Schätzung des atmosphärischen Wasserdampfes aus GPS-Messungen und anderen Sondierungsverfahren in der Antarktis

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The climate is affected by water vapor within the hydrological cycle as it is one of the most variable greenhouse gases (Elliot u. a., 1995). If the atmospheric temperature increases, then the atmospheric water vapor increases, too, as the atmosphere is able to store more water vapor with higher atmospheric temperature. The atmospheric feedback effect generates vice versa higher temperatur in the atmosphere due to higher water vapor content. Furthermore atmospheric water vapor transports energy in form of latent heat. Moreover decrease and increase of water vapor facilitates clouding. All these characteristics make water vapor to one of the most important indicator for a possible global warming (IPCC, 2007). Water vapor is an important indicator for development of ice mass balance in Antarctica beside its direct meteorological relevance. Out of this reason it gives also a hint for a possible sea level rise (Bromwich und Parish, 1998). Water vapor can occur as precipitation in the atmosphere due to condensation. The level of precipitation influences furthermore the development of the Antarctic ice shield because of accumulation of fallen precipitation. In addition Antarctica exerts with its low temperatures a huge influence on the global climate (King und Turner, 1997). The ice shield reflects the incoming insolation and avoids a conversion of this radiation towards heat radiation. Antarctica is very sensible in relation to climate changes because of this ice-albedo-effect in comparison to other regions (Oerlemans und Bintanja, 1995). Hence it is possible to detect global climate changes and estimate their impacts with help of long- and shortterm developments of atmospheric water vapor in Antarctica. Radiosondes are often used to detect atmospheric parameter (Kraus, 2001). It is very difficult to use radiosondes in Antarctica or in Polar Regions because of the extreme climate conditions. For this reason most scientific stations which use radiosondes are located at the Antarctic coast because of the moderate conditions comparing to the Antarctic continent. Additionally the temporal resolution is limited. Most stations only do one or two radiosonde measurements per day because of the high costs for one sonde. The high logical and financial efforts are common obstacles for using many meteorological measurement systems in extreme climate regions like Antarctica. GPS can be used in many ways besides positioning. Determination of atmospheric parameters to get information about the neutral or ionospheric part is one possibility (Bevis u. a., 1992). GPS is an approved method in temperate zones to get meteorological information (Schueler, 2001), but it has had its limits in arid regions in the past. It is necessary to take a closer look at the field of applications (like Polar Regions) for GPS-processing actually, because of new developments basically in software. GPS could be a serious alternative to established procedures like radiosondes because of its independence towards daytime and weather. Furthermore it can be installed as a remote system which needs a minimum of support. GPS-data of 12 Antarctic GPS-stations were analysed for the years 2000 and 2001 to test the suitability of this method for Antarctica. The fact that many GPS-stations are installed on the Antarctic ice-shield was also taken into account. These stations move with the velocity of this shield towards the coast. Precipitable water vapour values for each atmosphere were estimated out of these GPS-information with the help of atmospheric temperature and pressure. The precipitable water vapour values out of GPS-measurements were compared with radiosonde launches at six stations to make an examination of quality. The meteorological situation is stated in chapter 3 after the introduction. It is possible to get the estimated water vapor time series in a meteorological context by the description of temperature, air pressure and water vapor of the Antarctic atmosphere. The basics of the Global Positioning System are described in chapter 4 in order to be able to reconstruct following processing steps and to point out all possibilities of attaining tropospheric parameters out of the configuration of this system. Radiosonde measurements and further systems to get the atmospheric water vapor are presented in chapter 5. The accuracy of the GPS-based values is also estimated beside the conversion of the zenit total delays into atmospheric water vapor values. Individual error sources are clarified in chapter 6 regarding the estimation of atmospheric water vapor. Solutions for their minimization are determined. The estimation of the coordinate reference frame is described also apart from the suitable strategie to get correct tropospheric values. At least the range of accuracy of coordinates and velocities are specified. The results of the GPS-based water vapor estimation in Antarctica are analysed in chapter 7. These results are also sorted into a meteorological context. The GPS-based water vapor values are finally validated with radiosonde measurements of 6 stations in chapter 8, in order to test the suitability of these GPS-values. Finally GPS is evaluated, if it is usable as a meteorological sensor in Antarctica. The future of the system regarding the estimation of atmospheric water vapor under extreme climate conditions is prognosticated.

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Thesis (PhD)
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Schulte, D. (2009): Schätzung des atmosphärischen Wasserdampfes aus GPS-Messungen und anderen Sondierungsverfahren in der Antarktis , PhD thesis, Universität Bremen.

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