Tropospheric Zenith Delays Determined by VLBI as a Contribution to Climatological Studies

As consistent VLBI observations at various stations over the whole globe have been carried out since 1984, it is possible to determine long time series not only of baseline vectors and Earth orientation parameters, but also of tropospheric parameters. Time series of wet zenith delays provide information about trends and periodic variations of the amount of water vapour in the troposphere. At Wettzell (Germany) there is a trend of ~ +0.7 mm/year in the wet zenith delay which corresponds to ~ +0.1 mm/year precipitable water vapour. Additionally, periodic variations in the time series are revealed by Fourier and wavelet analyses, and information about the precipitable water provided by the ECMWF (European Centre for Medium-Range Weather Forecasts) is used to evaluate the VLBI estimates.