Stable discontinuities in the atmosphere over South Africa
Recent research has highlighted the lack of information on elevated absolutely stable layers per se throughout the troposphere over the southern African subcontinent. Hence a climatology of elevated absolutely stable layers is derived for each of four predominant synoptic circulation types over Southern Africa; namely semi-permanent continental anticyclones, transient mid-latitude ridging anticyclones, westerly wave baroclinic disturbances and barotropic quasi-stationary easterly waves. The horizontal and vertical structures as well as the temporal persistence of elevated absolutely stable layers are analysed using radiosonde data from nine South African aerological stations from the period 1986-1993. In addition, a climatology is derived for the mid-winter stability maximum, during the month of July from 1989-1993, in order to gain greater insights into the influence of anticyclonic circulation on the elevated absolutely stable layers. Four non-surface stable layers are identified over the country, at approximately the 800 hPa, 700 hPa, 500 hPa and 300 hPa levels. The lower of these layer occurs only over the coastal regions. All of the discontinuities exhibit a high degree of persistence and appear to be rapidly reconstructed subsequent to synoptic scale disturbances. The presence of these layers has obvious implications for local as well as global pollution transport, since stable discontinuities trap pollutants below their bases and act as upper air boundaries. As a consequence, global-scale transport of aerosols and trace gases in the free air is controlled to a large degree by these discontinuities. Greater insights into the mechanisms controlling such large-scale transport are essential in ascertaining southern Africa's contribution to greenhouse gas concentrations. Moreover, the persistence and strength of the discontinuities have implications for rainfall over the subcontinent, with the lower layers in particular acting as a vertical boundary to turbulence and mixing, and thus hindering the development of convective precipitation.
Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg for the Degree of Master of Science.
Geography -- South Africa., Atmosphere -- Research -- South Africa., Troposphere., Climatology -- Research -- South Africa., Air -- Pollution.