Browsing by Author "Blamey, Ross"

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    Open Access
    An analysis of heavy rainfall events over the Limpopo River Basin in southern Africa, their moisture sources and pathways
    (2020) Rapolaki, Ramontsheng Sakia; Reason, Christopher; Hermes, Juliet; Blamey, Ross
    Severe flooding events in subtropical southern Africa are not uncommon and can cause damage to infrastructure, lead to huge economic losses, and fatalities. Although extreme rainfall events can have far-reaching negative consequences, they can also provide large amounts of freshwater within a short time span, which supports the rain-fed farming upon which much of the population depends. However, the mechanisms through which extreme rainfall is produced in southern Africa are still not well understood. In particular, relatively little is known about where the moisture, a key ingredient in the rainfall, is sourced and how it is transported into the region. This thesis aims to address some of the gaps in this understanding by examining the moisture sources and subsequent moisture transport moisture into one of the key river basins in southern Africa, the Limpopo River Basin (LRB). The LRB, located in eastern southern Africa and spanning four countries, has experienced a number of extreme flooding events over the last three decades. Using CHIRPS satellite merged rainfall data for 1981-2016, the thesis identified the top 200 heavy extreme rainfall events in the LRB and the associated weather systems. It was found that tropicalextratropical cloud bands account for almost half of the events and tropical lows are responsible for just over a quarter. The remaining quarter of the events are associated with mesoscale convective systems and cut-off lows, the latter more important during transition seasons. Most of the events occur in the late summer when tropical lows and cloud bands are more common. Some relationships between the frequency of heavy rainfall events over the LRB and interannual climate modes of variability such as ENSO, SIOD, and SAM were found. Having examined the annual cycle of the top 200 heavy rainfall events, the analysis then applied the Lagrangian trajectory model HYSPLIT, with NCEP II reanalysis data as input, to backtrack air parcels from the LRB to their moisture source on seasonal scales and in terms of the types of weather systems involved. The resulting trajectories show that the seasonal transport of moisture over the LRB originates from seven moisture source regions; namely, local continental, tropical southeast Atlantic Ocean, midlatitude South Atlantic Ocean, tropical Northwest Indian Ocean, tropical southwest Indian Ocean, subtropical southwest Indian Ocean, and the Agulhas Current. Important differences in moisture source regions and pathways exist between early (OctoberDecember) and late (January-April) summers, with the tropical northwestern Indian Ocean and the northern Agulhas Current sources more prominent during JFMA than OND. Generally, moisture v source regions and transport pathways for LRB tend to be influenced by both the regional summer season circulation and the synoptic systems involved. Thus, it was found that cloud band and tropical low events within the top 200 tend to have the Congo Basin as an important moisture source whereas this source is less evident for cut-off low events. To help assess the robustness of the composite synoptic approach, the final part of the thesis applied the Lagrangian analysis to the most severe case in the top 200 events over the LRB (11- 21 January 2013). It was found that this case was largely linked to three main moisture sources: (1) tropical northwest Indian Ocean, (2) the Agulhas Current / Mozambique Channel, subtropical Southwest Indian Ocean, and (3) continental sources over the Congo Basin and northern Tanzania. Generally, the moisture source regions and pathways for the January 2013 event agreed with the climatological moisture source regions over the LRB, apart from the obvious absence of the tropical southeast Atlantic source in this case. In general, the thesis has provided a better understanding of the characteristics of heavy rainfall events over the LRB in terms of their associated weather systems, seasonality, interannual variability, and moisture source regions and trajectories.
