Ozone maxima over Southern Africa : characteristics and mechanisms.
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This thesis aims to clarify the sources of, and mechanisms associated with, the generation of ozone maxima over the southern African region. Inasmuch as, tropospheric ozone concentration is a function of both chemistry and meteorology, this thesis concentrates on the role of atmospheric dynamics. Firstly, a statistical analysis of the relationship between total ozone and meteorological parameters revealed a generally weak negative relationship between total ozone and the height of the 500, 300 and 100 hPa geopotential surfaces. The relationship is best expressed by the passage of a mid-latitude cyclone while anticyclonic conditions exhibited a weak relationship. An examination of the spatial distribution of total ozone and potential vorticity (PV), during the passage of westerly troughs, prompted a more thorough investigation of the exchange of ozone between the stratosphere and troposphere. The relationship between tropospheric ozone, and low pressure and anticyclonic systems is investigated further using data obtained during the South African Fire-Atmospheric Research Initiative (SAFARI) conducted in 1992. Ozone concentrations, as expressed by ozonesonde data, reveal different characteristic profiles for the two scenarios. Explanations for the differences observed are sought in the observed circulation patterns during the experiment. Case studies at Okaukuejo (Namibia), Irene (South Africa) and Brazzaville (Congo), which were utilised as ground stations during SAFARI, are presented in an attempt to gain insight into the vertical distribution of ozone over the entire expanse of the study region. The role of convective systems in the generation of short-lived upper tropospheric ozone maxima at tropical latitudes is illustrated while the different vertical ozone signatures, expressed under cyclonic and anticyclonic systems as described earlier, are reconfirmed by the Okaukuejo and Irene data. An attempt is made to investigate dynamic links between the troposphere and stratosphere and the concomitant exchange of ozone during the passage of westerly trough systems. Particularly deep troughs or cut off low pressure systems are identified as important mechanisms in the generation of upper tropospheric ozone maxima. An examination of the vertical distribution of ozone at Irene during the passage of a COL, using data obtained from the SA'ARI 1994 experiment, suggests concurrence with Danielsen's (1968) model of tropopause folding. The intrusion of high PV and dry stratospheric air, coupled with downward flow near the tropopause, in the vicinity of the upper tropospheric disturbance, promotes the transport of ozone-rich air to tropospheric altitudes. The limited availability of data has severely hampered the understanding of tropospheric ozo~e in southern Africa in the past. This study demonstrates the value of daily vertical ozone data, even for very short periods.