over Cape Town. The sulphur flux budget analysis shows that Cape Town can be a net source of or sink for sulphur during an extreme pollution event. This study has application potential in developing policies to reduce sulphur pollution in Cape Town and in other areas of South Africa.
Open Access
Understanding the variability and predictability of seasonal climates over West and Southern Africa using climate models
(2015) Lawal, Kamoru Abiodun; Abiodun, Babatunde Joseph; Stone, Dáithí A
A good understanding of seasonal climate and the limit to which it can be predicted is crucial in addressing various socio-economic challenges in Africa. However, how to improve the capability of the dynamical models of the climate system in reproducing the regional seasonal climate variability and in replicating the role of various atmospheric circulation anomalies on the regional variability remains a major challenge. Thus far, understanding of seasonal climate over these regions, as well as the ability of climate models to predict them, has focused on the agreement of simulations of dynamical models of the climate system, rather than considering outliers as potentially vital contributors to understanding and predictability. This thesis uses discrepancy in a large ensemble of climate simulations as a tool to investigate variability in dominant seasonal rainfall and temperature patterns (i.e. classes) over West and Southern Africa, to examine the capability of climate models in reproducing the variability, and to study the predictability of the seasonal climates over South Africa. The dominant classes of variability (of rainfall and maximum temperature fields) in both regions are examined based on the Self-Organizing Map (SOM) classifications. The sequences in which each class occurs cannot be linked simply to a single common index of global scale atmospheric circulation anomalies, implying that the chaotic regional atmospheric circulations that modulate the global scale modes of variability are indispensable. The climate model examined adequately reproduces the dominant classes of seasonal climate over West and Southern Africa.

Browsing by Author "Abiodun, Babatunde Joseph"

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