The advantages offered by Super High Frequency (SHF) and Extremely High Frequency (EHF) bands such as large bandwidth, small antenna size, and easy installation or deployment have motivated the interest of researchers to study those factors that prevent optimum utilization of these bands. Under precipitation conditions, factors such as clouds, hail, fog, snow, ice crystals and rain degrade link performance. Rain fade, however, remains the dominant factor in the signal loss or signal fading over satellite and terrestrial links especially in the tropical and sub-tropical regions within which South Africa falls. At millimetre-wave frequencies the signal wavelength approaches the size of the raindrops, adversely impacting on radio links through signal scattering and absorption. In this work factors that may hinder the effective use of the super high frequency and extremely high frequency bands in the Southern African region are investigated. Rainfall constitutes the most serious impairment to short wavelength signal propagation in the region under study. In order to quantify the degree of impairment that may arise as a result of signal propagation through rain, the raindrops scattering amplitude functions were calculated by assuming the falling raindrops to be oblate spheroidal in shape. A comparison is made between the performance of the models that assume raindrops to be oblate spheroidal and those that assume them to be spherical. Raindrops sizes are measured using the Joss-Waldvogel RD-80 Distrometer. The study then proposes various expressions for models of raindrops size distributions for four types of rainfall in the Southern Africa region. Rainfall rates in the provinces in South Africa are measured and the result of the cumulative distribution of the rainfall rates is presented. Using the information obtained from the above, an extensive calculation of specific attenuation and phase shift in the region of Southern Africa is carried out. The results obtained are compared with the ITU-R and those obtained from earlier campaigns in the West African sub region. Finally, this work also attempts to determine and characterize the scattering process and micro-physical properties of raindrops for sub-tropical regions like South Africa. Data collected through a raindrop size measurement campaign in Durban is used to compare and validate the developed models. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2014.

The inability of service providers to constantly meet the design target of 99.99 % availability of the line-of-sight (LOS) microwave links has caused concern among both the operators and consumers. The non-availability of the links is predominantly due to propagation impairments along the propagation link. These propagation effects include cloud, snow, fog, gas attenuation, rain and atmospheric scintillation. Various studies have shown high vulnerability of radio communication systems operating at microwave (3-30 GHz) and millimeter wave (30-300 GHz) to rainfall attenuation especially in the tropical regions characterized by heavy rainfall and relatively large rain drops when compared to the temperate regions. In order to understand the effects of attenuation due to rain on communication systems in any locality (region), a good knowledge of the raindrop size distribution (DSD) and the rainfall rate estimates is necessary for accurate prediction and estimation of the rainfall attenuation. For this study, experimental raindrop size measurements gathered over a period of three years, using the Joss-Waldvogel RD-80 disdrometer installed at the roof top of the Electrical, Electronic and Computer Engineering building, University of Kwa-Zulu Natal, Durban, a subtropical location in South Africa, is analysed. Disdrometer measurements, sampled at one-minute rate over a period of nine months from Butare, an equatorial site in Rwanda, is also analysed for the purpose of comparison. The estimated R0.01 values for Durban and Butare are employed for the purpose of analysis. Based on the statistical analysis of the measured data samples, DSD parameters are proposed from the negative exponential, modified gamma, Weibull and the lognormal models. The DSD models are compared to models from other countries within and outside the region. The Mie scattering approximation at temperature of 20oC for spherical raindrop shape is adopted for the estimation of the scattering functions. The study further investigates the influence of critical raindrop diameters on the specific rain attenuation for the annual, seasonal and various rainfall regimes in southern Africa. This is achieved analytically by integrating the total rainfall attenuation over all the raindrop sizes and observing the differential change in the attenuation over a given range of drop size diameters. The peak diameter at which the specific rainfall attenuation is maxima is determined for different rainfall regimes. Finally, the cross-polarisation discrimination (XPD) due to rain over Durban is computed at two elevation angles. The results of this study will be helpful for the proper design and allocation of adequate fade margins to achieve the expected quality of service (QoS) in a radio communication system operating in the Southern Africa region. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2014.

