South African Plateau Research Articles

Environmental conditions affecting air pollution dispersion over Richards Bay, South Africa

Air pollution dispersion over Richards Bay, South Africa is studied using satellite, reanalysis and in-situ measurements. Near-surface concentrations of trace gases and particulates reach high concentrations under dry stable ‘berg’ winds. Temporal records of CO, NO2, O3, PM2.5, SO2 (2000-2023) exhibit no trend and short-term dose exceedances of individual pollutants are rare and confined to winter mornings. Regional point-to-field correlation maps of a Richards Bay winter air pollution index found that anomalous warming of the west Indian Ocean accelerates the jet stream over Madagascar and pulls smoke- and dust-laden north-westerly winds across the South African plateau, implicating long-range transport. At daily and hourly time scales dewpoint temperature and boundary layer height best indicate changes in local air pollution dispersion that impact respiratory health. A case study of 15-17 June 2021 revealed a 10ºC thermal inversion caused by negative sensible heat fluxes of -40 W/m2 through the night, leading to SO2 concentrations above 100 ppb in the morning along the main access road (28.8ºS, 32ºE). Poor dispersion seldom happens in the summer when turbulent mixing and rainfall cleanse the air.

Late Cretaceous erosion and chemical weathering record in the offshore Cape Basin: Source-to-sink system from Hf[sbnd]Nd isotopes and clay mineralogy

Source-to-sink research has often used sediment fluxes as a fundamental parameter when seeking to understand the perturbations caused by tectonics or climate. This parameter is often interpreted only in terms of erosion rates, dismissing the component exerted by chemical weathering. In this study, we characterize sediments from the Deep Surface Drilling Program core 361 (DSDP 361) located in the Cape basin to show the evolution of both, physical erosion and silicate chemical weathering, during the late Cretaceous (100.5–66 Ma) in the source-to-sink system consisting of the South African plateau and the Cape Basin. Transmission Electron Microscopy (TEM) images and trace element analyses indicate that detrital clays predominate within the site and thus reflect variations in surface continental processes. Clay mineralogy shows a predominance of detrital smectite (60–98%) throughout the section. However, an increase in primary clay minerals, in particular illite (20%), indicates a relative enhancement in physical erosion, while an increase in palygorskite (10%) indicates regional climate aridification during the Campanian–Danian interval (ca. 77–62 Ma). Concomitant to this enhancement, we observe both, more radiogenic Hf isotopic compositions and less radiogenic Nd isotopic compositions in the clay fractions (< 2 μm). These variations can be respectively associated to an increase in silicate chemical weathering and the incorporation of unradiogenic material, likely related to the orogenic belts present on the western-south margin of Southern Africa. The regional aridification and the global climate cooling trend observed on δ18O benthic foraminifera records do not appear as suitable mechanisms for the increase in physical erosion and silicate chemical weathering observed. Our results, together with the reported increase in sedimentation rates and the reported tectonic uplift between 80 and 70 Ma, suggest that it is tectonic activity and not climate the main driver in the enhancement of denudation processes. Furthermore, our data suggests that such uplift episode could have impacted the regional climate by causing a rain-shadow effect, with an inland aridification suggested by the increase in palygorskite proportions. The increase in silicate chemical weathering suggested by our new dataset begins in the Campanian (ca. 77 Ma), later than the onset of the global climatic cooling trend that occurs from the Turonian onward. If enhanced weathering of the South African Plateau may not have triggered the recorded climatic cooling, it may still have contributed to maintain cooler conditions in the Campanian-Danian interval.