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    Climate and environmental change along the East Coast of South Africa: perspectives from a local marine resource- dependent community and scientific researchers
    (2019) Duba, Tania; Hermes, Juliet; Blamey, Ross; Raemaekers, Serge
    Coastal areas are very susceptible to environmental problems such as sea-level rise, coastal flooding, increased frequency and intensity of extreme events, and changes in marine ecosystems that are arising from global climate change and variability. In the South African context, the Agulhas Current is important for its crucial role in regional climate and weather as well as the fishing livelihood of the coastal communities along the east coast of South Africa. Despite the efforts made to understand the Agulhas Current and the impacts of climate and environmental change, the shelf region remains poorly understood mostly due to the difficulties associated with observing and modelling such strong currents. The marine resource users in the fishing communities along the east coast of South Africa show long term dependence on the neighbouring ocean going back at least three generations. These communities provide long term, rich, detailed, and contextualized environmental knowledge from their daily interactions with the sea. This study seeks to investigate the local climate and environmental change knowledge of the fishers based on their own observations, perceptions, and experiences. The convergence/divergence of the marine resource user’s knowledge with the traditional scientific findings is explored using a broad, participatory methodology including desktop literature analysis, interviews and an adopted version of the Rapid Vulnerability Assessment (RVA). Results show that fishers in Tshani-Mankosi have observed changes in the rainfall, sea surface temperature and wind patterns in their community. According to the fishers, sea surface temperature and annual rainfall seem to have decreased while winds and rainfall related extreme events have increased. Similar observations were noticed in the scientific research at a larger spatial and temporal scale. Key differences and similarities between the two types of knowledge come from factors such as knowledge construction processes, scales, type of data output and parameters of interest. Finally, the study reveals opportunities and challenges of research collaboration between the community and scientific researchers.
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    A climatology of potential severe convective environments across South Africa
    (Springer Verlag, 2016-11-15) Blamey, Ross; Middleton, C; Lennard, C; Reason, Chris
    Severe thunderstorms pose a considerable risk to society and the economy of South Africa during the austral summer months (October–March). Yet, the frequency and distribution of such severe storms is poorly understood, which partly stems out of an inadequate observation network. Given the lack of observations, alternative methods have focused on the relationship between severe storms and their associated environments. One such approach is to use a combination of covariant discriminants, derived from gridded datasets, as a probabilistic proxy for the development of severe storms. These covariates describe some key ingredient for severe convective storm development, such as the presence of instability. Using a combination of convective available potential energy and deep-layer vertical shear from Climate Forecast System Reanalysis, this study establishes a climatology of potential severe convective environments across South Africa for the period 1979–2010. Results indicate that early austral summer months are most likely associated with conditions that are conducive to the development of severe storms over the interior of South Africa. The east coast of the country is a hotspot for potential severe convective environments throughout the summer months. This is likely due to the close proximity of the Agulhas Current, which produces high latent heat fluxes and acts as a key moisture source. No obvious relationship is established between the frequency of potential severe convective environments and the main large-scale modes of variability in the Southern Hemisphere, such as ENSO. This implies that several factors, possibly more localised, may modulate the spatial and temporal frequency of severe thunderstorms across the region.
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    Drylines over the interior of subtropical southern Africa
    (2024) Van Schalkwyk, Lynette; Reason, Christopher; Blamey, Ross
