OpenUCT :: Browsing by Author "Hewitson, Bruce"

Browsing by Author "Hewitson, Bruce"

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Open Access
Air quality and climate change in the greater Cape Town area
(2009) Kalognomou, Evangelia-Anna; Hewitson, Bruce; Tadross, Mark
The work presented in this dissertation stems from the link that exists between meteorological conditions and the significant accumulation of air pollutants in large urban agglomerations. The research focuses on the Greater Cape Town Area (GCTA), where temperature inversions lead to high air pollutant concentrations and episodes of air pollution. As local meteorological conditions are often manifestations of larger weather producing phenomena (e.g. anticyclones), the work presented studies the changes that may occur in the synoptic conditions associated with temperature inversions, which will consequently affect the rate of occurrence of air pollution episodes. After a brief introduction of the topic, background information on the relevant legislation and the actions taken towards an air pollution abatement strategy and a detailed literature review, the high levels of air pollution in the GCT A during winter and especially during the morning peak hour traffic and their link to temperature inversions are studied in detail for the year 2002. The large scale circulation and its link to temperature inversions are studied through the application of the Self-Organizing Maps technique using NCEP-DOE Reanalysis 2 data and making use of the findings for the year 2002. The synoptic states most associated with temperature inversions are found to be the anticyclonic conditions caused by the South Atlantic High Pressure (SAHP) system and the west coast trough associated with berg winds bringing dry continental air towards the GCT A. The 2002 data also show that more air pollution episode days are associated with west coast troughs than with the SAHP system and the average strength of the temperature inversions associated with west coast troughs is found to be approximately 50 % higher than that associated with the SAHP system. The Global Circulation Models (GCMs) ECHAM5, CNRM-CM3 and CSIRO-MK3.5 are used to study the potential changes in the future climate of the area under the IPCC A2 emissions scenario. ECHAM5 shows a small increase in the synoptic states associated with anticyclonic influence over the south western part of South Africa and CNRM-CM3 shows a small increase in both the synoptic states associated with anticyclonic influence and those associated with a west coast trough. Both models show a small decrease in the synoptic states associated with cold fronts. CSIRO-MK3.5 was not found to adequately reflect the current climatology in the domain, making it difficult to distinguish between model bias and future climate trends.
Open Access
Antarctic sea-ice extent, Southern hemisphere circulation and South African rainfall
(1998) Hudson, Debra Alison; Hewitson, Bruce
The study examines the response of an atmospheric general circulation model (OCM) to a reduction in Antarctic sea-ice extent during summer and winter, with emphasis on non-polar and southern African climates. Following an evaluation of the OCM, the control and perturbation simulations are analysed. The controls are forced by prescribed, observed sea surface temperatures (SSTs) and sea-ice extents, while in the perturbation simulations sea-ice is reduced and replaced with SSTs. The introduced anomalies are derived from an algorithm based on observed ice variability. The simulations are restarts of an AMIP (Atmospheric Model Intercomparison Project) configured simulation, and two summers (1979/80 and 1984185) and two winters (1980 and 1985) have been selected for the study. Three replicates have been performed for each time period for both the control and perturbation conditions.