Highly Contrasted Geochemical Pattern in Sediments of the Okavango Delta, Botswana Driven by Dust Supply, Hydrological Heritage and Biogeochemical Reactions

AbstractThe Okavango Delta in North Botswana is a hot‐spot of biodiversity within the semi‐arid central part of the South African plateau. This endorheic ecosystem is highly dependent on the annual flood that brings freshwater from the highlands of Angola to the North. However, in many places, the groundwater of the Delta is saline and contains very high concentrations of metal and metalloid elements, making it toxic to the flora and fauna. These saline waters have been largely studied and their formation is generally explained through evaporation and evapotranspiration processes. However, no studies have investigated the geochemical composition of the sediments that host the aquifers. Here, we provide a complete sedimentological and geochemical investigation (major, traces and rare earth elements, carbon and oxygen stable isotopes ratios, Sr and Nd isotope ratios) of the various geomorphological and ecological compartments that form the landscape of the SW Okavango Delta. We demonstrate that the non‐connected underground aquifer is capped by a clay layer corresponding to a major event of dust deposition from the Makgadikgadi pan. We suggest that this aquifer may extend outside the island, below the floodplain and that the hydrological structure of the Nxaraga area is indeed composed of two non‐connected aquifers of different origins. Challenging the model based on evaporation and evapotranspiration, we propose that the composition of that allochtonous clay and the in situ reactions between sediment, water and organic matter in a confined aquifer explain the geochemical enrichment of the water.

The Zambezi deep-sea fan: mineralogical, REE, Zr/Hf, Nd-isotope, and zircon-age variability in feldspar-rich passive-margin turbidites

ABSTRACT We here present the first comprehensive provenance study of the Zambezi deep-sea fan, based on integrated petrographic, heavy-mineral, elemental-geochemistry, isotope-geochemistry, and detrital-zircon-geochronology analyses of middle Pleistocene to Holocene turbidites. The Zambezi Valley and Fan represent the submarine part of an ∼ 5000-km-long sediment-routing system, extending from the heart of the South African Plateau to the abyssal depths of the Indian Ocean. Sediment is derived not only from the African side, but also from Madagascar Island mostly via the Tsiribihina Valley. Being shed by two dissected rifted margins, detritus supplied from opposite sides of the Mozambique Channel shares similar feldspar-rich feldspatho-quartzose composition, although with significant differences in heavy-mineral and geochemical signatures. The εNd values of Madagascar sand are markedly more negative and TNd model ages notably older. Zircon grains yield mostly Irumide (late Stenian) U-Pb ages in African-derived sand and mostly Pan-African (Ediacaran–Cryogenian) U-Pb ages in Madagascar-derived sand, which also yields a few grains as old as Paleoarchean and many discordant ages reflecting Pan-African reworking of Archean cratonic rocks. Lower Valley and Lower Fan deposits have intermediate fingerprints, indicating that sediment supply from Madagascar is not much less than from Africa despite a much smaller catchment area, which can be explained by deposition of a conspicuous part of Africa-derived sediment in the Intermediate Basin confined between the Zambezi Shelf, the Beira High, and the Îles Éparses. By assuming that compositional differences between Quaternary submarine deposits and modern Zambezi River sands primarily resulted from sediment impoundment by large dams, we could evaluate the anthropogenic impact on natural sediment fluxes. Quaternary turbidites are somewhat higher in quartz and poorer in heavy minerals with higher relative amounts of durable ZTR species, and yield more Ediacaran, Neoarchean, and Carboniferous detrital-zircon ages than modern river sands. The Orosirian peak characterizing the Intermediate Basin sample points to prominent supply from the middle and upper parts of the Zambezi catchment in the middle Pleistocene. Rough calculations suggest that pre-dam Zambezi sediments were generated ≤ 10% in the upper catchment, ∼ 60% in the middle catchment, and only ≥ 30% in the lower catchment that provides the totality of sediment reaching the Indian Ocean today.