Hydrological variability involving rainfall and streamflows in southern Africa have been often studied separately or have used cumulative rainfall and streamflow indices. The main objective of this study was to investigate spatio-temporal variations of rainfall, their influences on streamflows and their relationships with climatic variations with emphasis on indices that characterise the hydrological extremes, floods and droughts. It was found that 60-70% of the time when it rains, daily rainfalls are below their long-term averages and daily amounts below 10 mm are the most frequent in southern Africa. Spatially, climatologies of rainfall sub-divided the southern African subcontinent into the dry western/southwestern part and the “humid” eastern and northern part. The daily amounts below 20 mm contribute significantly to annual rainfall amounts in the dry part while all types of daily rainfall exceeding 1 mm have comparable contributions in the humid part. The climatologies indicated the highest likelihood of experiencing intense daily events during the core of the wet seasons with the highest frequencies in central Mozambique and the southern highlands of Tanzania. Interannual variations of rainfall indicated that significant changes had occurred between the late-1940s and early-1980s, particularly in the 1970s. The changes in rainfall were more evident in the number of daily rainfall events than in rainfall amounts, led generally to increasing early summer and decreased late summer rainfall. It was also found that intra-seasonal dry day sequences were an important parameter in the definition of a rainy season’s onset and end in southern Africa apart from rainfall amounts. Interannual variations of the rainy season characteristics (onset, end, duration) followed the variations of rainfall amounts and number of events. The duration of the rainy season was affected by the onset (Tanzania), onset or end (tropical southern Africa - southwestern highlands of Tanzania, Zambia, northern Zimbabwe and central Mozambique) and end (the remaing part of southern Africa). Flow duration curves (FDCs) identified three types of rivers (ephemeral, seasonal and perennial) in southern Africa with ephemeral rivers found mainly in the dry western part of the region. Seasonal streamflow patterns followed those of rainfall while interannual streamflow variations indicated significant changes of mean flows with little evidences of high and low flow regime changes except in Namibia and some parts of northern Zimbabwe. It was, however, not possible to provide strong links between the identified changes in streamflows and those in rainfall. Regarding the influences of climate variability on hydrological variability in southern Africa, rainfall variations in southern Africa were found to be influenced strongly by ENSO and SST in the tropical Indian ocean and moderately by SST in the south Madagascar basin. The influence of ENSO was consistent for all types of daily rainfall and peaks for the light and moderate (< 20 mm) events in the southern part and for the intense events in the northern part. SST in the tropical Indian ocean influence the light and moderate events while SST close to the region influence the heavy events. However, the relationships experienced significant changes in the mid-1950s and in the 1970s. The former changes led to improved associations while the latter deteriorated or reversed the relationships. The influences of climatic variables on streamflows and rainy season characteristics were inferred from the rainfall-streamflow and rainfall-climatic variables relationships.