Open Access
Approaches to quantifying and reducing uncertainty in GCMs over Southern Africa
(2008) Carter, Suzanne; Hewitson, Bruce
Global Circulation Models (GCMs) are the primary tool for simulating future climate changes. These models by necessity make use of various assumptions and simplifications due to computational constraints, and in so doing introduce biases and systematic error. Along with other sources of uncertainty regarding our understanding of the climate system and given the quasi-chaotic nature of the climate, climate projections differ between models whose climate simulation skill is poorly quantified. A new methodology is presented to assess the regional biases in GCMs and to, in part, compensate for some aspects of these biases. The study will focus on the Southern African region but could be replicated for other regions. Using Self-Organising Maps (SOMs), synoptic archetypal patterns are identified and the distribution and frequency of these patterns assessed. The use of synoptic archetypes to quantify model metrics presents a novel approach with many benefits over standard metrics, such as errors and means per variable. SOMs add a spatial and multi-variable dimension to the analysis as each metric is calculated based on its synoptic circulation pattern and associated to a set of atmospheric variables. Some persistent biases in the models are notable based on comparisons between the NCEP and GCM SOM node mapping, such as an overall cool bias in the models and a shift of the dominant high pressure cells and thus the westerly wave to the south. The weighting techniques provide insight into how much of the model bias is contributed by differences in synoptic frequency and what part is attributable to systematic biases in the models which result in a different mean state for a given synoptic process. The frequency correction enabled a correction of up to 25% of the difference between model and reanalysis data, but in most cases the change was far smaller than this. The differences in mean conditions remained the largest component of the bias. To correct for this the weighting was applied to the climate change anomaly (difference between future and control projections) per synoptic process to create a multi-model climate change component that is added to the NCEP baseline. This provides the most accurate depiction of future climate from the data provided. The models generally have different strengths, therefore the weighted multi-model solution allows models to give a greater contribution where they are skilful and less where they do not match the observed dynamics. Comparison of the magnitude of the climate change signal showed that winter states in the weighted multi-model composite had a smaller temperature increase and reduced rainfall compared to the unweighted results. In summer states there is greater warming and increased rainfall, especially over the oceans. This suggests the models are over estimating changes in temperature in winter and underestimating the increases in summer. Synoptic events are the primary driver of climate change impacts. Therefore errors in synoptic state will have a notable influence on the climate change projections and need to be fully considered in any climate change impact study. The use of the weighting technique helped to identify and reduce uncertainties in the climate change projections over Southern Africa.
Open Access
Assessing climate change impacts and agronomic adaptation strategies for dryland crop production in southern Africa
(2016) Zinyengere, Nkulumo; Hewitson, Bruce; Tadross, Mark; Crespo, Olivier
Dryland farmers in southern Africa operate under harsh conditions; infertile soils, erratic rainfall regimes, sub-optimal input levels etc. Crop yields have generally been low, negatively affecting food security and livelihoods. Climate change is anticipated to aggravate these already existing challenges. In the recent past, a wide range of studies has sought to understand how climate change will affect crop production. However, there are only few detailed localised studies that focus on understanding climate change impacts and adaptation under heterogeneous conditions that dryland farmers in southern Africa operate. This study sought to understand how climate change will affect food crop production in southern Africa's drylands and to provide insight on the potential of on-farm agronomic management strategies for adaptation. The study focused on three locations representing some of the agro-ecological conditions of southern Africa i.e. Big Bend in Swaziland (low altitude, hot and dry), Mohale's Hoek in Lesotho (high altitude, cool and wet and dry), and Lilongwe in Malawi (mid altitude, wet with moderate temperatures). The study was performed largely using a climate-crop model simulation approach supported by a review of similar approaches in the region, data collected from reported agricultural experimental trials, regional experts, downscaled climate projections (using up to 9 GCMs) and surveys.
Open Access
Assessing maize water requirements in the context of climate change uncertainties over southern Africa
(2011) Hachigonta, Sepo Promise; Hewitson, Bruce
Climate change studies are subject to high uncertainties partly resulting from data reliability. This study investigates the challenges of using statistical downscaled climate data to examine the likely impacts of climate change on maize growth in southern Africa in the context of these uncertainties.
