Stratosphere Troposphere Exchange in the Southern Hemisphere    

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 Central Otago weekly with a record going back to 1986. An ozonesonde is a balloon borne instrument package that takes vertical profiles of pressure, temperature, humidity and ozone content.The use of tracer-tracer correlations to identifty Troposphere-Stratosphere Transport (TST) and Stratosphere-Troposphere Transport (STT) will be examined.

 

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 University of Canterbury radar (CUSTAR). While these data sets are sparse and location dependent, they may provide valuable data for the study.

 


Back to Home Page