Rainfall climatology over the Gauteng Province in South Africa

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Synoptic circulation patterns associated with heavy rainfall over Gauteng

Preface

This Chapter describes the synoptic circulation associated with heavy rainfall over Gauteng. A monthly synoptic climatology is provided first and is then compared to the monthly composite fields complied from days with heavy rainfall and days which remain dry. In Chapter 2 the Gauteng daily rainfall were calculated for a 35-year period and heavy rainfall and dry days were identified and are used here to construct the average monthly maps. Lastly horizontal wind divergence and relative vorticity advection is calculated over Gauteng and vertical profiles of these variables are discussed on heavy rainfall and dry days. In Chapter 4 the thermodynamical profile of the atmosphere is investigated during heavy rainfall at a single location (Irene) but understanding the properties of the larger environment in which heavy rainfall develops is explored in Chapter 3.

Background

Taljaard (1996) provides a thorough description of the character of rainfall in South Africa. Roughly 85 % of SA receives more than half of its rainfall in summer (October-March). The 50 mm isohyet advances westward from KwaZulu-Natal in August to the Northern Cape Province in March. Gauteng falls within the summer rainfall area and on average receive more than 50 mm of rain in all summer months. Taljaard (1996) stressed the importance of abundant moisture in the production of rainfall and explained how in summer months at surface levels moisture is advected into the interior of South Africa by a high pressure system centred close to Maputo. The Indian Ocean is the primary source of moisture in summer but the southward penetration of air from the ITCZ also plays a significant role in the production of rainfall over Gauteng. For rainfall and indeed heavy rainfall to occur over the interior of South Africa humid tropical maritime air should invade the subcontinent from the east. However, this penetration is inhibited by the semi-permanent anti-cyclone from 700- 200 hPa located over the subcontinent. The humid air is injected into the interior when ridging anticyclones reach the east coast or as the continental high moves, weakens or strengthens. The most important weather systems producing rainfall in summer in South Africa were identified by Taljaard (1996) and Tyson and Preston-White (2000) as

  • The westerly trough (or TTT)
  • The cold front trough
  • The southward extending tropical low or V-shaped troughThe Cut-off low (COL)
  • Tropical cyclones (TC)
  • The ridging high
  • The blocking high or long wave ridge