Water vapour content and transport over southern Africa and adjacent oceans are examined. Early summer rainfall over the northern and central interior of South Africa tends to be associated with baroclinic controls whereas late-summer rainfall is barotropic in origin. This is reflected in the northwesterly water vapour transport from an Atlantic Ocean source by middle and upper tropospheric westerly waves in early summer. A thermally indirect Ferrel cell, indicated-from energetics, COpIU1nSthe· temperate nature of the early-summer atmosphere over southern Africa. Late summer water vapour transport, in contrast, is strongly from the tropics, with' a reduced eddy component, indicating an important tropical control on late SUmmerrainfall especially in terms of fluctuations in the position of the ascending limb of .the Walker cell Over southern Africa. The Hadley cell is of importance to the late summer rainfall in that dry (wet) years are associated with an anomalous cell OVereastern (central) South Africa such that low level vapour transport is southerly (northerly). The anticyclone over the eastern parts of southern Africa, coupled with. a trough over the interior (especially at the 700 hPa pressure level), is important for the introduction of water vapour over the subcontinent in wet and dry years and for tropical-temperate trough case studies. Water vapour source regions differ from early summer (Atlantic Ocean) to late summer (Indian Ocean), which reflects the temperate. control on early and the tropical control on late summer circulation. The convergence of water vapour over southern Africa in wet years and during tropical-temperate troughs is not only important for cloud formation and precipitation, but also for latent heat release associated with convergent water vapour. Diabatic heating decreases the stability of the tropical atmosphere thereby resulting in increased vertical motion. It also forces an anomalous Badley circulation during wet late summers and tropical-temperate trough .cases as a result of complex energy transformations. Heating increases eddy available potential energy which is converted to zonal available potential energy by a thermally indirect circulation found in the tropics. The zonal potential energy is then converted to kinetic energy by the thermally direct Badley cell. Water vapour and its variations are thus important for the precipitation, heating and SUbsequent energy of the subtropical southern African atmosphere, / GR 2017

Dissertation submitted to the Faculty of Science, University of the Witwatersrand, for completion of the Degree of' Master of Science / The atmospheric response of the Colorado State University Regional Atmospheric Modelling System (RAMS) to sea-surface temperature anomaliesis investigated. A period of four days was chosen from 21 to 24 January 1981, where focus was placed on the development and dissipation of a tropical-temperate trough across Southern Africa. Previous experimenting this mesoscalenumerical model have detemined the kinematic, moisture, and thermodynamic nature of these synoptic features. The research in this dissertation focuses specifically on the sensitivity of the numerical model's simulated responses to positive sea-surface temperature anomalies. Three separate experiments were devised, in which positive anomalous temperatures were added to the ocean surface north of Madagascar (in the tropical Indian Ocean), at the region of the Agulhas Current retroflection, and along the tropical African west coast (in the Northern Benguela and Angola currents). The circulation aspects of each sensitivity test were investigated through the comparison of simulated variables such as vapour and cloud mixing ratios, temperature, streamlines and vertical velocity, with the same variables created by a control simulation. The results indicate that for the first sensitivity test, (the Madagascar anomaly), cyclogenesis was initiated over the area of modified sea temperatures which resulted in a marginal decrease in continental precipitation. The second sensitivity test (over the Agulhas retroflection) produced a much smaller simulated response to the addition of anomalously warm sea temperatures than the tropical Indian Ocean anomaly. Instability and precipitation values increased over the anomalously warm retroflection region, and were slowly transferred along the westerly wave perturbation and the South African east coast. The third sensitivity experiment showed a predominantly localised simulated increase in precipitation over Gabon and the Congo, with the slow southward progression of other simulated circulation differences taking place. The small perturbations in each of the simulated meteorological responses are consistent with the expected climate response to anomalously warm sea-surface temperatures in those areas. / AC 2018

It has become accepted that long-term global mean temperatures have increased over the twentieth century. However, whether or not climate change can be detected at a local or regional scale is still questionable. The numerous new record highs and lows of temperatures recorded over South Africa for 2003, 2004 and 2005 provide reason to examine whether changes can already be detected in southern Africa's temperature record and modelled hydrological responses. As a preface to a temperature detection study, a literature reVIew on temperature detection studies, methods used and data problems encountered, was undertaken. Simple statistics, linear regression and the Mann-Kendall non-parametric test were the methods reviewed for detecting change. Southern Africa's temperature record was thereafter examined for changes, and the Mann-Kendall non-parametric test was applied to time series of annual means of