    Drylines are surface airmass boundaries separating regions of very dry air from regions of moist air. Due to changes in airflow that occur along the interface of these boundaries, they are known to act as zones of surface convergence which can trigger convection. For this reason drylines have been studied globally. Drylines regularly form over the interior of subtropical southern Africa and have also been associated with thunderstorm development and often severe thunderstorm initiation, but their seasonal characteristics, formation mechanisms and the extent to which they influence convection, are largely unknown. By utilizing ERA5 reanalysis surface specific humidity and temperature data and an objective moisture and temperature gradient detection algorithm, a drylines dataset is compiled between 1979 and 2020 for the subtropical southern African interior plateau (SAP). Drylines occur most frequently during spring and summer and almost daily in December. In this region dryline locations are very sensitive to surface moisture variations and therefor a westward shift in peak dryline frequency occurs through summer as moisture increases from the east after the dry winter months. Drylines peak first over the eastern SAP during November, but over the central and western parts in December. During years with increased surface moisture weaker moisture gradients occur in the east during mid-summer, resulting in fewer drylines, but during years with decreased surface moisture, more drylines tend to occur. During mid-summer, dryline frequencies in the east are negatively correlated with dryline frequencies in the west so that during years with high dryline frequencies in the east, low dryline frequencies tend to occur in the west and vice versa. The dryline climatology also establishes that drylines occur most frequently over the climatologically drier western parts of the SAP (western plateau) during summer. Drylines on the western plateau are then further investigated and lightning data from 2010-2021 are used to distinguish between drylines which resulted in convective storm development (convective drylines) and those that did not (non-convective drylines). It is found that 76% of drylines which occur over the western plateau during the late afternoons, when convection is most likely in the west, are convective. Convective drylines peak during December months when 12 occur on average over the western plateau region. The formation mechanisms of convective vs. non-convective drylines are determined by constructing synoptic composites during early summer (October-December) and late summer (January-March). Low-level moisture flow composites for the two days leading up to a convective dryline show that the Limpopo River Valley and Zambezi River Valley are important moisture pathways through which moisture from the southwest Indian Ocean travel in the days leading up to a convective dryline. During early summer convective drylines tend to occur after sharp increases 7 in surface moisture ahead of the dryline, in the presence of a deep Kalahari Heat Low and an upper air trough to its west. During late summer the pressure gradients are not as intense, but the Kalahari Heat Low continues to be a key component of the synoptic circulation on convective dryline days. Compared to early summer, a higher number of days with moderate CAPE (>1000 J.kg-1 ) occur in the same location that has the highest frequency of drylines during late summer, and this results in a greater proportion of dryline-related thunderstorms in southwestern Botswana, northeastern Northern Cape and western Northwest. Dryline climatology results confirm that drylines frequently occur over the SAP and that they have a meaningful impact on thunderstorm development in the drier western interior. This drylines study is completed by an investigation of the month with the highest number of convective drylines since the convective dryline record began in 2010 and an analysis of its synoptic characteristics. During December 2021, 20 convective drylines occurred on the western plateau, 8 more than the average. Record rainfall occurred over the central interior of South Africa while the drier western interior also recorded above average rainfall. A detailed analysis is performed of a 6-day convective dryline event where thunderstorms developed along the dryline daily, before being steered towards the central interior by anomalously strong mid-level winds. It is shown that a significant positive correlation exists between dryline days on the western plateau and rainfall within the domain, but also further downstream. In this thesis, drylines are shown to be an important climatological feature of subtropical southern Africa throughout summer. Dryline locations are dependent on the seasonal influx of moisture from the southwest Indian Ocean via the Limpopo and Zambezi River valleys and are often located at the western boundary of the tropical rain band edge. Drylines frequently act as triggering mechanisms for convection and therefor also influence rainfall further downstream
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    Early summer rainfall variability in the Congo Air Boundary Region
    (2024) Maphugwi, Mulalo; Blamey, Ross; Reason, Christopher