Not all of these weather systems cause heavy rainfall over South-Africa and only those weather systems that have been shown to be associated with heavy rainfall in summer will be expanded on here. The TTT is a wave in the west-wind regime which connects tropical convective systems to mid-latitude weather systems and is clearly visible as elongated cloud bands on satellite imagery which extends southeastward (Hart et al., 2010). The TTT is part of an important process which causes humid tropical air to flow from the ITCZ into South Africa. Moist tropical air has a large potential for instability as indicated by Convective Available Potential Energy (CAPE) CAPE is dealt with in detail in Chapter 4.3.5. Williams and Reno (1993) calculated CAPE values to be in the order of 2000 – 5000 Jkg-1 over much of the tropics. The TTT therefore enhances the potential for convective development over South Africa by causing the inflow of air with greater potential for convective instability. The TTT may cause heavy rainfall over the interior of South Africa as was illustrated by Hart et al. (2010). They stressed the importance of local convection in the production of the heavy rainfall. Furthermore De Coning et al. (1998) identified that a TTT was partially responsible for the very heavy rainfall over the central interior of South Africa during February 1996.
Taljaards’ (1996) southward extending tropical low or V-shaped trough is also referred to as a continental tropical low pressure system (Dyson and van Heerden, 2002). This low pressure system is characterised by a low pressure system which stands upright form the surface to 400 hPa but is replaced by a high pressure system in the upper troposphere. The low pressure system is warm cored from 500 hPa upwards but with relatively cool temperatures in the surface to 700 hPa layer. The surface dew point temperatures associated with these systems are in the order of 18-20 °C with precipitable water values of 20 mm. Some noteworthy and very heavy rainfall events were caused by these tropical weather systems for example the Free State floods of February 1988 (Triegaardt et al., 1991) as well as the heavy rainfall and floods over northeastern South-Africa in February 2000 (Dyson and Van Heerden, 2001). Part of the reason for the flooding which may be associated with these weather systems is that they move very slowly- causing heavy rainfall over the same area on consecutive days. In February 1988 heavy rainfall occurred over the central Free State for 3 consecutive days (19-22 February). The convective cells which develop in association with tropical low pressure systems are semi-stationary and associated with little or no vertical wind shear.
Cut-off lows (COLs) are truly significant weather systems of the extra-tropics and are responsible for wide spread rainfall over South Africa and 20 % of COLs produce heavy rainfall which may produce floods (Taljaard, 1985). Most COLs have life-spans of longer than 2 days but may influence the rainfall over South Africa for up to 5 days (Singleton and Reason, 2007a). All together COLs influence the weather over South Africa about 40 days per annum. COLs occur most frequently in March/April and September/October months when approximately 2 systems occur per month. They are least frequent in January and July when on average only 1 system occur per month. These systems are recognisable on synoptic maps by the following:
• Low pressure in the middle troposphere is cut-off from the general westerly circulation and lies     equator ward of westerly flow
• Closed cyclonic circulation throughout the troposphere
• Cold cored in the middle troposphere.
These weather systems are clearly baroclinic in nature and heavy rainfall from COLs occur when they are in a development stage. The atmosphere is baroclinic when the atmospheric density is a function of both temperature and pressure and a thermal wind therefore exists (Holton, 1992). A thermal wind indicates vertical wind shear and a COL will be classed as developing when the low in the middle and upper troposphere lies westward of the surface low. Under these conditions vertical wind shear will be present in the atmosphere which should be visible on upper air sounding data over the summer rainfall area (such as at Irene). Singleton and Reason (2007b) described the heavy rainfall associated with a COL over South Africa during March 2003. They concentrate their study on the south coast and adjacent interior but Gauteng also received heavy rainfall during this period (on the 23rd of March 2003 the average rainfall over Gauteng was 11 mm).The COL was lying westward with height on the 23rd when the highest rainfall total occurred over Gauteng for the days under investigation in this study. On the 24th there was a decrease in the vertical wind shear over Gauteng and the average rainfall over Gauteng was only 1 mm. There are on average 11 tropical depressions over the South West Indian Ocean per year (Jury and Pathack, 1991) but these systems do not make landfall over the southern African subcontinent every year (Malherbe et al., 2012). The normal displacement of tropical cyclones (TC) is to the southwest, south and eventually the southeast when they reach 25° S (Taljaard, 1996). When a system makes landfall it may either move further westward into the interior of southern Africa or it may deflected to the south or north. The influence of tropical cyclones on the rainfall over the eastern interior can be quite contrasting depending on the location of the TC. If centred at 500 km or more from the east coast subsidence results over the eastern parts of the subcontinent with a deep southerly to southeasterly flow. However, when these systems make landfall torrential rain may fall especially from the coastal belt up to the escarpment. TC Domonia made landfall close to Maputo on 29 January 1984 resulting in 24-h rainfall totals of more than 200 mm over the eastern escarpment (Poolman and Terblanche, 1984). But there was no rainfall over the Gauteng during this event. On the 17th of January 2012 TC Dando made landfall south of Maputo and moved in a northwesterly direction over the subcontinent while weakening (Meteo France, 2012). Daily rainfall in excess of 200 mm occurred over the escarpment of Mpumalanga on the 17th of 18th of January 2012. Gauteng also received no rainfall during this event. Conversely TC Eline made landfall north of Beira, on the Mozambique coast, on 22 February 2000 and tracked slowly westward over Zimbabwe and Botswana during the next few days. More than 100 mm of rain occurred at some stations over Gauteng on the 22nd and about 50 mm on the 23rd and 24th. It is clear that the position of the TC plays a governing role on the amount of rainfall over Gauteng.
The ridging high and blocking high pressure systems contribute to the development of rainfall over the summer rainfall area of South Africa by aiding in the advection of moisture from the Indian Ocean to interior plateau of South Africa. They are not directly responsible for heavy rainfall although if the blocking high occurs in conjunction with a COL, heavy rainfall may result as was the case in September 1988 (Triegaardt el al., 1991).
On the synoptic-scale upward vertical motion, required for the formation of precipitation, is associated with horizontal wind convergence and the advection of relative vorticity (Holton, 1992). The continuity equation (with pressure as vertical coordinate) (equation 3-1) provides the relationship between the change of vertical velocity with pressure (left hand side of equation) and
horizontal wind divergence (right hand side of equation).

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Data and Method

The daily rainfall over Gauteng were analysed for all summer months from 1977 to 2012 and days with heavy rainfall were identified as described in Chapter 2. Two heavy rainfall classes are considered namely single station heavy rainfall (SHR) and area-average heavy rainfall (AHR) which is the 90th percentile of the daily area-average and single station rainfall events. National Centre for Environmental Prediction (NCEP) data used to investigate the synoptic circulation were found to be unreliable to accurately recreate atmospheric fields prior to 1979 due to the absence of observational data in the Southern Oceans (Tennant, 2004). The analysis preformed here will therefore only include data from 1979-2012. Table 3.1 lists the number of the AHR and SHR events per month from 1979-2012 as well as the number of days when no rain (No Rain) occurred over Gauteng. January months had the most days with AHR and SHR, followed by December and February months. There were only 63 days in December months when no rainfall occurred over Gauteng, this is just 6 % of all December days. The number of No Rain days was less than AHR and SHR in December, January and February months.
NCEP reanalysis data (Kalney et al., 1996) were used to investigate the monthly synoptic circulation associated with AHR, SHR and No Rain days. NCEP data have a horizontal resolution of 2.5 degrees and have 17 levels in the vertical. Variables utilized were geopotential height, the zonal and meridionial winds and mixing ratio. The average height of the interior of South Africa is approximately 1500 m above mean sea level and it is therefore common practice in the forecasting offices in South Africa to use the geopotential heights of the 850 hPa level to represents the surface synoptic circulation; this is also done here. Only 8 of the 17 vertical levels available in the NCEP data set are utilized namely the 850, 700, 600, 500, 400, 300, 250, and 200 hPa levels. Horizontal and vertical resolution of the NCEP data are ideal to research weather systems on the synoptic-scale but small or mesoscale features will not be adequately identified by this data set.
NCEP data were first used to calculate the long-term monthly mean circulation over Southern Africa. The average monthly circulation for AHR, SHR and No Rain days were consequently calculated and these results are discussed. Due to the similarity in the maps for AHR and SHR, only the maps for AHR will be discussed in section 3.4.
In the NCEP data Gauteng is situated approximately in the middle of four grid points (Fig. 3-1). Horizontal wind convergence and relative vorticity advection was calculated and displayed as vertical profiles at these four grid points.

