Ozone Dynamics and Seasonal Variability over Africa
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Date
2011-07
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Addis Ababa University
Abstract
In this thesis results of satellite and in-situ airplane ozone data analysis are presented.
Total Ozone Column (TOC) from Total Ozone Mapping Spectrometer (TOMS), and
ozone enhancements sampled on the cruise flight route from Johannesburg to Vienna
by Measurement of OZone by Inservice AIrCraft (MOZAIC) at Equatorial Africa and
North Africa are investigated. The work in this thesis is focused on the study of ozone
transport during stratosphere-troposphere exchange (STE) events over intense STE areas,
and seasonal variability of ozone over Africa. On very few occasions, at upper troposphere
Equatorial Africa ozone spikes are observed at a flying altitude of 250-200 hPa. To understand
the events that attribute to spiky MOZAIC ozone and relative humidity observations,
additional data sets from different data sources were used to examine the observed
events. Vertical wind fields in the regions of low Outgoing Long-wave Radiation (OLR),
very high latent heat and cloud liquid water content transport have indicated presence
of convection and thunderstorm events. Potential vorticity intrusion over equatorial region
introduced enhanced ozone of stratospheric origin, as revealed by ERA-interim ozone
data. Both downwelling and upwelling induced by convection and intrusion transported
enhanced ozone from the stratosphere all the way down to the boundary layer.
We have distinguished source and regions of discharge of MOZAIC ozone enhancements
observed at a flying altitude of 250-200 hPa over North Africa. Different data
sets from ECMWF-ERA-Interim are used to examine the dynamics associated with the
events. Relative humidity from MOZAIC measurement, longitudinal cross-section of potential
vorticity (PV) and ozone volume mixing ratio (VMR) over the region of MOZAIC
ozone enhancements confirmed that the observed enhancements are measured within lower
stratosphere. This is mainly due to massive shift in the tropopause level. Longitudinal
cross-section of PV, potential vorticity in isentropic surfaces (IPV) and ozone VMR revealed
that the first mode of transport is large scale airmass subsidence from the stratosphere
over mid-latitude. Some of these events lead to cross tropopause ozone transport
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to upper troposphere North Africa. The second case is subsidence at polar regions during
North hemisphere winter seasons. IPV is strongly modified as revealed by wind fields.
In addition zonal winds and Rossby waves, which are active during winter seasons, have
strong contribution in modifying potential vorticity and ozone spatial and temporal distribution.
TOC is analyzed from TOMS satellite data to study its seasonal variability over Africa.
The seasonal variability comparisons in the latitude regions 0-34.50 N and 0-34.50 S show
that ozone concentration peaks shift from April at North Africa to September near the
equator. Ozone concentration begins to raise from May to October, and decreasing afterwards
in the Southern part of Africa with highest peaks only between September to
October. This result shows that ozone concentration seasonal duration is longer in the
Northern part of Africa than the South. Low total column ozone concentration around
200 N and 200 S was observed which could be related to dynamical factors such as convection
while the seasonal variability of very high ozone VMR is related to photochemical
production
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Keywords
Ozone Dynamics