Open Access
Assessing the representation of teleconnective drivers of rainfall over Eastern Africa in global and regional climate models and projected future changes
(2017) Endris, Hussen Seid; Hewitson, Bruce; Lennard, Chris
Climate variability is an important characteristic of regional climate, and a subject to significant control from teleconnections. An extended diagnosis of the capacity of climate models to represent remote controls of regional climate (teleconnections) is vital for assessing model-based predictions of climate variability, understanding uncertainty in climate projections and model development. An important driver of climate variability for Africa is the sea surface temperature (SST) - rainfall teleconnection, such as the El Ni˜no/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). In this study, an assessment of the teleconnection between tropical SSTs and Eastern African rainfall in global and regional climate models is presented, with particular attention paid to the propagation of large-scale teleconnection signals (as represented by model reanalyses and Coupled Global Climate models (CGCMs)) into the domain of the Regional Climate Models (RCMs). The teleconnection-rainfall relationship with the Eastern Africa region is assessed in two rainfall seasons (June-July-August-September and October-November- December) under present and future periods. Evaluation runs (RCMs driven by reanalysis datasets) and historical simulations (RCMs driven by CGCMs) are assessed to quantify the ability of the models to capture the teleconnection relationship. The future analysis is performed for two Representative Concentration Pathway scenarios (RCP4.5 and RCP8.5) to assess future change in this relationship as a result of global warming. Using ERA-interim reanalysis as perfect boundary conditions, the RCMs adequately simulate the spatial and temporal distribution of rainfall in comparison with observations, although the model performance varies locally and seasonally within the region. Furthermore, the RCMs correctly capture the magnitude and spatial extent regional-scale seasonal rainfall anomalies associated with large-scale oceanic modes (ENSO and IOD). When the lateral boundary conditions are provided by CGCMs, RCMs barely capture the regional teleconnection patterns associated with large-scale modes, and mostly depend on the selection of the driving CGCM. Comparison of the CGCM-driven RCM simulations with the reanalysis-driven RCM simulations revealed that most of the errors in teleconnection found in the RCM simulations are inherited from the host CGCMs. The ERA-Interim driven downscaled results show better agreement with observed spatial teleconnection patterns than the CGCM driven downscaled results. Analysis of the CGCMs and corresponding downscaled results showed that in most cases both the CGCM and the corresponding downscaled simulations had similar teleconnection patterns, but in some cases the RCM results diverge to those of the driving CGCM results. It has been demonstrated that similarities in SST-rainfall teleconnection patterns between the RCM simulations and respective driving CGCM simulations are noted over the equatorial and southern part of the region during OND season, where the rainfall is primarily controlled by large-scale (synoptic-scale) features, with the RCMs maintaining the overall regional patterns from the forcing models. Di↵erences in RCM simulations from corresponding driving simulations are noted mainly over northern part of the domain during JJAS, which is most likely related to mesoscale processes that are not resolved by CGCMs. Looking at the model projections of the future, although the spatial pattern of teleconnections between ENSO/IOD and rainfall still persist, important changes in the strength of the teleconnection have been found. During JJAS, ENSO is an important driver of rainfall variability in the northern parts of the region where dry anomalies are associated with El Ni˜no and wetter anomalies with La Ni˜na. Both regional and global ensemble projections show higher rainfall during La Ni˜na and lower rainfall during El Ni˜no over the northern part of the region compared to the present period. During OND, the teleconnection between ENSO/IOD and rainfall is projected to strengthen (weaken) over Eastern horn of Africa (southern parts of the region) compared to the present period. This implies heavy seasonal rains associated with positive phases of ENSO and IOD will increase in future across the Eastern horn of Africa. The change OND rainfall teleconnections are stronger and also more consistent between the models and scenarios as compared to the change in JJAS teleconnections. These findings have an important implication for the water and agricultural managers and policies in the region to tackle the anticipated droughts and floods associated anthropogenic climate change. Finally, the analysis demonstrated that the largest source of uncertainty in the regional climate model simulations in the context of teleconnective forcing of rainfall over Eastern Africa is the choice of CGCM used to force the RCMs, reinforcing the understanding that the use of a single GCM to downscale climate predictions/projections and using the downscaled product for assessment of climate change projections is insufficient. Simulations from multiple RCMs nested in more than one GCM, as is undertaken in the Coordinated Regional Downscaling Experiment (CORDEX), are needed to characterize the uncertainty and provide estimates of likely ranges of future regional climate changes.