1 INTRODUCTION 
1.1 Background
1.2 Aim and Objectives
1.3 Outline of the document
1.4 References
2 Rainfall climatology over the Gauteng Province in South Africa 
2.1 Preface
2.2 Heavy daily-rainfall characteristics over the Gauteng Province
2.2.1 Introduction
2.2.2 Data and methods
2.2.3 Quality control of rainfall data
2.2.4 Calculation of average daily rainfall
2.2.5 Defining heavy rainfall
2.2.6 Seasonal rainfall characteristics over Gauteng
2.2.7 Monthly and daily rainfall characteristics over Gauteng
2.2.8 Synoptic circulation in wet and dry seasons
2.2.9 Daily area-averaged heavy rainfall characteristics over Gauteng
2.2.10 Heavy rainfall at individual stations
2.2.11 Location of heavy rainfall at individual stations
2.2.12 Major and extreme rain events
2.2.13 Discussion
2.2.14 Acknowledgements
2.2.15 References
2.3 Overview
3 Synoptic circulation patterns associated with heavy rainfall over Gauteng 
3.1 Preface
3.2 Background
3.3 Data and Method
3.4 Synoptic climatology of heavy rainfall over Gauteng
3.4.1 October months
3.4.2 November months
3.4.3 December months
3.4.4 January months
3.4.5 February months
3.4.6 March months
3.5 Horizontal wind convergence and advection of relative vorticity
3.6 Summary of main synoptic-scale features during wet and dry months
3.7 Overview
3.8 References
4 Sounding-derived parameters associated with heavy rainfall over Gauteng 
4.1 Preface
4.2 Background
4.3 Sounding-derived parameters
4.3.1 Temperature parameters
4.3.2 Moisture variables
4.3.3 Equivalent potential temperature parameters
4.3.4 Wind parameters
4.3.5 Convective indices
4.4 Data and Method
4.4.1 Irene sounding data as a proximity sounding for Gauteng
4.4.2 Sounding data
4.4.3 Quality control of upper air data
4.4.4 Elimination of soundings done in cloud
4.4.5 Methodology
4.4.6 Self-organizing maps
4.5 A monthly climatology of basic variables
4.6 A monthly climatology of sounding-derived parameters.
4.6.1 Temperatures and temperature lapse rates
4.6.2 Geopotential thickness and freezing levels
4.6.3 Moisture variables
4.6.4 Equivalent potential temperatures
4.6.5 Winds
4.6.6 K-Index, Elevated K-Index, Total Totals Index and Elevated Total Totals Index
4.6.7 Convective variables
4.7 Sounding-derived parameters associated with heavy rainfall and dry days
4.7.1 Moisture
4.7.2 Temperatures
4.7.3 Temperature lapse rates and related variables
4.7.4 Wind speed and direction
4.7.5 Wind shear
4.7.6 Average tropospheric Ɵe and ΔƟe
4.7.7 Convective variables
4.8 A heavy rainfall sounding climatology using Self-organizing maps
4.8.1 Average heavy rainfall sounding climatology
4.8.2 Single station heavy rainfall climatology
4.8.3 Summary of the SOM heavy rainfall climatology
4.9 Overview
4.10 References
5 Self-organizing maps as a predictive tool. 135
5.1 Preface
5.2 Data and methodology
5.3 The training period
5.4 Predicting the frequency distribution of rainfall in a validation period
5.5 Summary
5.6 Overview
5.7 References
6 Summary conclusions and recommendations 
6.1 Study area
6.2 Characteristics of heavy rainfall over Gauteng: Objective 1
6.3 Synoptic circulation associated with heavy rainfall over Gauteng: Objective 2
6.4 Sounding-derived parameters associated with heavy rainfall over Gauteng: Objective 3
6.5 A heavy rainfall sounding climatology using Self-organizing maps: Objective 4
6.6 Self-organizing maps as a predictive tool: Objective 5
6.7 Assessing the scientific contribution of this study
6.8 Recommendations for future research
6.9 References

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