TROPOSPHERIC PARAMETERS OVER TWO DECADES DETERMINED BY VLBI AS A CONTRIBUTION TO CLIMATOLOGICAL STUDIES Harald Schuh, Johannes Boehm Institute of Geodesy and Geophysics (IGG) Vienna University of Technology 1040 Vienna, Gusshausstrasse 27-29, Austria harald.schuh@tuwien.ac.at ABSTRACT. 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 of baseline vectors and Earth orientation parameters, and also of tropospheric parameters. Time series of wet ze- nith 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 ~ +0.8 mm/year in the wet zenith delay which corresponds to ~+0.1 mm/year precipitable water vapour. Addition- ally, 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 Me- dium-Range Weather Forecasts) is used to evaluate the VLBI estimates. 1 INTRODUCTION The total path delay for an observation at the elevation angle ε consists of the hydrostatic and the wet part. Each of these parts is the product of the delay in zenith direction and the corre- sponding mapping function. Assuming azimuthal symmetry at a VLBI station, the total path delay in the neutral atmosphere ∆L(ε) can therefore be modelled as: (1) w h () () () ε ⋅ + ε ⋅ = ε ∆ mf WZD mf HZD L HZD hydrostatic zenith delay, WZD wet zenith delay, mf h (ε) hydrostatic mapping function, mf w (ε) wet mapping function. In standard VLBI analyses, the wet zenith delay (WZD) is estimated, while the other three pa- rameters (HZD, mf h , mf w ) are assumed to be known. Since at some stations consistent VLBI observations have been carried out for more than 20 years, long time series of the wet zenith delays at various stations can be determined and used for climatological studies. Table 1 gives an overview of the VLBI stations that have been used for these investigations. On the aver- age, 24 h geodetic VLBI sessions have been performed every 4th to 5th day, which yields a temporal coverage between 19% and 25%.