Tropical stratosphere-troposphere exchange is examined by the use of water vapour data and trajectory calculations. Water vapour data from the Microwave Limb Sounder (MLS) instrument and two-dimensional trajectories calculated from the assimilated data of the United Kingdom Meteorological Office (UKMO) are used to examine the evolution of water vapour upon iseiltropes near the 68 hPa surface. Three-dimensional trajectories calculated from the assimilated data of the European Centre for Medium-Range Forecasts (ECMWF) are used to examine the flow of air across the tropopause. Combination of the MLS and UKIVIO data allows the construction of purelyisentropic evolutions of water vapour. On tirnescales less than those characteristic of diffusion and of methane oxidation, comparisons of these evolutions to the observed cases can be used to show where vertical motions must cross isentropic surfaces. As such, a comparison of monthly-averaged, water-vapour maps on isentropic surfaces near 68 hPa to monthly-averaged maps of the same data after ten-day iserltrol)icadvections is presented for most of the operation time of the 183 GHz antenna of the MLS instrument November 1992 to March 1993. The comparison shows that the two evolutions rarely match each other and thins that vertical airflows must be present over most of the tropical region, throughout the year, near 68 hPa. The use of three-dimensional trajectories allows for a more-direct and more-detailed analysis of tropical, cross-tropopause motion. Though the lesser reliability of vertical wind-data does mean that only those three-dimensional-trajectory results that were in basic agreement with the results of the two-dimensional case described above could be considered useful, such an agreement was generally found during this study. Three-dimensional trajectories are presented to show that though cross- trop opause motion does occur throughout the tropical region a persistent longitudinal-preference in the motion existed for all times investigated the winters of 1980, 1981 and 1993 and all seasons of 1992. Crossing frequencies are shown to be much higher over the mid-Pacific throughout the year and much lower over the Indonesian and Asian monsoons, both extrema being visible at the ECMWF model-levels at 110 and 90 hiPa. The peak crossing was stronger during the 1993 winter than in any other winter and stronger in autumn than in any other season of 1992. Furthermore, whenever the peak crossing showed an increase in magnitude it was accompanied by an increase in the concentration of particle crossings into the mid-Pacific. Taken together, the results of the two- and three- dimensional experiments also suggest that longitudinal variations in the 68 hPa, MLS water-vapour-concentrations occur not through differential uplift but by variations in the height of the tropopause.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:669290 |
| Date | January 2003 |
| Creators | Billingham, Aaron |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/10811 |
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