Ozone maxima over Southern Africa : characteristics and mechanisms.

Abstract

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.