Long-term PPP-derived ZTD series for regional climate studies

Monitoring the atmosphere is essential for understanding short-term variability and long-term climate trends. Global Navigation Satellite Systems provide continuous information about the troposphere by estimating the zenith tropospheric delay. This study investigates a multi-year series of zenith tropospheric delays at six stations of the ASG-EUPOS network in Central Europe. The objective is to assess seasonal and interannual variability and evaluate these series' potential for regional climate monitoring. The observations were processed using the GipsyX software package with the Precise Point Positioning technique. The processing relied on precise satellite orbits and clocks, Earth orientation parameters, and the Vienna Mapping Function 1 for tropospheric modelling. The outputs included time-tagged estimates of total, hydrostatic, and wet delays, horizontal gradients and station coordinates. Quality control and homogenization procedures were applied to ensure stability and consistency. The results demonstrate a clear annual cycle at all stations, with maxima during summer and minima in winter. The wet delay exhibits substantial variability reflecting atmospheric moisture, while the hydrostatic component remains relatively stable. Differences between stations are linked to local climatic conditions, but the overall patterns are coherent across the network. In conclusion, long-term zenith tropospheric delay series derived from Precise Point Positioning are homogeneous and reliable geophysical records. They provide valuable information for regional climate studies and establish a robust basis for future derivation of integrated water vapour and for linking GNSS meteorology with hydrological and climatological applications.