    Strong meridional rainfall gradients exist between the tropics and subtropics in southwestern Africa, bounded to the north by the moist Congo basin and the south by the Kalahari Desert. This region termed the tropical-subtropical divide (TSD) here, has recently faced one of the worst droughts in the last 40 years, contributing to local food insecurity. Compared to the rest of southern Africa, relatively little scientific attention has focused on the domain, partly due to long-term conflict preventing reliable observations. In this study, focus is placed on understanding rainfall characteristics and variability during the austral summer (October – April) across southwestern Africa using daily fifth generation of European Centre for Medium-Range Weather Forecast Reanalysis (ERA5) and Climate Hazard Group InfraRed Precipitation with stations (CHIRPS) data. Evaluation of gridded rainfall products in the region found that CHIRPS and ERA5 compare well with limited station data in the region. Using the ERA5 data, results reveal a significant decreasing trend of early summer (October-December) rainfall totals as well as rainy days since 1979 to present. The importance of the early summer rainfall is that it accounts for up to 60% of the total summer rainfall. There also appears a statistically significant long-term decreasing trend in rainfall onset (i.e., later onset), which typically occurs during the late October – early November months. A late onset could lead to dry early summers, a decrease in rainy season length, and severe droughts in the region in future. However, the late summer (January – April) increasing trend in rainfall totals might offset any large changes in summer rainfall. Correlation analysis reveals that although there is a significant relationship between early summer rainfall and rainy days and some of the main modes of climate variability and regional circulation systems, although it was highly variable across the domain. Thus, two sub-domains termed western and eastern region (“WR” and “ER”) were also investigated. A significant relationship between sea-surface temperature from the Niño 3.4 region and Indian Ocean Dipole (IOD) with rainfall and rainy days was only limited to the extreme southeast of Zambia. For late summer, when El Niño-Southern Oscillation (ENSO) has matured, there is a strong contrasting signal of significant positive (negative) relationship with rainfall totals and rainy days in the Angola Highlands (the rest of subtropical southern Africa). Angola Low index significantly and positively (negatively) correlates with early summer rainfall in the western (eastern) parts of the domain. The relationship between the Botswana High with early summer rainfall and rainy days is only limited to the eastern parts of the region. Lastly, a composite analysis of wet and dry years for the whole tropical-subtropical divide region as well as two sub-domains (western and eastern region) were investigated. Given the considerable variability within the region, anomalously dry and wet years are not consistent for the tropical-subtropical divide region, western and eastern region. Composites of wet years for the tropical-subtropical divide region and both sub-domains shows that the western Indian Ocean appears to act as the main source of moisture for the region. Typically, wet composites over the eastern region (western region) are characterized by stronger (weaker) Angola Low, while Botswana High influence over wet and dry years is only limited to the eastern region.
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    Open Access
    Extreme rainfall events over the Pongola-Mtamvuna Water Management Area of South Africa
    (2022) Mpungose, Nomvula Bongekile; Reason, Christopher; Blamey, Ross
    Subtropical southern Africa experiences substantial rainfall variability both spatially and temporally, due to regional orography, geographic position, and local sea-surface temperatures. Extreme weather conditions such as droughts and floods are not uncommon and can result in both positive and negative socio-economic impacts. The Pongola-Mtamvuna Water Management Area (PM) located over north-eastern South Africa consists of communities that depend on rain-fed agriculture, as well as an inter-linked ecosystem and fresh water bodies that are dependent on rainfall. Extreme rainfall events and the systems that produce them are still not well understood, therefore, a detailed analysis of such events can contribute to an improved understanding and management of their associated risks. Here, the main focus is on the summer rainy season (October – March), rainfall variability is examined using CHIRPS daily rainfall data covering a period of thirty-seven years from 1981 – 2018. Extreme rainfall events are identified and classified for the PM area. The analysis points to the highest rainfall amounts typically occurring over low-lying coastal areas and near mountainous regions. About 60% of the extreme rainfall events were associated with tropical lows (40%) and MCS (20%). Cut-off lows (18%), cloud bands (16%), and tropical cyclones (6%) contributed to the remaining proportion. The highest frequency of events occurred during late summer months (January – March) when tropical lows and occasionally, tropical cyclones are more common. Rainfall over the PM has a statistically significant relationship with ENSO, most of the seasons with below-average rainfall and extreme events coincided with El Niño conditions. Odd cases where this was the opposite were more influenced by regional circulation anomalies which acted to enhance or reduce moisture over the land-mass thereby increasing conditions favourable/unfavourable for rainfall over the region.
