Trends in the atmospheric water vapour estimated from GPS data for different elevation cutoff angles

Abstract. We have processed 20 years of GPS data from 8 sites in Sweden and 5 sites in Finland, using two different elevation cutoff angles 10° and 25°, to estimate the atmospheric integrated water vapour (IWV). We have also tested three additional elevation-angle-dependent parameters in the GPS data processing, i.e., (1) two different mapping functions, (2) with or without second order corrections for ionospheric effects, and (3) with or without elevation dependent data weighting. The results show that all these three parameters have insignificant impacts on the resulting linear IWV trends. We compared the GPS-derived IWV trends to the corresponding trends from radiosonde data at 7 nearby (< 120 km) sites and the trends inferred from the European Centre for Medium-RangeWeather Forecasts (ECMWF) reanalysis data (ERA-Interim). The IWV trends given by GPS elevation 10° and 25° solutions show similar results when compared to the trends from the nearby radiosonde data, with correlation coefficients of 0.71 and 0.74, respectively. In addition the 25° solution gives a slightly lower root-mean-square (RMS) difference (0.15 kg/(m²·decade)) than the 10° solution (0.17 kg/(m²·decade)). When compared to the IWV trends obtained from ERAInterim, the GPS solution for the 25° elevation cutoff angle gives a higher correlation (0.90) and a lower RMS difference (0.09 kg/(m²·decade)) than the ones obtained for the 10° solution (0.53 and 0.18 kg/(m²·decade)). The results indicate that a higher elevation cutoff angle is meaningful when estimating long term trends, and that the use of different elevation cutoff angles in the GPS data processing is a valuable diagnostic tool for detection of any time varying systematic effects, such as multipath impacts.