Browsing by Author "Tadross, Mark"

Now showing 1 - 12 of 12
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    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.
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    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.
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    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.
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    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.
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    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.
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    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.
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    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.
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    Future changes in extreme rainfall events and circulation patterns over southern Africa
    (2015) Pinto, Izidine S de Sousa; Lennard, Chris; Tadross, Mark; Hewitson, Bruce
    Changes in precipitation extremes are projected by many global climate models as a response to greenhouse gas increases, and such changes will have significant environmental and social impacts. These impacts are a function of exposure and vulnerability. Hence there is critical need to understand the nature of weather and climate extremes. Results from an ensemble of regional climate models from the Coordinated Regional Downscaling Experiment (CORDEX) project are used to investigate projected changes in extreme precipitation characteristics over southern Africa for the middle (2036-2065) and late century (2069-2098) under the representative concentration pathway 4.5 (RCP4.5) and 8.5 (RCP8.5). Two approaches are followed to identify and analyze extreme precipitation events. First, indices for extreme events, which capture moderate extreme events, are calculated on the basis of model data and are compared with indices from two observational gridded datasets at annual basis. The second approach is based on extreme value theory. Here, the Generalized Extreme Value distribution (GEV) is fitted to annual maxima precipitation by a L-moments method. The 20-year return values are analyzed for present and future climate conditions. The physical drivers of the projected change are evaluated by examining the models ability to simulate circulation patterns over the regions with the aid of Self-Organizing Maps (SOM).
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    Investigating the relationships between wheat-specific rainfall characteristics, large-scale modes of climate variability and wheat yields in the Swartland region, South Africa
    (2014) Kloppers,Pierre-Louis; Johnston, Peter; Tadross, Mark
    Wheat producers in the South Western Cape (SWC) of South Africa need to cope with biophysical and socio-economic systems exposing farmers to a multidimensional decision- making environment. The rain fed wheat production in the Swartland region is highly susceptible to the interannual variability of winter rainfall. Producers, therefore, need relevant climatic information to identify ways to improve profitability and to make sound economic decisions. Seasonal forecasting has the potential to provide wheat producers with invaluable information regarding the climatic conditions. However, due to the complex nature of the atmospheric dynamics associated with winter rainfall in South Africa, seasonal forecasting models have been found to have very little skill in predicting the variability of winter rainfall. Such a shortfall has created a gap for which this study has attempted to bridge. This study aimed to investigate the relationship between wheat-specific rainfall characteristics, large-scale modes of climate variability and wheat yields in the Swartland region to assess whether these relationships could provide useful climatic information to the wheat farmers. Six wheat-specific rainfall characteristics (total rainfall ; number of wet days ; number of ‘good’ rainfall events; number of heavy rainfall events; percentage ‘good’ rainfall ; and the number of dry dekads ) on various time scales (winter; seasonal; monthly and dekadal) were correlated against wheat yield records over a 17 year period from 1994 to 2010. From this analysis, the distribution and timing of the rainfall throughout the wheat growing season (April to September) emerged as an important determinant of wheat yield. An accurate statistical wheat prediction model was created using farmer stipulated rainfall- wheat yield thresholds. Three teleconnections (El Niño-Southern Oscillation [ENSO], Antarctic Oscillation [AAO] and South Atlantic sea surface temperatures [SSTs]) represented by eight climate indices (Nino 3.4 Index, Ocean Nino Index [ONI], Southern Oscillation Index [SOI], AAO index, Southern Annular Mode Index [SAM], South Atlantic Dipole Index [SADI], South Western Atlantic SST Index [SWAI] and South Central Atlantic SST Index [SCAI]), were correlated against wheat yield data over a 17 year period from 1994 to 2010. The relationships between the three teleconnections and wheat yield in the Swartland were established. Teleconnection-wheat yield correlations were found to be limited, with regards to the application of this information to farmers, due to the lack of a comprehensive understanding of the dynamics of how the three teleconnections influence the local climate and, therefore, the wheat yield in the Swartland. The eight climate indices, representing the three teleconnections, were correlated against the six wheat-specific rainfall characteristic indices from each of the three study areas over the period from 1980 to 2012. The state of ENSO during the first half of the year was shown to be correlated with rainfall characteristics during both the first (April to July) and second (July to September) halves of the wheat growing season; however, these correlations differ ed in their sign. Correlations suggested a negative phase of AAO was associated with above normal rainfall throughout the year across the Swartland region. Sea surface temperatures in the central South Atlantic during March to October showed significant negative correlations with rainfall during the latter half of the wheat growing season (July to October) across the Swartland region. This study presented evidence supporting the plausibility and validity for the use of the state of large-scale modes of variability in the prediction of wheat-specific rainfall characteristics and aggregated yields in the Swartland region. This has the potential to provide useful information to wheat farmers in the Swartland to aid in their decision making process
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    Modelling the impact of future climate change on subregional wheat production in the Western Cape
    (2013) Wallace, Michael; Tadross, Mark
    Climate change is evident in the Western Cape province of South Africa, particularly in observed trends in average temperatures. Further increases are expected in the future, based on General Circulation Model (GCM) projections, as highlighted in the Inter-governmental Panel on Climate Change (IPCC) 4th (and previous) assessment reports. Whilst it is recognised that rises in temperature coupled with changes in rainfall will impact wheat yields (the province’s dominant field crop), little information exists to guide adaptation planning, especially on the potential range of climate change impacts on dryland winter wheat production. Furthermore the Western Cape is a highly diverse region with regard to geology, soils, topography, climatic influences and agricultural systems. Future climate change therefore, is likely to have different impacts in different zones of the province where wheat is produced. To address this heterogeneity, the APSIM crop model was applied to assess future climate impacts on wheat in 21 relatively homogeneous farming areas (RHFAs) across the province.