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    Long-lived mesoscale convective systems over Eastern South Africa
    (2022) Morake, Dedricks Monyai; Reason, Christopher; Blamey, Ross
    Previous studies on severe weather in South Africa have often focused on synoptic-scale systems such as cut off lows, tropical extratropical cloud bands, and tropical cyclones, with little attention placed on the smaller mesoscale convective systems (MCSs). On a global scale, MCSs are not only important as key rainfall producers, but are often responsible for flooding, severe winds, hail and sometimes tornadoes. MCSs in South Africa remain poorly understood with there being little evidence of any long-term climatology studies of these systems over the region. A climatology of large, long-lived MCSs over eastern South Africa for the extended austral summer (September-April) from 1985-2008 is presented. On average, 63 MCSs occur here in summer, but with considerable interannual variability in frequency. The systems mainly occur between November and March, with a December peak. This seasonal cycle in MCS activity is shown to coincide with favorable convective available potential energy (CAPE) and vertical shear profiles across the domain. Most systems tend to occur along the eastern escarpment with its sharp topographic gradients close to the warm waters of the northern Agulhas Current. The eastern escarpment can act as a convective trigger by mechanically uplifting sufficiently moist and unstable air masses. In addition, strong latent heat fluxes from the northern Agulhas Current help to create high-CAPE conditions. Typically, initiation begins in the early afternoon, MCS status is reached mid-afternoon, maximum extent early in the night and termination around midnight or shortly thereafter. The analysis also considered MCSs that developed over land versus those over the adjacent ocean. It is found that most MCSs initiate over land, but systems that initiate over the ocean tend to last longer than those that develop over land. The results also show that there are differences in the seasonal cycle between continental and oceanic MCSs, with oceanic systems containing two intraseasonal peaks (December and April). There is a relatively strong positive relationship between the southern annular mode (SAM) and early summer MCS frequency. For the late summer, the frequency of MCSs appears related to the strength of the Mascarene high and Mozambique Channel trough, which modulate the inflow of moisture into eastern South Africa and the stability of the lower atmosphere over the region. The 6 results indicated that there is considerable variability in the long-lived MCS frequency on interannual time scales and such variability can have considerable impacts on regional rainfall totals during the summer months over eastern South Africa. MCSs are known to produce heavy rainfall that is often associated with floods, which can be devastating to livestock, crops, infrastructure, and humankind. However, these systems also provide important rainfall within a short time span that is significant for rain-fed agriculture for a semi-arid country as South Africa. Using Climate Hazards Infrared Precipitation with Stations (CHIRPS) satellite rainfall data for 1985-2008, this study identified 38 daily extreme rainfall events which account for 40% of the top 50 extreme rainfall events over the two sub-domains of the eastern parts of South Africa that are linked to long-lived MCSs. Of the 38 events, systems duration ranged between 6-51 hours with 23 systems lasting for longer than 12 hours. Individual MCS-associated extreme daily rainfall events contribute between 21% - 54% to the total seasonal amount of rainfall over eastern South Africa. There is also noticeable interannual variability of seasonal rainfall over the northern and the southern domain, and considerable spatial variability in seasonal rainfall of MCS-related extreme rainfall events over the two sub-domains. In general, the thesis has contributed to a better understanding of the wider group of large, longlived MCS characteristics over eastern South Africa in terms of their distribution, frequency, life cycle, seasonal cycle and large-scale environmental conditions and the relationship between MCS frequency and interannual climate mode of variability such as El Niño-Southern Oscillation (ENSO), the subtropical south Indian Ocean dipole (SIOD), and SAM. The study also contributed to a better understand the role MCSs play in eastern South Africa summer rainfall and particularly extreme rainfall in the region.
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    Mesoscale convective complexes over southern Africa
    (2012) Blamey, Ross; Reason, Chris
    A combination of numerous factors, including geographic position, regional orography and local sea surface temperatures, ensures that subtropical southern Africa experiences considerable spatial and temporal variability in rainfall and is prone to both frequent flooding and drought events.
