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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/36598 |
| Date | 04 July 2022 |
| Creators | Morake, Dedricks Monyai |
| Contributors | Reason, Christopher, Blamey, Ross |
| Publisher | Faculty of Science, Department of Oceanography |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, Doctoral, PhD |
| Format | application/pdf |
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