Open Access
Atmosphere-vegetation interactions over South Africa
(1997) Shannon, Debbie Anne; Hewitson, Bruce
This study examines the sensitivity of the atmospheric circulation to vegetation change over South Africa in the context of the portended global warming. This is achieved using a vegetation model driven by climate change information and subsequently incorporated within a general circulation model (GCM). The stand-alone vegetation model is driven using precipitation, temperature and relative humidity derived from downscaling using artificial neural networks. The vegetation model is then run with perturbed precipitation, temperature and relative humidity from downscaled model data from lxCO₂ and 2xCO₂ GCM simulations. The resultant vegetation perturbation response to climate change is then examined and incorporated into the GCM in order to ascertain the atmospheric sensitivity to vegetation changes. The off-line results of the vegetation model indicate a moderate degree of sensitivity of the vegetation to perturbations in precipitation, temperature and relative humidity. The general trend in response to the CO₂ climate is a westwards and altitudinal shift of lowland vegetation over the eastern part of the country, and a southwards and eastwards shift of the more dryland vegetation in the west. These shifts are in accordance with expected responses, since lowland vegetation responds more to temperature changes and the dryland vegetation to precipitation changes. Nonetheless, the use of the model provides a physically justifiable scenario on which to base the GCM studies, and at a finer resolution than otherwise available. A GCM simulation with the perturbed vegetation was then performed using sea surface temperature boundary conditions for 1980 and compared to an identical GCM run without the perturbation. 1980 was chosen since this year does not represent either a strong El Niño or La Niña year. The atmospheric sensitivity to the vegetation perturbation has been examined in terms of climatic variables such as temperature, precipitation, pressure, specific humidity, horizontal divergence, and sensible and latent heat fluxes. The results show that the atmosphere is quite sensitive to relatively small vegetation changes. Atmospheric response to vegetation perturbations indicates greater sensitivity over the NW and SE regions of southern Africa. The perturbation indicates a reduction in precipitation over the SE interior, related to less moisture feeding in over the interior from the SE Indian Ocean. Wind speed changes over the adjacent ocean were also evident, and are probably related to the changes in the South Atlantic and Indian high pressures. A southwards extension of the Hadley Cell was also suggested, as well as changes in sensible and latent heat fluxes, relating to precipitation and temperature changes. It is suggested that changes may be in response to the general drying out of the country and the associated increase in aridity. This research forms the preliminary investigation for further work incorporating the atmospheric perturbation response back into driving the vegetation model in order to examine the direction of the feedback -- whether this is positive or negative in the longer term. Thus, this study has demonstrated that the atmosphere is significantly sensitive to vegetation changes over South Africa and reinforces the need for improved land surface parameterization schemes and vegetation models in general circulation models.
Open Access
Climate change effects on land degradation and agriculture in the Swartland, South Africa
(2005) Barrable, Anne; Hewitson, Bruce; Meadows, Michael E
The Swartland is a region of sparse natural vegetation, consisting of primarily dry-land crop farming. The area is particualarly sensitive to the changing Meditterranean-type climatic conditions and is characterised by undulating terrain and a history of land degradation and soil erosion. This thesis therefore considers how future climate change may impact on soil loss in such a climatically sensitive region of central economic significance for southern Africa.
Open Access
Climatic trends and soil moisture feedbacks over Zimbabwe
(2005) Mdoka, Marshall Lison; Hewitson, Bruce; Tadross, Mark
The research focuses on an objective analysis of austral summer rainfall variability over Zimbabwe as well as characterization of rainfall patterns and frequency analysis over southern Africa region. A statistical analysis of historical trends in climate extreme events is used and lays a foundation of projecting into future climates. A trend analysis done on rainfall patterns attained from SOMs approach compliments the RClimdex statistical approach and strengthens some of the historical trends findings on climate extremes. Thereafter, some exploratory research seeks to explain the trends observed using the land-atmosphere interactions and shows the response of rainfall to anomalous soil moisture conditions during an extreme wet and dry seasons using RegCM3. Finally, some radiation effects results are presented from these soil moisture perturbations experiments. Results show drying out patterns over the region from the historical records analysed. The trend analysis done with SOM arrays revealed a positive trend towards drier conditions and a negative trend for wet conditions. The climate extremes indices analysis complimented these findings as shown in the decrease in total precipitation and an increase in the number of dry spells. This is supported by the circulation patterns showing an increase in frequency of the 500hPa anticyclones and a decrease of low pressures. However, some high altitude stations showed an intensification of precipitation events. This would exacerbate need for proper planning of future water resource management and farming strategies. Soil moisture rainfall feedback mechanisms were not fully explored. However drier conditions experiments showed a stronger response to soil moisture perturbations than in wetter conditions experiments. No consistent response to soil moisture initialisation over southern Africa was found. The altitude does modulate these feedback mechanisms with low-lying areas depicting a stronger response. A better understanding of the observed rainfall patterns, historical climate trends and soil moisture-rainfall feedback mechanisms are essential for improved short-term and seasonal forecasting and will aid the generation of plausible climate change impact predictions.