Solid sedimentation rates history of the Southern African continental margins: Implications for the uplift history of the South African Plateau

AbstractThe Cretaceous and Cenozoic fill of the continental margins of southern Africa (South‐East Atlantic and Agulhas Margins) contains a continuous record of sediment supplied from the South African Plateau (SAP) for the past 134 million years. Estimates of solid sediment volumes deposited offshore were calculated from isopach maps and extrapolated vertical cross‐sections derived from a large amount of industrial geophysical data. Solid phase volumes and accumulation rates were calculated for six epochs: Lower Cretaceous (134–113 Ma), Mid Cretaceous (113–93.5 Ma), Upper Cretaceous (93.5–81 and 81–66 Ma), Palaeogene (66–25 Ma), Neogene (25–0 Ma). Our new compilation demonstrates the existence of two periods of elevated flux. The most important one occurs in the late Cretaceous (93.5–66 Ma) and was synchronous with an acceleration of onshore denudation as shown by thermochronometric data. After a period of extremely low accumulation rate, the second phase of elevated flux started in the Oligocene (~30–25 Ma) until present‐day. From these observations we suggest that the main phase of uplift of the SAP took place during the Upper Cretaceous. Two mechanisms, namely uplift caused by lithospheric delamination or by dynamic topography caused by the continent moving over the African Superplume, are viable explanations for our observations. The more recent and lower amplitude episode of enhanced accumulation rates is likely to correspond to a second period of uplift, potentially associated with the onset of uplift and extension along the East African Rift System.

Cenozoic deformation of the South African plateau, Namibia: Insights from planation surfaces

Passive margins and associated inland areas display complex vertical displacements that have been abundantly described offshore but much less so onshore due to erosion. Planation surfaces are ubiquitous markers inland that can be used as a proxy both climatic conditions and changes in vertical displacements induced by both short (faults) and long (tilting, flexure) wavelength deformations. We propose 1) a synthetic typology of the planation surfaces driven by a genetic process (weathering versus mechanical erosion), and 2) a method to use these surfaces to map short and long wavelength deformations. This methodology was applied to the Namibian margin and inland plateau to quantify the Cenozoic deformation. The results show that the Namibian margin was affected by bulging during the Oligocene and by an E-W to NW-SE extension during the Late Miocene and Pliocene. The bulge is parallel to the shoreline with a wavelength of 300km and an amplitude up to 500m. After investigating the available deformation processes, we propose that an increase in the spreading rate along the mid-oceanic ridge during the Oligocene generated this bulging. The vertical displacement is partially maintained afterwards via isostasy because of mass loss generated by scarp retreat. A minimum average rate of scarp retreat of 5km/Myr was calculated, which is high compared to the rates estimated in other places in the world. We ascribe this high value to the prior intense weathering period. Indeed, the alteration largely degraded the bedrock and facilitated the formation of the subsequent scarp. This study also reveals that the high reliefs of the Damara domain existed before the Cenozoic, induced by a reactivation of the Damara structures.

Post-rift stratigraphic evolution of the Atlantic margin of Namibia and South Africa: Implications for the vertical movements of the margin and the uplift history of the South African Plateau

Abstract The aim of this study is to constrain the post-rift deformations of the Atlantic passive margin of Namibia and South Africa using an extensive industrial 2D reflection seismic dataset calibrated by wells and onshore outcrops that have been revaluated in age (biostratigraphy) in order to discuss the evolution of the South African Plateau uplift. The first-order evolution of the margin is tectonically driven and can be divided into three principal phases. The first (131–93.5 Ma) comprises an overall retrogradational trend that results from a rate of accommodation, created by the thermal flexure of the margin, that is higher than the sediment supply. The second (93.5–66 Ma) comprises an overall aggradational-progradational trend that results from a relative increase in sediment supply due to an uplift and subsequent erosion of the margin and the inland domain. The third (66–0 Ma) is retrogradational again in response to a decrease in sediment supply induced by a relative stability of the margin and climate conditions. This study demonstrates that the present-day configuration of the margin is acquired during the Late Cretaceous (from 81 to 66 Ma) with a strike, long-wavelength and seaward tilt of the margin which could be related to the growth of the onshore main escarpment. We also characterize a regional Oligocene unconformity marked by a significant downward shift of the shoreline which suggests a moderate uplift of the inland domain and could be associated with a reactivation of the relic late Cretaceous relief.