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    Numerical simulation of a mesoscale convective system over the east coast of South Africa
    (2007) Blamey, Ross; Reason, Chris
    Weather stations across the northern KwaZulu-Natal coastline recorded over 100 mm of rainfall over the 11112 February 2005, with Cape St. Lucia and Richards Bay measuring 111 mm and 96.8 mm, respectively. This heavy rainfall was associated with a mesoscale convective system (MCS) that initiated through small convective storms beginning early in the afternoon on 11 February 2005 and eventually decayed in the early morning hours on the 12th. The high-lying topography of the eastern escarpment and high diurnal surface heating possibly provided the trigger for the event. It was also identified that a combination of synoptic features in and around South Africa contributed to the evolution of the system. This particular MCS is investigated with a non-hydrostatic numerical model (MM5) to help determine which processes were important in its initiation and development, as well as what factors contributed to the associated heavy rainfall. The model is also used to conduct sensitivity tests to determine the role that local features, such as the regional topography and sea surface temperature, played in the evolution of the system. Through the various MM5 simulations, it was evident that the eastern escarpment played a key role in triggering the convective event, while it also had an influence on the low level winds that advected moisture into the region. A sea surface temperature sensitivity simulation highlighted the important role that the Agulhas Current plays in supplying moisture to fuel extreme precipitation events in South Africa. The significance of resolving large-scale features in the mid-latitudes in numerical simulations of weather events in South Africa was identified when excluding these features from the simulation. Through these simulations it was identified that the development of the MCS and the heavy nocturnal precipitation was due to a combination of the continuous moisture supply into the region, a conditionally unstable atmosphere, and uplift due to low level convergence and the local topography.
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    Variability and trends in rainy season characteristics of the Eastern Cape
    (2023) Mahlalela, Precious; Reason, Christopher; Blamey, Ross
    Forming part of south-eastern South Africa, the Eastern Cape province has been prone to extreme weather events such as floods and droughts. The region also displays considerable interannual rainfall variability with a tendency towards prolonged dry periods in recent decades. There is generally a poor understanding of the factors contributing to this rainfall variability. This is concerning considering the recent prolonged (2015 - 2020) drought, that has had major socio-economic effects particularly on the large impoverished rural population as well as on some urban areas where supplied water services have broken down in several cases. Even with some rainfall relief in the province during 2022, water shortages persist, particularly in the largest metropolitan area of Nelson Mandela Bay. The region is influenced by both midlatitude and tropical systems leading to a complex regional meteorology that hitherto has not been much studied compared to other parts of South Africa. Here, variability and trends in rainfall characteristics for the Eastern Cape are examined. Focus is placed on the spring (September-November) and summer (December- February) as these seasons contribute the largest proportion to annual totals. The spring season contributes between about 25-35% of the annual rainfall total, while the summer season contributes about 40-45%. Due to limited available station data, the Climate Hazards Infrared Precipitation with Stations (CHIRPS) data set is used. Comparisons with the available station data, provides confidence in the CHIRPS-derived results. On interannual time scales, the results indicate that dry (wet) springs over the Eastern Cape are associated with a cyclonic (anticyclonic) anomaly southeast of South Africa as part of a shift in the zonal wavenumber 3 pattern in the midlatitudes. Over the landmass, a stronger (weaker) Botswana High is also apparent with increased (decreased) subsidence over and near the Eastern Cape which is less (more) favourable for cloud band development and hence reduced (enhanced) rainfall during dry (wet) springs. The summer season shows significant El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM) influences as well as from the Botswana High. Composites show that dry (wet) summers tend to be associated with a negative (positive) SAM pattern superimposed with a wave number 4 anomaly. According to CHIRPS data, the spring season has shown a significant decreasing trend in total rainfall as well as the number of light, moderate, and heavy rainfall days over most of the province since 1981. The summer signal is less consistent, with a significant increase in rainfall in some inland areas but a decrease in rainfall near the coast which is not found to be statistically significant. The observed summer trends are attributed to an increase in light and moderate rainfall days inland and a decrease in heavy rainfall days near the coast. An examination of the regional rainfall features suggests that the observed decrease in spring rainfall may be related to an observed decrease in the number of cloud bands during the spring while, the observed increase in rainfall inland during the summer might be associated with increased ridging along the south coast. Analysis of mid-century (2040-2060) CMIP5 rainfall projections suggests that there may be a flattening of the annual cycle over the Eastern Cape with the winter becoming wetter and parts of the summer drier. There is a large spread in CMIP5 model projections over the region with the multi-model mean projecting a very slight drying in both seasons. It is suggested that existing climate models may find representing the Eastern Cape region particularly challenging given its sharp gradients in land surface and ocean conditions and its complex meteorology