Open Access
Direct and semi-direct aerosol effects on the southern African regional climate during the austral winter season
(2011) Tummon, Fiona; Solmon, Fabien; Tadross, Mark; Hewitson, Bruce
The regional climate model RegCM3 is used to investigate the direct and semi-direct aerosol effects on the southern African climate during the austral winter season (June-September). The sensitivity of simulated aerosol-climate effects to different biomass burning inventories, boundary conditions and sea surface temperature (SST) feedbacks is tested to assess the range of uncertainty associated with these parameters.
Open Access
The diurnal cycle of cloud cover over southern and central Africa
(2008) Coop, Lisa Jane; Hewitson, Bruce; Tadross, Mark
The current understanding of the temporal and spatial distribution of clouds over southern and central Africa is poor and the regional processes governing cloud occurrence is only weakly understood. This study seeks to improve the current understanding of cloud diurnal variability over this region by providing a base-line diurnal climatology of lowlevel, mid-level and high-level cloud cover. Diurnal variations of cloudiness are examined using ten years of cloud data from latest version of the International Satellite Cloud Climatology Project (ISCCP-D1). The broad seasonal average diurnal variability is explored across the region. Thereafter a more detailed analysis of regionally specific variability is made using a Self-Organising Map. The findings of this study are in broad agreement with previous work. Cloud over the southern and central African region shows clear spatial organisation, most significantly associated with the location of the Intertropical Convergence Zone. The diurnal variation of high-level cloud is large, closely correlated to its mean and is enhanced by orographic features. Minimum high-level cloud occurs at 1100 LST and maximum extent is reached during the evening around 1800 LST, except in locations experiencing deep convection which displayed a redevelopment of cloud in the early morning (0300 LST). This redevelopment of HLCA is hypothesised to be due to the destabilization of the upper troposphere through nighttime cloud radiative cooling. Mid-level cloud exhibits smaller diurnal variations, reaching maximum coverage at approximately 0300 LST. Clouds at this level are severely obscured by higher clouds and therefore the detected diurnal variation is due to both real and artificial signals and care needs to be taken in interpreting the results. Low-level cloud shows strong diurnal variations when not obscured by higher clouds, reaching a maximum just after midday. The results of this study are interpreted in terms of the life-cycle of deep convective cloudiness. A number of mechanisms are suggested to explain the regional differences in diurnal variations with land surface heating being the primary mechanism.
Open Access
Dynamics of co-behaviour of climate processes over Southern Africa
(2021) Quagraine, Kwesi Akumenyi; Hewitson, Bruce; Jack, Christopher; Lennard, Christopher
Large-scale climate processes such as El Niño-Southern Oscillation (ENSO), Antarctic Oscillation (AAO), and many others, play varying roles in regional climate variability across the world. While the role of singular processes have been explored in many studies, the combined influence of multiple large-scale processes has received far less attention. Key to this is the challenge of developing methodologies to support the analysis of multiple processes interacting in potentially non-linear ways (co-behaviour) in a particular region. This study details the development of such a methodology and demonstrates its utility in the analysis of the co-behaviour of largescale process interactions on regional precipitation and temperature variability over southern Africa. The study defines co-behaviour as the interaction of large-scale processes that may influence regional circulation leading to climate variability. A novel methodology which involves a combination of analysis techniques such as Self-Organizing Maps (SOM) and Principal Component Analysis (PCA) is developed to identify and quantify such co-behaviour which accommodates potentially non-linear interactions. This methodology is evaluated in the context of southern African regional climate using three key processes, namely ENSO, AAO and Inter-tropical Convergence Zone (ITCZ), and characterizations of regional circulation, and temperature and rainfall variability. Analysis of co-behaviour under observed conditions identifies results that concur with prior studies, in particular the dominant regional response to ENSO, but also establishes key examples of co-behaviour such as the role of the AAO in moderating and altering the regional response to ENSO which is important for understanding regional climate variability. Application of the approach to Global Climate Model (GCM) simulations of past climate reveals that while many GCMs are able to capture individual processes, in particular ENSO, they fail to adequately represent regional circulation variability and key observed co-behaviour. The study therefore clearly demonstrates the importance of co-behaviour in understanding regional climate variability as well as showing the usefulness of the new methodology in investigating co-behaviour. Finally, the new insights into evaluating model performance through the lens of core climate processes and their interaction provides a significant step forward in both model development and application for decision making.