Uplift history of a transform continental margin revealed by the stratigraphic record: The case of the Agulhas transform margin along the Southern African Plateau

The south and southeast coast of southern Africa (from 28°S to 33°S) forms a high-elevated transform passive margin bounded to the east by the Agulhas-Falkland Fracture Zone (AFFZ). We analysed the stratigraphic record of the Outeniqua and Durban (Thekwini) Basins, located on the African side of the AFFZ, to determine the evolution of these margins from the rifting stage to present-day. The goal was to reconstruct the strike-slip evolution of the Agulhas Margin and the uplift of the inland high-elevation South African Plateau. The Agulhas transform passive margin results from four successive stages:(1)Rifting stage, from Late Triassic to Early Cretaceous (~200?–134 Ma), punctuated by three successive rifting episodes related to the Gondwana breakup;(2)Wrench stage (134–131 Ma), evidenced by strike- and dip-slip deformations increasing toward the AFFZ;(3)Active transform margin stage (131–92 Ma), during which the Falkland/Malvinas Plateau drifts away along the AFFZ, with an uplift of the northeastern part of the Outeniqua Basin progressively migrating toward the west;(4)Thermal subsidence stage (92–0 Ma), marked by a major change in the configuration of the margin (onset of the shelf-break passive margin morphology).Two main periods of uplift were documented during the thermal subsidence stage of the Agulhas Margin: (1) a 92 Ma short-lived margin-scale uplift, followed by a second one at 76 Ma located along the Outeniqua Basin and; (2) a long-lasting uplift from 40 to 15 Ma limited to the Durban (Thekwini) Basin. This suggests that the South African Plateau is an old Upper Cretaceous relief (90–70 Ma) reactivated during Late Eocene to Early Miocene times (40–15 Ma).

Contrasting Mesozoic evolution across the boundary between on and off craton regions of the South African plateau inferred from apatite fission track and (U‐Th‐Sm)/He thermochronology

AbstractThe timing and mechanisms involved in creating the elevated, low‐relief topography of the South African plateau remain unresolved. Here we constrain the thermal history of the Southwest African plateau since 300 Ma by using apatite fission track (AFT) and (U‐Th‐Sm)/He (AHe) thermochronology. Archean rocks from the center of the Kaapvaal Craton yield AFT ages of 331.0 ± 11.0 and 379.0 ± 23.0 Ma and mean track lengths (MTLs) of 11.9 ± 0.2 and 12.5 ± 0.3 µm. Toward the southwest margin of the craton and in the adjacent Paleozoic mobile belt, AFT ages are significantly younger and range from 58.9 ± 5.9 to 128.7 ± 6.3 Ma and have longer MTLs (&gt;13 µm). The range of sample AHe ages complements the AFT ages, and single‐grain AHe ages for most samples are highly dispersed. Results from joint inverse modeling of these data reveal that the center of the craton has resided at near‐surface temperatures (&lt;60°C) since 300 Ma, whereas the margins of the craton and the off‐craton mobile belt experienced two discrete episodes of cooling during the Cretaceous. An Early Cretaceous cooling episode is ascribed to regional denudation following continental breakup. Late Cretaceous cooling occurs regionally but is locally variable and may be a result of a complex interaction between mantle‐driven uplift and the tectonic setting of the craton margin. Across the entire plateau, samples are predicted to have remained at near‐surface temperatures throughout the Cenozoic, suggesting minimal denudation (&lt;1 km) and relative tectonic stability of the plateau.

