CHAPTER 3 STUDY AREA
3. The Context of the Study Area
3.5. Rainfall and climatic systems
The various planetary and mesoscale atmospheric circulation systems interact in such a way that repeatable weather systems are created. Eleven such weather systems that affect Southern Africa have been identified (Tyson, 1986), of these seven can produce significant quantities of rainfall that can result in flooding. These are Easterly Waves and Tropical/Subtropical Lows, Westerly Waves and Cut-off Lows, Thunderstorms and Tropical Cyclones/depressions. All these systems produce summer rainfall (November to February), however the Cut-off Lows most often associated with flooding (Preston-Whyte & Tyson 1997) predominate from September to May. It must be noted that any one of these systems can overlap on the same day,
making it difficult to distinguish between the system predominantly responsible for the rainfall (Tyson 1986). These weather systems are summarised in Table 3.1.
Table 3.1. Weather systems of Southern Africa (After Tyson 1986). The light grey shading indicates weather systems which may produce flooding. Dark grey shading represents cut-off lows most associated with flooding (Preston-Whyte &
Tyson 1997) Circulation
Direction
Weather System
Description Rainfall Region Period
Easterlies
Waves Travelling cyclonic waves
Heavy rains Eastern portion of Southern Africa
December to January Tropical Lows Associated with the
Inter-Tropic Convergence Zone
Heavy rains that may last a few days
Eastern and central portion and Southern Africa
December to January
Subtropical
Lows Associated with
strong anticyclonic circulation over the continent
Can produce
heavy rainfall Various parts of
Southern Africa September to March
Westerlies
Waves Travelling
anticyclonic waves Can produce
heavy rainfall Southern Coastal
area October to
April but concentrated October to November and January to March Cut-off Lows Intense closed
circulation unstable depression
Heavy rainfall associated with flooding
March to May and
September to November.
Lower frequency December to February Ridging
Anticyclones
Advection of moist air from the Indian Ocean over the land
Widespread general rainfall
Eastern parts of Southern Africa
October to February Southerly
Meridional Flow
Circulation pattern over the southern ocean with a strong pressure gradient from west to east
Light rains Coastal belt September to November
West Coast Troughs
Low pressure system off the western coast of Southern Africa
Widespread general rainfall
Western and Central Southern Africa
December to March Cold Fronts Movement of cold air
from the south in winter producing cold weather
Dry conditions preceding the front with possible serve weather behind the front
Southern Africa April to August
Thunderstorms Thunderstorms Associated with
many weather
systems that will allow of local convection systems
Intense localised rain
Southern Africa November to January
Tropical
Cyclones Tropical Cyclones/
Depressions
Develop in the Indian Ocean and recurve south. May follow Mozambique Channel
Intense
concentrated rain along storm path
Eastern and South-eastern coastal region
December to March
3.5.1. Monthly prevalence of weather systems
Tyson (1986) compiled a series of weather system frequency observations first made by Vowinckel (1956) and Taljaard (1982). These data are reproduced graphically in Figure 3.5.
These data show that the Easterly air flow predominates in the period from October to April mainly in the form of Easterly lows and Easterly waves and lows. Westerly air flow weather systems are less prevalent compared to the Easterly air flow. The Westerly weather systems tend to have higher frequencies from September to November and February to April. Westerly cut-off lows predominate from March to November.
Figure 3.5. Graph of synoptic w eather system frequencies over the eastern part of the Republic of South Africa (After Tyson 1986).
3.5.2. Small to medium scale weather systems
Small to medium scale weather systems can also have a significant impact by producing rainfall that results in flooding. Tropical cyclones and tropical depressions have produced extensive flooding along the eastern portion of Southern Africa. Localised thunderstorm activity can similarly produce copious amounts of rain in a short space of time, often leading to flash floods.
3.5.3. Tropical cyclones and tropical storms
Tropical cyclones form in the Indian Ocean and follow a westerly path which curves south and can affect the Republic of South Africa when they cross over Madagascar or move south down the Mozambique Channel (Fig. 3.6) (Preston-Whyte & Tyson 1997). These develop as weak low pressure cells that develop into well-defined high intensity low pressure cyclonic systems.
Tropical cyclones develop in summer and autumn and can last between two and eight days (Preston-Whyte & Tyson 1997). The high wind speeds and copious rainfall associated with these systems make them very destructive (Preston-Whyte & Tyson 1997). When tropical cyclones encounter a land mass they slow down and degrade from cyclones to tropical depressions. Tropical storms have lower wind speeds but still produce significant volumes of rain.
According to Jury & Pathack (1991), on average 11 tropical cyclones/tropical storms develop during a South West Indian Ocean (SWIO) cyclone season. Only a limited number make landfall where they deflect north or south or continue west (Malherbe et al 2012). The bulk of the tropical cyclones/tropical storms that make landfall do so along central Mozambique and move inland into the Republic of South African Lowveld region. Malherbe et al (2012) documented 43 tropical cyclones/tropical storms that made landfall between 1948 and 2007. In the same period only three tropical cyclones/tropical storms made landfall that affected the KwaZulu-Natal study area (Claude, 1966; Eugenie, 1972 and Domoina, 1984) (Kovács et al.
1985; Preston-Whyte & Tyson 1997; JTWC 2014).
3.5.4. Thunderstorms
Thunderstorms can range from single cells to multi-cells (Preston-Whyte & Tyson 1997). These may take the form of line storms, scattered cells or isolated single cells (Preston-Whyte &
Tyson 1997). These storms are formed by vertical uplift of air resulting from localised surface heating. Multi-cell storms are the result of surface heating and mesoscale air mass circulation (Preston-Whyte & Tyson 1997).
Single cell storms usually cover a small geographic area (5 – 10 km) and last less than an hour (Preston-Whyte & Tyson 1997). Apart from localised surface heating, daytime upslope winds also produce horizontal convergence and vertical uplift (orographically driven). As the warm air rises it expands and starts to cool and water vapour condenses (Preston-Whyte & Tyson 1997).
Precipitation particles grow while being supported by the updraft. When the updraft cannot support the weight of the particles and precipitation occurs. The falling particles force a downdraft to develop creating cooling and unstable conditions and increasing wind speeds
Figure 3.6. Tropical c yclone tracks in the SWIO (1982 – 2013). Coloured lines represent different annual tracks. Data from the Joint Typhoon Warning Centre
(JTWC 2014).
(Preston-Whyte & Tyson 1997). Rainfall can exceed 100 mm/hour during the storm (Preston- Whyte & Tyson 1997).
Multi-cell thunderstorms can be 30 - 50 km in extent and develop as single cell storms merge or develop as a grouping of cells within a storm (Preston-Whyte & Tyson 1997). The multi-cell storm systems can last many hours and can similarly produce rainfall in excess of 100 mm/hour (Preston-Whyte & Tyson 1997). Thunderstorms (either single or multi-celled) can result in flash flooding and thus have a large potential impact over a small geographic area (Foody et al. 2004;
Carpenter & Georgakakos 2006; Ntelekos et al. 2006; Kobiyama & Goerl 2007; Kim & Choi 2011).