Open Access
Effects of biomass-burning aerosol loading on Southern African climate
(2013) Maúre, Genito Amós; Tadross, Mark; Hewitson, Bruce
This study highlights the importance of including all biomass burning emissions, from the aerosols directly released from fires to the precursor gases, as different radiative forcings will be obtained from different chemical species, and, therefore, different circulation patterns are likely to be induced over all seasons, regardless of how large the emissions loading and/or concentrations are.
Open Access
Effects of global climate change on the recruitment of Anchovy in the Southern Benguela upwelling system
(2001) Young, Shona Linda; Hewitson, Bruce; Richardson, Anthony
Changes to global climate patterns, as predicted by many climatologists, will impact on ecosystems in numerous ways. The nutrient-rich waters of marine upwelling environments enable prolific plankton growth, which in turn supports vast shoals of pelagic fish. The nutrient supply is dependent on the strength and direction of winds, which govern the upwelling process as well as turbulence. A change in climate may thus affect the food supply and feeding conditions required by pelagic fish populations. This thesis investigates predicted changes in wind patterns in the southern Benguela system and assesses how these changes may impact on the recruitment of the Cape anchovy, Engraulis capensis. A general circulation model (NCAR Climate System Model) is used to compare a future simulation under double C02 conditions with a simulation of the present day wind regime. Climate change effects on anchovy in the other main upwelling systems, off the coasts of California, Peru and Morocco are also examined. Wind speeds and turbulence off the Agulhas Bank in the southern Benguela system are expected to remain suitable and potentially become even more conducive to spawning in the future. The results show similar mean wind speeds to present day values, a decrease in mean turbulence, a decrease in the frequency of extreme wind speeds and a decrease in the frequency of extreme turbulence during the anchovy spawning season (i.e. September-February). An increase in Lasker events is expected at the Eastern Bank Grid Cell, which suggests that this area may become the preferred spawning habitat in the future. The Cape Town Grid Cell also shows suitable conditions in the future simulation for anchovy spawning and may thus become an alternative spawning location. The West Coast shows an increase in alongshore wind stress and thus an increase in upwelling in the future simulation from November - February. This is likely to increase planktonic food availability and ultimately anchovy recruitment. Future simulations from the Climate System Model show that the upwelling systems off the coasts of California, Peru and Morocco are likely to continue supporting anchovy spawning and may become even more suitable in terms of wind and turbulence regimes.
Open Access
Event characteristics of intra-seasonal climate circulations
(2002) Tennant, Warren James; Hewitson, Bruce
An analysis of rainfall characteristics over the summer rainfall areas of South Africa is done in order to find links between rainfall variability and the general circulation. Seasonal rainfall totals are clearly linked to significant rain days, indicating the importance of evaluating synoptic-scale event characteristics. Rank ordered rainfall characteristics reveal that normal rainfall may be considered as the middle three quintiles, with the outer quintiles representing wet and dry conditions. The general circulation in tenns of atmospheric state (humidity and temperature), moisture and heat transport, and energy exchange - with attention to tropical-extratropical cloud-bands, are central to wet seasons in South Africa. These are generally associated with deep convection that is driven by vertical instability. Transport of moisture into South Africa generally takes place from the east with the Indian Ocean being a major source of water vapour. Although important, fluctuations in moisture transport are not clearly associated with rainfall. This is because moisture is a necessary but not a sufficient condition for rainfall. Atmospheric dynamics producing rainfall are a combination of disturbances of mid-latitude and tropical origin. These disturbances often initiate the tropical-temperate cloud-band where eddy available potential energy, generated through surface heating over land, is converted to eddy kinetic energy. It is generally mid-latitude disturbances with stronger vertical shear that are associated with rainfall events. Increased baroclinic activity in the Southern Ocean south of South Africa, as such, is generally associated with dry seasons. During these seasons there is usually a greater amount of available potential energy which strengthens the southern branch of the Hadley Cell. The effect of this is to increase the strength of the subtropical jet through enhanced poleward flux of angular momentum. The resulting increase in baroclinicity in the South Atlantic Ocean then disrupts rainfall over South Africa through the advection of dry air from the west by the vertical-mean wind component. General circulation models are shown to capture inter-annual variability such that forecasts of regional rainfall, of useful skill at a seasonal scale, may be made. However, the skill level with regard to daily circulation, restricts their use to the large-scale circulation. Nonetheless, the analysis of rainfall and the links to large-scale circulation, discussed in the thesis, provide information to produce more skilful seasonal forecasts without having to model small-scale features directly. Such forecasts may also include additional information on rainfall characteristics, such as number of rain-days and length of dry spells.
