Master Thesis
Wasserdampfkonzentrationsmessungen in großen Höhen und bei niedrigen Temperaturen
Daniela Pfab (05/2010-03/2011)
Support: Thomas Foken, Johannes Lüers
The following text is about humidity measurement on the Tibetan Plateau in the winter. The Tibetan Plateau is located in the Himalaya Mountains, 4700 m a.s.l., with very cold climate and low humidity values in winter. The this thesis is embedded in a framework of a project which is sponsored by the European Union. In this project satellite and surface observations (e.g. Eddy–covariance measurements) are required. Eddy-covariance measurements were conducted on the Nam Co Comprehensive Research Station of the Chinese Academy of Sciences. In addition to the CSAT-3 anemometer of the company Campbell Sci., an infrared gas analyzer (LI-7500, company LiCor Inc.) for humidity and carbon dioxide measurements and a HMP for reference measurements of humidity and temperature were installed. Previous studies observed that a water vapor oversaturation occurred especially during the winter months. This led to the as-sumption that a measurement with LI 7500 might not be correct. That is why the Department of Micrometeorology of the University of Bayreuth started this new experiment in the frame-work of the project in February 2010. A UV-hygrometer (KH20, company Campbell Sci.) was added to the already available measurements of air humidity with the LI 7500. The advantage of the KH20 is the newly developed mobile calibration unit of the department of micromete-orology of the University of Bayreuth in cooperation with the Gesellschaft für Akustik und Fahrzeugmesswesen mbH, Zwickau. The calibration unit enables in situ calibration through alteration of the distance of the krypton tubes of the KH20 with constant water content of the air. Through this calibration a good adjustment of the humidity values to the reference values of the HMP can be achieved. In contrast to that the analysis of the humidity values of the LI 7500 shows too high results of absolute and relative humidity in comparison to the reference values of the HMP. This result reflects the oversaturated humidity found in previous studies on the Nam Co station. A shift of the measured values of the LI 7500 seems to be caused by a sudden decrease of temperature in November 2009. Further analysis concerning the calibra-tion polynomial was conducted, as is conventional practice in an area such as the Tibetan Plateau with low humidity, temperature and pressure. This analysis shows that the error can be 6 % and more in the low humidity sector of the calibration polynomial. Previous studies are based on the assumption that only the measurements of absolute and relative humidity are incorrect and the measurement of the latent heat flux is affected. But a comparison of the measured latent heat flux with the LI 7500 and the KH20 reveals a 30 % higher latent heat flux of the KH20. The conclusion of this study and previous experiments is that either the humidity measurement has to be done with another humidity analyzer or the calibration frequency of the LI-7500 should be increased.