Morphology and long-term landscape evolution of the South African plateau in South Namibia

The South Namibian Plateau, which belongs to the South African Plateau, is situated between 25°‐29°S and 15°‐20°E. This study examines the relationships between this portion of the plateau and the development of planation surfaces and scarps bounding the plateau. Geomorphological analysis through morphometric indices extracted from a DEM (90m) and its derived maps as well as field observations supported by the literature data permit the demonstration that tectonics, climate and base level are the main factors that control the formation and evolution of the planation surfaces and scarps that separate them. From west to east, the South Namibian Plateau consists of four main morphological provinces: the coastal province and the intermediate, upper and Kalahari provinces in the interior plateau. These provinces include five Cenozoic planation surfaces that are differentiated by their elevations, regional slopes, relative reliefs and erosion processes. These surfaces have been formed by etchplanation and pediplanation. The drainage pattern and river length profiles show that the coastal province evolved differently than the interior plateau. The development of the coastal province is controlled by eustasy and tectonics, whereas climate-related processes and tectonics control that of the interior plateau. The presence of several scarps separating these surfaces, the most degraded and older of which are inside the plateau, reflects a polyphase evolution. This evolution can be highlighted by two models: (1) an eastward migration mantle dynamics causing a slight uplift and rejuvenation of the system by destabilizing the coupled weathering/erosion equilibrium and (2) an internal propagation of isostatic rebound induced by the retreat of the western scarp leading to a significant flattening in the central part of the plateau.

Cretaceous–Cenozoic sedimentary budgets of the Southern Mozambique Basin: Implications for uplift history of the South African Plateau

In this study, data from 41 wells were used to quantify the evolution of the sedimentary budget in the Southern Mozambique passive margin basin, with a high temporal resolution for the Cenozoic period. We found that the drainage areas, which supplied sediments to the Southern Mozambique Basin, were eroded in two episodes. The first, of Mid–Late Cretaceous in age, is concordant with both thermochronological datation and sedimentary fluxes estimated by other studies in the Namibian and South African and Northern Mozambique margins. This erosion episode ended when the African surface, as defined by Burke and Gunnel (2008), had become flat and low-lying over most of the South African Plateau by ∼65Ma. Carbonate sediment deposition became more important in the shallow waters of the Mozambique basin after that time. The second erosion episode began at ∼23Ma and is likely due to an uplift event of the North-eastern part of the South African Plateau. It seems that the Limpopo catchment and the whole area sourcing the studied basin have inherited their present relief from two epeirogenic uplift pulses of Late Cretaceous and Miocene ages.

Post-breakup evolution and palaeotopography of the North Namibian Margin during the Meso-Cenozoic

Passive margins display various structural and morphological patterns resulting in a complex coupling between local effects. We performed an integrated analysis from the ocean to the continent at the crustal scale of the post-breakup evolution of the northern Namibian margin in terms of vertical displacements and palaeotopography. A main topographic scarp that disappears where the Palaeo-Proterozoic Damara belt crosses the coast separates the Namibian margin and the South African plateau. We analysed this transition using a synthetic crustal cross-section based on a compilation of published data as well as new field data.The thinned margin displays two domains separated by a deep sedimentary trough. This trough separates a proximal part with landward tilted blocks from the distal part with seaward tilted blocks. This structural pattern reveals a complex flowing pattern of the ductile crust controlled by magmatism. Onshore, the basalts and Cenozoic conglomerates recorded a progressive seaward tilting of the coastal zone.Post-breakup vertical motions were estimated with subsidence curves in the offshore domain and from isostatic adjustments induced by the variations of cover thickness (depth of magmatic intrusions and Apatite Fission Tracks) in onshore domain. Three domains are recognised: a subsiding offshore domain, a uplifting continental domain and a transitional domain, located on both sides of the current shoreline with successive phases of uplift and subsidence. The offshore subsidence displays variations along the margin controlled by the thinning crust generated by the continental breakup. The onshore palaeotopogaphy is approximately assessed since the end of rifting by integrating the isostatic adjustment induced by denudation: the mean elevation of the plateau has remained above 1000m since continental breakup. No significant additional uplift was pointed out during the late Cretaceous or late Neogene. Therefore, we conclude that the South African plateau has been present in northern Namibia since 130Ma, at least.

The Fish River canyon (Southern Namibia): A record of Cenozoic mantle dynamics?