Open Access
An examination of the spatially extensive heavy precipitation events over South Africa and the associated moisture trajectories
(2002) Walawege, Ruwani Priyanthika; Hewitson, Bruce
Precipitation is possibly the most important climatic variable in southern Africa. The absence of a good rainy season is often marked with low productivity and in some cases starvation. However, excessive rainfall can also bring with it disaster and destruction. Investigating the causes of such events has been the aim of a number of studies in the past. The interest shown in extreme events, including that of precipitation has been growing in the last couple of years. This comes as a result of understanding that the change in the mean of variables can have a large influence on the extremes of these same variables. In fact, changes in the extremes can be disproportionate and often have more impact on both society and the environment. This raises concerns especially in poorer nations who often lack the financial resources to deal with such impacts. The primary aim of the present study is to investigate spatially extensive heavy precipitation events in South Africa. It explores the possible spatial patterns that exists within these events and also investigates the non-local sources of moisture for them. In order to do this, the study firstly utilises a categorisation technique called Self Organising Maps (SOMs). A number of groups were identified through this process and sample events from this were then further analysed with the use of a kinematic trajectory model. The results indicated that moisture for these events were most likely to be transported from the south Indian Ocean. Although this finding has been shown previously in other similar studies, the present investigation shows that the possible source of moisture to be further south that previously thought. Further investigation was done by examining a case event more closely. The results also show that an area in the south Indian Ocean, east of Madagascar to be the possible source of moisture for this particular event.
Open Access
An exploration of South Africa's wind climate using station records and reanalysis data
(2016) Argent, Brendan; Hewitson, Bruce; Lennard, Christopher James; Hahmann, Andrea
Sparse information about the wind climate of South Africa behooves an exploration of the drivers of surface wind speed, especially in the context of wind resource assessment. This work quantifies the coupling between the synoptic circulation states and station-scale flows to develop a process-based regionalisation of wind regimes over the country .A thorough inspection of available South African Weather Service (SAWS) wind records is conducted and a quality control procedure is applied. The procedure reveals a large proportion of the data are missing and existing data contain numerous errors such that only107 of the original 960 stations passed the quality control criteria. However, data from these107 stations only overlap temporally 2% of the time, which makes the data inappropriate fora regionalisation procedure. To ameliorate this, a method for incorporating bias-corrected time series data from a reanalysis data set is developed. Data from the 0.3◦ resolution hourly Climate Forecast System Reanalysis (CFSR) be-tween 1989-2010 is selected to improve the temporal coverage of the station data. The raw CFSR data overestimates wind speeds and underestimates the temporal variability and long-term trends. A bias correction method based on the wind speed and direction, time of day and month of the year is developed which successfully removes the mean error on wind speed and direction and improves the correlation with station records. This is achieved without disrupting spatial correlation patterns. Corrected and extended wind time series from each station site are used for the regionalisation. The regionalisation uses a self-organising map (SOM) to define the archetypal synoptic circulation patterns in the reanalysis data set and the influence of these on the local wind climate is quantified. 12 representative atmospheric states are defined by the SOM that are consistent with the existing literature and capture the major synoptic circulation states. A hierarchical clustering is then used to define wind climate regions based on the coupling between these circulation states and the extended station data. Six relatively cohesive spatial wind-climate groupings are identified that are physically consistent with the driving synoptic environment and are characteristic in terms of terrain and response to synoptic drivers. This process-based regionalisation facilitates a future assessment of potential changes in the wind climate of South Africa as a result of a warming world.
