Browsing by Author "Keen, Cecil"

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    A climatology of the coastal low in the SW Cape
    (1987) Heydenrych, Clive Malcolm; Keen, Cecil
    The Coastal Low is a shallow cyclonic mesoscale weather 'disturbance' which migrates around the southern African subcontinent on a fairly regular basis. It is generated and maintained by the synoptic scale circulations. The movement and surface characteristics have been well documented by a number of authors but few detailed studies have been undertaken on its vertical structure in southern Africa. In addition to this, most of the previous work has been of a meteorological nature. This study has concentrated on a more climatic approach in its investigation of the vertical and· surface features of the Coastal Low as it migrates through the South Western (SW) Cape. The SW Cape is a 'transition region' for the migration of the Coastal Low; situated between the west and south coasts with a distinct local climate due to the complex topography of the region. This fact tends to alter the characteristic features of the Coastal Low system but appears not to prevent the Coastal Low from migrating through the region. The Coastal Low is regarded as being an internal trapped Kelvin wave and corrected surface pressure values best indicate its migration characteristics. However upper air analysis indicates that temperature values (between 950- 900mb) at the level of the inversion, produce one of the best signatures of the Coastal Low's passage. This is related to the strong subsidence from above the 850mb level in the pre-Low period. This strong divergence dynamically compresses the lower layers into low level wind speed maxima on either side of the centre of the system. The Coastal Low appears to have a very complex structure, and two results from this study in the. SW Cape bear particular mention. Firstly the offshore flow at the escarpment level is weakly defined. Secondly also, the longshore spatial extent of the Coastal Low system has been estimated to have an 'inner' diameter of 150-200km and an 'outer' diameter of approximately 1000km.
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    The formulation of a classification procedure for specific use on cumulus cloud weather modification experiments
    (1980) Erasmus, David Andre; Keen, Cecil
    The central theme of this study concerns the use of classification schemes on weather modification experiments designed to investigate the possibility of increasing rainfall from individual cumuli or cumulus cloud systems. The principal objectives of these experiments are the evaluation of treatment effects and the identification of situations where seeding with artificial ice-nuclei is likely to have positive results. The classification of experimental units into categories that are associated with significantly different physical processes aids the evaluation process and the formulation of seeding strategies in the desired manner. As part of this study a classification scheme, which stratifies convective events on the basis of the synoptic situations which give rise to and maintain the convection, is formulated. In chapter seven and eight this scheme and another scheme presently being employed on a cumulus cloud weather modification experiment are examined statistically. Investigations show that the formulated scheme attains the objectives of classification to a greater degree. Certain attributes of the second scheme, permit the development of a classification procedure whereby the most effective stratification of experimental units can be accomplished.
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    Growth, Structure and Prediction of the Thermal Internal Boundary Layer
    (1988) Comrie, Andrew Charles; Keen, Cecil
    The Thermal internal Boundary Layer (TIBL) is a dynamic and turbulent mesoscale feature of the coastal atmosphere that forms over the land during conditions of onshore flow. The TIBL develops as an adjustment of the atmospheric boundary layer to the discontinuities of temperature and roughness that occur at the interface between the underlying marine and terrestrial surfaces. The resulting formation of a characteristically convex mixed layer below relatively stable air aloft has serious implications for the dispersion of pollutants in shoreline environments. Although a wide range of research relating to various features of the TIBL may be found in the literature, relatively few broadly-based studies have been performed. This study has employed both airborne and surface measurements to obtain a comprehensive spatial and temporal data set, in order to elucidate aspects of the characteristic structure and behaviour of the TIBL. TIBL growth was found to follow a diurnal pattern, the initially irregular boundary becoming more uniform during the day as a steady balance between various factors was achieved. The TIBL was associated with a layer of uniform wind speed anti direction flowing perpendicular to the coastline, within which warmer temperatures and changes in relative humidity and moisture content were observed. The temperature structure of the onshore flow strongly influenced the intensity of turbulence encountered in the TIBL and the degree of entrainment aloft. Patterns of turbulent properties displayed significant increases in the TIBL, which were relatively abrupt near the surface and more gradual towards the top of the TIBL. Measurements of sensible heat flux revealed strong undulations in TIBL structure due to transitory eddies and thermal upcurrents. Certain theoretically based predictive equations of TIBL height displayed the best overall performance out of eight selected models, and some promise was shown by an empirical formulation. TIBL development was generally complex and irregular within the first few kilometres of the shore, while further inland more regular TIBL formation enabled the relatively accurate observation and prediction of TIBL height.
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    Photochemical smog in greater Cape Town
    (1988) Loewenheim, L; Keen, Cecil; Dutkiewicz, Ryszard Karol
    Photochemical smog is the name given to a complex sequence of chemical reactions that occurs in the presence of sunlight. These reactions comprise a mix of organic and inorganic compounds, including a number of toxic secondary pollutants such as ozone (O3) and peroxyacetyl nitrate. These substances are commonly referred to as oxidants and are the result of numerous reactions of primary pollutants or precursors (including nitrogen oxides (NOx) and non- methane hydrocarbons (NHHC)) emitted from vehicle exhausts and to some extent industry, O3 is the major constituent of the photochemical oxidants, and its concentration is often used to determine the severity of photochemical smog. Limited research on photochemical smog in Cape Town has been undertaken, and this study has concentrated on providing a more detailed understanding of photochemical precursor and oxidant levels in the urban atmosphere of Greater Cape Town. This was approached by the investigation and assessment of the spatial and temporal behaviour of photochemical pollutants, making use of automatic monitor data collected from 1984 to 1986, and supplemented by data collected during a spatial survey in April and Hay of 1987. Precursor levels were found to be strongly influenced by the seasonal cycle of the weather and were highest in winter when stable atmospheric conditions prevailed, particularly during morning rush hours, O3 behaviour was complex and lacked any definite relationship to season or to selected meteorological variables, although the limited data indicated high levels during the early spring months. Peak levels were generally experienced on fair weather days during the early afternoon hours at the time of maximum ultraviolet radiation, O3 levels did not exceed the USEPA 1-hour standard of 0.12ppm during 1985 and 1986. The spatial distribution of precursor and oxidant concentrations showed the NOx levels to be spatially dependent, following the major arterial roads. NHHC levels were spatially less well defined than NOx, and O3 levels were spatially relatively uniform, exhibiting depletion due to scavenging by nitric oxide (NO) in areas close to main traffic routes. High NOx levels were experienced predominantly in the city centre, while the suburbs tended to experience the higher O3 levels. Cape Town was not considered to have a photochemical smog problem of the same magnitude as Los Angeles or Sydney, ( due to a number of factors which contributed to the complex situation (such as high NO levels, relatively low NMHC levels and strong winds in summer). However in the Northern Suburbs, the absence of high NO levels together with additional NMHC emissions from nearby industry led to the recognition of this area as one of potential photochemical smog formation.