Intracontinental elevated plateaus remain geomorphologic features which are poorly studied. Their genesis requires a coupling between climate and deformation. The Fish River canyon (southern Namibia), the second largest canyon of the Earth, carved the South-African plateau on 550 m along 65 km. This study reveals that the upper and the lower segments are shaped by NE–SW and north-south grabens, respectively. These deformations increased the meandering and the vertical incision mainly in the lower canyon. However the river main trend was not drastically modified attesting that the river was ancient and that the rate of the vertical displacement was slow compared to the erosion rate. The main incision occurred during a NW–SE stretching of Plio-Pleistocene. These stretching episodes belong to two deformation phases previously poorly described in the South-African plateau. These widespread stretching phases are interpreted as a result of deep mantle dynamics affecting the inner African continent.

Sediment supply to the Orange sedimentary system over the last 150 My: An evaluation from sedimentation/denudation balance

The South African plateau is bordered by passive margin basins preserving the terrigeneous sediment produced during onshore erosion. As such, these basins potentially provide a record of the variation in onshore elevation and relief over time. Here we bring new constraints on the uplift and erosion of the Southern African plateau over the last 150 Ma from the perspective of the stratigraphic architecture of these basins. We review published data to quantify the terrigeneous supply eroded in the drainage area and preserved in the basins. The novel aspect of our approach is the integration of the evolution of the whole domain in sedimentation (i.e. not only the platform) as well as the onshore eroding region. Along the South African and Namibian Atlantic margins, we determined the long-term signal of sedimentary supply (at the 10 Ma scale) from 3D mass balance calculations comparing sedimentary volumes deduced from offshore isopach maps (with a particular attention to associated uncertainties) on one hand and denudation volumes deduced from thermochronology data on the other. We show that, in this case, the volumes of denudation and of sediments preserved in the basin are consistent both in total and incremental amount. The latter is therefore a reliable estimation of the denudation in the associated continental domain, subject to certain assumptions such as the drainage area over time. The long-term signal of sedimentary supply of the South African and Namibian Atlantic margins shows that the major part of the denudation occurred during the post-rift Lower Cretaceous and the Upper Cretaceous and subsequently decreased. This is not the expected behavior for the simple relaxation of topography acquired during rifting that would show a progressive decrease of the sediment supply over time. This suggests a significant rejuvenation of relief during the Upper Cretaceous, 50 Ma after the rifting event. If erosion is directly linked to elevation or relief variation, this implies 2 generations, the first before break-up (i.e. before 130 Ma; presumably during rifting) and the second during the Upper Cretaceous.

Evidence for large-scale transport of biomass burning aerosols from sunphotometry at a remote South African site

We present the results of sunphotometry measurements at De Aar, a remote site on the central South African plateau, during and after the intensive dry season field campaign of SAFARI 2000. We determine a 6-month-long time series of aerosol optical depths over the site. Twelve haze events are identified, for which we derive Angström exponents and their derivatives, and, through cross-plots of these parameters, typical aerosol sizes and levels of hydration. These results, in conjunction with meteorological data and air trajectory calculations, show biomass burning to be the main aerosol generating source for 8 of the 12 events, and responsible for the 5 cases with the highest turbidity. While the bulk of the biomass emission is clearly of African origin, we identify several possible South Atlantic crossings of aged smoke from fires in the Amazon basin. We define the southern edge of the main aerosol transport route over southern Africa during the austral winter. We estimate that, for the half-year investigated, 84% of the losses of visible solar irradiation over our experimental location are caused by biomass burning haze, and conclude that these types of aerosols have the most critical impact on solar irradiation and atmospheric albedo over the entire southern Africa.

