Atmospheric water vapour-a natural greenhouse gas of importance in the climate system-remains poorly monitored for some regions of the Earth. This study presents an analysis of Global Positioning System (GPS) data that results in a new precipitable water (PW) data set for Antarctica. In order to obtain optimal estimates of PW, the most up-to-date models, including the VMFI tropospheric mapping function and absolute antenna phase centre variations, are used. Particular emphasis is placed on the accurate modelling of Ocean Tide Loading (OTL) displacements. Initially, a precise point positioning GPS•.'analysis is presented, In which three dimensional ground displacements are estimated at eight diurnal and semi-diurnal tidal frequencies. A global comparison with measurements from Very Long Baseline Interferometry (VLBI) and with estimates computed from numerical ocean tide models is undertaken. GPS is shown to be capable of measuring OTL displacements with comparable accuracy to VLBI, particularly for lunar tidal constituents, N2 and Ql. Systematic GPS biases remain for the Kl and K2 constituents, however. Comparison of GPS and modelled OTL displacements in Antarctica identifies TPX06.2 to be the overall best fitting model. A global reprocessing of a 60 station GPS network is undertaken for the 1995-2006 period. Estimated zenith delays are converted to PW for twelve, mainly coastal, Antarctic locations. Comparison with radiosonde derived PW time series shows the reprocessed GPS measurement technique exhibits good temporal stability. GPS I radiosonde biases are small, at the sub-millimetre level; correlations are of the order of 0.95. Comparison with PW data sets obtained from MODIS, AIRS and AMSR-E satellite instruments shows the AIRS instrument gives the best agreement with GPS, again with sub-millimetre biases. The PW time series capture the dry climate of Antarctica. Periodic signals observed include a strong annual signal at all locations, and a semiannual signal at coastal East Antarctic locations. A 24-hourlY (S 1) periodic variation is observed in summer PW measurements at many sites. There is an apparent increase in summer-time PW over the 1995-2006 period in coastal East Antarctica and at the South Pole. It is concluded that globally reprocessed GPS solutions can provide accurate measurements of PW that are potentially useful for meteorological and climatological applications, particularly in remote, data sparse regions such as Antarctica.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:485862 |
| Date | January 2008 |
| Creators | Thomas, Ian David |
| Publisher | University of Newcastle Upon Tyne |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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