minimum and maximum temperature, summer means of maximum temperature and winter means of minimum temperature. Furthermore, changes in the upper and lower ends of the temperature distribution were examined. The Mann-Kendall test was applied to numbers of days and numbers of 3 consecutive days abovelbelow thresholds of 10th and 90th percentiles of minimum and maximum temperatures, as well as abovelbelow threshold values of minimum (i.e. 0°) and maximum (i.e. 40°C) temperatures. A second analysis, using the split sample technique for the periods 1950 - 1970 vs 1980 - 2000, was performed for annual means of daily maximum and minimum temperatures, summer means of daily maximum temperatures, winter means of daily minimum temperatures and coefficients of variability of daily maximum and minimum temperatures. Two clear clusters of warming emerged from almost every analysis, viz. a cluster of stations in the Western Cape and a cluster of stations around the midlands ofKwaZulu-Natal, along with a band of stations along the KwaZulu-Natal coast. Another fmding was a less severe frost season over the Free State and Northern Cape. While certain changes are, therefore, evident in temperature parameters, the changes are not uniform across southern Africa. Precipitation and evaporation are the primary drivers of the hydrological cycle, with temperature an important factor in the evaporation process. Thus, with changes in various temperature parameters having been identified over many parts of southern Africa, the question arose whether any changes were evident as yet in hydrological responses. The ACRU model was used to generate daily streamflow values and associated hydrological responses from a baseline land cover, thus eliminating all possible human influences on the catchment and channel. A split-sample analysis of the simulated hydrological responses for the 1950 - 1969 vs 1980 - 1999 periods was undertaken. Trends over time in simulated streamflows were examined for medians, dry and wet years, as well as the range between wet and dry years. The seasonality and concentration of streamflows between the periods 1950 - 1969 and 1980 - 1999 were examined to determine if changes could be identified. Some trends found were marked over large parts of Primary Catchments, and certainly require consideration in future water resources planning. With strong changes over time in simulated hydrological responses already evident in certain Primary Catchments of South Africa using daily rainfall input data from 1950 1999, it, therefore, became necessary to examine the rainfall regimes of the Quaternary Catchments' "driver" rainfall station data in order to determine if these hydrological response changes were supported by changes in rainfall patterns over time. A splitsample analysis was, therefore, performed on the rainfall input of each Quaternary Catchment. Not only were medians considered, but the higher and lower ends of the rainfall distributions were also analysed, as were the number of rainfall events above pre-defined daily thresholds. The changes evident over time in rainfall patterns over southern Africa were found to vary from relatively unsubstantial increases or decreases to significant increase and decreases. However, the changes in rainfall corresponded with the changes noted in simulated streamflow. From the analyses conducted in this study, it has become clear that South Africa's temperature and rainfall, as well as hydrological responses, have changed over the recent past, particularly in certain identifiable hotspots, viz. the Western Cape and KwaZulu-Natal where significant increases in temperature variables and changes in rainfall patterns were detected. These detected changes in climate need to be considered in future water resources planning. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.

Water is recognised as a key driver for social and economic development in the Zambezi basin. The basin is riparian to eight southern African countries and the transboundary nature of the basin’s water resources can be viewed as an agent of cooperation between the basin countries. It is possible, however, that the same water resource can lead to conflicts between water users. The southern African Water Vision for ‘equitable and sustainable utilisation of water for social, environmental justice and economic benefits for the present and future generations’ calls for an integrated and efficient management of water resources within the basin. Ensuring water and food security in the Zambezi basin is, however, faced with challenges due to high variability in climate and the available water resources. Water resources are under continuous threat from pollution, increased population growth, development and urbanisation as well as global climate change. These factors increase the demand for freshwater resources and have resulted in water being one of the major driving forces for development. The basin is also vulnerable due to lack of adequate financial resources and appropriate water resources infrastructure to enable viable, equitable and sustainable distribution of the water resources. This is in addition to the fact that the basin’s economic mainstay and social well-being are largely dependent on rainfed agriculture. There is also competition among the different water users and this has the potential to generate conflicts, which further hinder the development of water resources in the basin. This thesis has focused on the Zambezi River basin emphasising climate variability and climate change. It is now considered common knowledge that the global climate is changing and that many of the impacts will be felt through water resources. If these predictions are correct then the Zambezi basin is most likely to