A synoptic climatology of Continental Tropical Low pressure systems over southern Africa and their contribution to rainfall over South Africa

Abstract

Tropical weather systems are known to cause havoc due to heavy rain which leads to flooding. Continental Tropical Low pressures (CTLs) are one of such systems. Very little research has been dedicated to tropical weather over southern Africa, with even less literature available on CTLs. As a result, there is little understanding of these weather systems and the potential danger associated with them. Using NCEP reanalysis data, an objective identification method which consists of four criteria is created to recognize CTLs by detecting atmospheric conditions that are conducive to the development of CTLs. These conditions include, the presence of an upright standing low pressure with a high pressure aloft, warm column temperatures, high precipitable water and high total energy values. These results are used to create a climatology of CTLs over southern Africa where it is found that CTLs occur most frequently over southern Zambia and Angola. CTLs do not often reach the central interior of South Africa, with a return period of 78 years, however, they occur more frequently over the northern parts of Limpopo province with a return period of 5 years. It is uncommon for CTLs to extend south of 22.5°S (South Africa) during December months, however by March there is a westward shift in the preferred CTL location. January has the highest number of CTL events (36%), followed very closely by February (34%). The average lifespan of a CTL is up to three days, but reaching 13 to 16 days on rare occasions. The rainfall contribution of CTLs to South Africa is calculated using daily observed rainfall station data from the South African Weather Service. It is found that the average rainfall associated with a CTL is far higher than the total rainfall of the region. CTLs are associated with extreme rainfall events and it is seen that there is an average of 110 occurrences per year where at least 50 mm is measured in a 24 hour period and 29 per year where at least 100 mm is recorded. The general rainfall distribution around a CTL largely occurs to the east of it, while the extreme rainfall amounts are found to the south of the CTL. It is also established that topography plays an important role in the rainfall distribution with higher rainfall amounts occurring along the escarpment. A case study is used to demonstrate the objective identification method presented in this dissertation. The CTL was accurately identified, however the NCEP data slightly misplaced the position of the low pressure when compared to the satellite image. The case study highlighted that due to the slow movement, broad extent of the rain bands and the possible long lifespan, CTLs can result in widespread impacts. It also demonstrated that using the criteria developed in this dissertation, a forecaster can accurately identify a CTL.