The Zonal Asymmetry of the Southern Hemisphere Winter Storm Track

Abstract Atmospheric general circulation model experiments have been performed to investigate how the significant zonal asymmetry in the Southern Hemisphere (SH) winter storm track is forced by sea surface temperature (SST) and orography. An experiment with zonally symmetric tropical SSTs expands the SH upper-tropospheric storm track poleward and eastward and destroys its spiral structure. Diagnosis suggests that these aspects of the observed storm track result from Rossby wave propagation from a wave source in the Indian Ocean region associated with the monsoon there. The lower-tropospheric storm track is not sensitive to this forcing. However, an experiment with zonally symmetric midlatitude SSTs exhibits a marked reduction in the magnitude of the maximum intensity of the lower-tropospheric storm track associated with reduced SST gradients in the western Indian Ocean. Experiments without the elevation of the South African Plateau or the Andes show reductions in the intensity of the major storm track downstream of them due to reduced cyclogenesis associated with the topography. These results suggest that the zonal asymmetry of the SH winter storm track is mainly established by stationary waves excited by zonal asymmetry in tropical SST in the upper troposphere and by local SST gradients in the lower troposphere, and that it is modified through cyclogenesis associated with the topography of South Africa and South America.

Intercontinental transport of nitrogen oxide pollution plumes

Abstract. We describe the first satellite observation of intercontinental transport of nitrogen oxides emitted by power plants, verified by simulations with a particle tracer model. The analysis of such episodes shows that anthropogenic NOx plumes may influence the atmospheric chemistry thousands of kilometers away from its origin, as well as the ocean they traverse due to nitrogen fertilization. This kind of monitoring became possible by applying an improved algorithm to extract the tropospheric fraction of NO2 from the spectral data coming from the GOME instrument. As an example we show the observation of NO2 in the time period 4--14 May, 1998, from the South African Plateau to Australia which was possible due to favourable weather conditions during that time period which availed the satellite measurement. This episode was also simulated with the Lagrangian particle dispersion model FLEXPART which uses NOx emissions taken from an inventory for industrial emissions in South Africa and is driven with analyses from the European Centre for Medium-Range Weather Forecasts. Additionally lightning emissions were taken into account by utilizing Lightning Imaging Sensor data. Lightning was found to contribute probably not more than 25% of the resulting concentrations. Both, the measured and simulated emission plume show matching patterns while traversing the Indian Ocean to Australia and show great resemblance to the aerosol and CO2 transport observed by Piketh et al. (2000).

Variability of Hailstorms on the South African Plateau

Detailed climatological studies of hailstorms on the South African plateau have been made using a dense network of voluntary observers. Hailstone structures have been investigated and, since 1971, radar observations have been used to study storm characteristics. Results are given for the frequency of hailstorm occurrence, characteristics of hailfalls at a point, hail paths, types of storms, radar characteristics and hailstone trajectories. Practically all features showed great variability, the magnitude of which is given whenever possible. Application of the results to the design of a hail suppression experiment is briefly discussed.

A pre-frontal squall line on 14 November 1975

The origin and life cycle of a squall line observed on the South African plateau are reviewed. The squall line was associated with a cold front whereas most squall lines in the subtropical part of the continent are not. It moved to the north-east and to the left of the upper winds at a speed of 35 km/h, which was considerably faster than that of the front. The squall line was at least 400 km long, with an unbroken central region of 110 to 130 km, on both sides of which were well-separated storms. In the area of Johannesburg and Pretoria, small patches of hail were produced by various cells. Radar echoes above 8 km AGL1 were rare. One cell of the unbroken portion while passing over Pretoria intensified rapidly, extended in all three dimensions (echo top up to 11 km AGL) and produced hailstones of up to 3 cm in diameter. Like its neighbouring cells, it moved exceptionally fast and almost parallel to the long axis of the squall line (eastewards at 65 km/h), following the winds between 600 and 500 mb. The echo core was tilted significantly towards east-south-east as a result of the strong wind shear. In the region of this overhang hailstones reached the ground before the onset of rain.

Description of the Unusual Behavior of a Prefrontal Squall Line in South Africa

Abstract Unusual features of a prefrontal squall line on 14 November 1975 observed by radar on the South African plateau are described. Some cells moved almost along the line at speeds of up to 65 km h−1.