suffer under such impacts since its economic mainstay is largely determined by the availability of rainfall. It is the belief of this study that in order to ascertain the impacts of climate change, there should be a basis against which this change is evaluated. If we do not know the historical patterns of variability it may be difficult to predict changes in the future climate and in the hydrological resources and it will certainly be difficult to develop appropriate management strategies. Reliable quantitative estimates of water availability are a prerequisite for successful water resource plans. However, such initiatives have been hindered by paucity in data especially in a basin where gauging networks are inadequate and some of them have deteriorated. This is further compounded by shortages in resources, both human and financial, to ensure adequate monitoring. To address the data problems, this study largely relied on global data sets and the CRU TS2.1 rainfall grids were used for a large part of this study. The study starts by assessing the historical variability of rainfall and streamflow in the Zambezi basin and the results are used to inform the prediction of change in the future. Various methods of assessing historical trends were employed and regional drought indices were generated and evaluated against the historical rainfall trends. The study clearly demonstrates that the basin has a high degree of temporal and spatial variability in rainfall and streamflow at inter-annual and multi-decadal scales. The Standardised Precipitation Index, a rainfall based drought index, is used to assess historical drought events in the basin and it is shown that most of the droughts that have occurred were influenced by climatic and hydrological variability. It is concluded, through the evaluation of agricultural maize yields, that the basin’s food security is mostly constrained by the availability of rainfall. Comparing the viability of using a rainfall based index to a soil moisture based index as an agricultural drought indicator, this study concluded that a soil moisture based index is a better indicator since all of the water balance components are considered in the generation of the index. This index presents the actual amount of water available for the plant unlike purely rainfall based indices, that do not account for other components of the water budget that cause water losses. A number of challenges were, however, faced in assessing the variability and historical drought conditions, mainly due to the fact that most parts of the Zambezi basin are ungauged and available data are sparse, short and not continuous (with missing gaps). Hydrological modelling is frequently used to bridge the data gap and to facilitate the quantification of a basin’s hydrology for both gauged and ungauged catchments. The trend has been to use various methods of regionalisation to transfer information from gauged basins, or from basins with adequate physical basin data, to ungauged basins. All this is done to ensure that water resources are accounted for and that the future can be well planned. A number of approaches leading to the evaluation of the basin’s hydrological response to future climate change scenarios are taken. The Pitman rainfall-runoff model has enjoyed wide use as a water resources estimation tool in southern Africa. The model has been calibrated for the Zambezi basin but it should be acknowledged that any hydrological modelling process is characterised by many uncertainties arising from limitations in input data and inherent model structural uncertainty. The calibration process is thus carried out in a manner that embraces some of the uncertainties. Initial ranges of parameter values (maximum and minimum) that incorporate the possible parameter uncertainties are assigned in relation to physical basin properties. These parameter sets are used as input to the uncertainty version of the model to generate behavioural parameter space which is then further modified through manual calibration. The use of parameter ranges initially guided by the basin physical properties generates streamflows that adequately represent the historically observed amounts. This study concludes that the uncertainty framework and the Pitman model perform quite well in the Zambezi basin. Based on assumptions of an intensifying hydrological cycle, climate changes are frequently expected to result in negative impacts on water resources. However, it is important that basin scale assessments are undertaken so that appropriate future management strategies can be developed. To assess the likely changes in the Zambezi basin, the calibrated Pitman model was forced with downscaled and bias corrected GCM data. Three GCMs were used for this study, namely; ECHAM, GFDL and IPSL. The general observation made in this study is that the near future (2046-2065) conditions of the Zambezi basin are expected to remain within the ranges of historically observed variability. The differences between the predictions for the three GCMs are an indication of the uncertainties in the future and it has not been possible to make any firm conclusions about directions of change. It is therefore recommended that future water resources management strategies account for historical patterns of variability, but also for increased uncertainty. Any management strategies that are able to satisfactorily deal with the large variability that is evident from the historical data should be robust enough to account for the near future patterns of water availability predicted by this study. However, the uncertainties in these predictions suggest that improved monitoring systems are required to provide additional data against which future model outputs can be assessed.