Open Access
Exploring the role of climate change risk perceptions in informing climate services for adaptation in East Africa
(2023) Steynor, Anna; Hewitson, Bruce; Pasquini, Lorena
The specificities of the African decision context are not well understood and are often not considered in the development, tailoring and supply of climate services for Africa. Yet, the utility of climate services relies on them being suitable for the decision context, so an understanding of the decision context should be central to climate services development. One unexplored approach to deepening an understanding of the decision context for climate services is through climate change risk perceptions, because perceptions of climate change risk influence whether decision makers take action on climate change as well as what information they seek and use to take action. Accordingly, this study uses data collected through a regional survey (N = 474) and semi-structured interviews (N = 36) to explore and better understand climate change risk perceptions amongst policy decision influencers in east Africa. The data informs three separate analyses. First, a climate change risk perceptions model is constructed, elucidating how various risk perception determinants interact to influence climate change risk perceptions and professional action on climate change in east Africa. This model shows that the pathway to climate change risk perceptions differs depending on individual value systems. Heightened climate change risk perceptions of those with primarily self-enhancing (inward looking) values are predominantly influenced by social norms, whereas heightened climate change risk perceptions of those with primarily self-transcending (outward facing) values are predominantly influenced by experience of extreme events and the psychological proximity of climate change. Second, the identified climate change risk perception determinants are quantified and explored to better understand the specificities of the decision context in which climate services are used. Climate change risk perceptions are found to be heightened, driven by observance of social norms, perceptions of climate change as a proximal risk, frequent experience of extreme weather events and a predominantly self-transcending value system among policy decision influencers. Lastly, the relationships between determinants of climate change risk perceptions and the use of climate services information are quantified and explored to evaluate gaps in currently available climate services. The analysis reveals three main gaps, namely the lack of long-term climate change projections disseminated through National Meteorological Services, the limited locally ground-truthed delivery of impact-based forecasts and inadequate capacity development of climate services users to understand and use complex climate information. The study culminates in a proposed framework for the enhancement of climate services for east Africa, based on understanding gained through the study.
Open Access
Future changes in extreme events in Mozambique as simulated using the PRECIS regional climate modeling system
(2011) Pinto, Izidine S de Sousa; Tadross, Mark; Hewitson, Bruce
Future climate change is generally believed to lead to an increase in climate variability and inthe frequency and intensity of extreme events. Mozambique is well known for its occurrenceof severe weather and extreme climate events such as floods, tropical cyclones and droughts.Such events have serious impacts on the livelihoods of most people who often rely on subsistence agriculture.This dissertation explores possible changes in extremes in temperature and precipitation over Mozambique, based on high-resolution (25 km) simulations of the regional climate model system PRECIS (HadRM3P), forced by the ECHAM4 global mode.
Open Access
The impact of global climate change on the runoff and ecological sustainability of the Breede River
(2004) Steynor, Anna C; Hewitson, Bruce; Archer, Emma
The Breede River catchment in the South Western Cape is already under pressure for its water resources due to its supporting a variety of different land uses. The predominant land use in this catchment is agriculture, which demands the majority of river water for irrigation. The Department of Water Affairs and Forestry are currently investigating the future demand for water from the river, in this respect it is important to know what effect climate change will have on the change in river flow. Self Organising Maps (SOMs) are used to identify changes in the circulation systems contributing to the rainfall of the region and from this the potential change is assessed for the Breede River flow under future climate change. It is assessed that the runoff in the Breede River is expected to change under all the models of ECHAM4, CSIRO and HadAM. The magnitude of this alteration is calculated by using the change in the SOM node frequencies between the present and the future data. This is then subtracted from the present runoff data supplied by DWAF. A source of runoff decrease in the future is agricultural irrigation. The increase in irrigation under climate change is determined by inserting future climate data into an agricultural model. Once the increased amount of water used in irrigation is determined, it is subtracted from the projected future runoff. From this it is determined whether the river will be ecologically sustainable under climate change.