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    Potential implications of climate change for Rooibos (A. linearis) production and distribution in the greater Cederberg region, South Africa
    (2015) Lotter, Johanna Magdalena; Van Garderen, Emma Archer; Tadross, Mark; Valentine, Alex
    Wild plants assist in supporting human livelihoods worldwide, both within traditional systems of medicine, and as economically useful plants. Indigenous to the Fynbos biome in the north-western part of the Western Cape, South Africa is the leguminous shrub, A. linearis (rooibos), which is extensively used as ethnomedicine by local communities, while also commercially grown and exported for the herbal tea market. Being a range-restricted species, climate change poses a threat to wild plants and their dependent communities, as well as the sustainability of the rooibos industry. Climate mediated impacts on rooibos are mostly substantiated by anecdotal evidence from commercial growers and local communities and have traditionally been insufficiently addressed. This study integrates predictive modelling and empirical data to provide important insights into rooibos' plant physiological functioning in the presence of climatic and environmental constraints. The aim is to determine whether there is evidence of climate change over the rooibos distribution area, how these climate anomalies are expected to affect the species distribution and to perform experimental studies by testing plant physiological functioning of A. linearis under changing climate conditions. Analysis of climate parameters important for rooibos production (rainfall frequency and intensity, temperature extremes and wind speed) have shown that plants will experience a shorter period of water availability during winter, and prolonged exposure to summer conditions (high temperatures and water stress) in the coming decades. Under these conditions, climate envelope modelling suggests that wild and cultivated rooibos types are at risk to lose between 49.8% and 88.7% in the extent of the bio-climatically suitable localities, most notably along the western and northern periphery of the rooibos production area by 2070. Plant physiological responses (growth analysis, gas exchange parameters and leaf carbon and nitrogen isotope ratios) to the assessed climate anomalies were measured in experimental studies at glasshouse and field scale. Specific adaptation mechanisms (increasing water use efficiency, developing a higher level of sclerophylly and altering the allocation of plant reserves) which helped seedlings to survive short term drought in the glasshouse were not able to offset more severe conditions in field settings. Finally, a comparison of wild and cultivated tea has shown an apparent adaptive advantage of wild tea to tolerate increased aridity with greater water economy, and more reliance on biological nitrogen fixation for N nutrition, indicating a potentially less severe scenario of range contraction for wild types than initially indicated. This study provides a more robust prediction of rooibos plant responses to climate change factors to enable more effective adaptive planning and conservation management in a changing climate.
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    The role of soil moisture on summer climate simulations over southern Africa
    (2016) Mdoka, Marshall Lison; Hewitson, Bruce; Tadross, Mark
    This study aims to increase our perspective of the responses of Southern African climate to soil moisture forcings by drying or moistening the land surface using a regional climate model version 3, RegCM3. The sensitivity and response capabilities to soil moisture perturbations of the model are investigated. This includes identification of regions that may be influenced differently by antecedent soil moisture conditions as well as understand the implications of soil moisture conditions on frequency and intensity of rainfall. Exploratory analyses of soil moisture retention and comparison of climate model parameters with available observations or re-analysis data is done. The study then seeks out the large-scale atmospheric forcings under which the regional climate explicitly responds to perturbations in soil moisture using self-organising map technique. To investigate these underlying processes of atmosphere-soil moisture interactions a series of RegCM3 model experiments utilizing wet, dry and normal soil moisture conditions were designed. The experiments are based on changing the soil moisture field capacity in the RegCM3. The control simulations are run with RegCM3 nested in NCEP/NCAR reanalysis 2 data and using Emanuel convective scheme for the selected six summers (dry seasons - 1991/92, 1994/95 and 1997/98; wet seasons - 1995/96, 1996/97 and 1998/99). September to March simulations are performed with August as the spin-up month. The respective dry and wet soil moisture perturbation simulations are then initialised at field capacities of 25% (wilting) and 75% (saturation) within the land surface model, Bio-sphere Atmosphere Transfer Scheme. From the sensitivity studies, anomalously dry (wet) conditions have positive feedbacks with similar dry (wet) synoptic forcings of the regional climate. Anomalous dry forcing persists for longer and exacerbates the changes in the regional anticyclonic circulation especially during a drought or dry period. Soil moisture perturbations mostly affect the lower troposphere. Surface variables analysed especially surface temperature show strong responses to the soil moisture perturbations under all synoptic forcings but rainfall characteristics are strongly influenced by large-scale synoptic circulations. However, in some areas over southwestern parts of the region a weak feedback which can be either positive or negative depending on geographical and climatological setting has been detected.