PhD Student: Kathleen Monahan
Joint Supervisor: Greg Bodeker (NIWA, Lauder)
Stratosphere
troposphere exchange (STE) describes the movement of air parcels between the two
lowest layers in the atmosphere. This movement can be tracked by examining the
characteristics of air parcels, as air from the stratosphere has different
characteristics to that of air in the troposphere. For instance, the troposphere
is moist with very low ozone content, whereas the stratosphere is dry, ozone
rich and very stable.
STE plays an
important role in defining the distributions
of ozone and water vapour in the Upper Troposphere/Lower Stratosphere (UT/LS)
region along with other constituents. The UT/LS region of the atmosphere plays a
crucial role in the climate system because of the radiative impacts of water
vapour, ozone, and aerosols in that region. STE has
also been shown to have a significant impact on the distribution of atmospheric
constituents in the troposphere. To date, very little work has been done on STE
in the Southern Hemisphere (SH). The SH is generally included in global studies
however more in-depth work has not been carried out because of the greater
quantity and quality of data available from the Northern hemisphere. The focus
of this work is to improve the understanding of STE in the SH. In addition,
analysis will focus on understanding the
transport processes which couple the stratosphere and troposphere on a number of
scales. This goal
will be realised by using a mixture of high-resolution measurements from
ozonesondes, satellite data and trajectory
modelling.
Ozonesondes are
released at Lauder in
It should be
noted that because of the rapid overturning in the troposphere,
signatures of STE are sometimes difficult to identify from in-situ trace gas
measurements. The higher vertical stability of the lower stratosphere, on the
other hand, conserves the signatures of troposphere to stratosphere exchange,
thus allowing the identification of a ‘‘mixing layer’’ above the local
tropopause, where the chemical composition is a mixture of stratospheric and
tropospheric air . Thus, examination of the differences in identified regions of
STE in the UT and LS may provide useful information on the relative scales of
mixing in these regions.
A particular
emphasis will be placed on tropopause fold events in the Southern hemisphere.
Tropopause folds have been found in different studies to occur with different
frequency and strength in different areas and altitude ranges. Through the use
of ozonesondes from various locations around the Southern Hemisphere, and
satellite data, it is hoped to build up a distribution and climatology of these
events. This analysis will be of use in identifiying how active and deep these
events are, that is how efficient these events are at transporting
chemicals.
Several
different instruments will provide auxiliary datasets for confirming location
and finer details of tropopause fold events. These instruments include
radiosondes, satellites (SAGE series, AIRS, CHAMP, EOS-MLS) and the