On the homogeneity and interpretation of precipitable water time series derived from global GPS observations

Observations of the Global Positioning System (GPS) were reanalyzed over the period from 1994 to 2004 in a joint project of the technical universities in Dresden and Munich. The estimated tropospheric parameters were converted into precipitable water (PW) using surface pressure observations from the World Meteorological Organization and atmospheric mean temperature fields from the European Centre for Medium‐Range Weather Forecasts. For the first time a systematic study of the homogeneity of global GPS‐derived precipitable water time series was carried out regarding the influence of changes in the GPS antennas and radomes as well as changes in the number of recorded observations. The focus of this study is on interannual changes in precipitable water. Over Europe, large parts of North America, and Iceland and in the region south of 30°S, these changes are very small. The range of the PW variations on interannual time scales is less than 2 mm in these areas. However, in the southeastern part of North America and north Australia, these anomalies in precipitable water show a range of up to 6 mm. In the tropics, PW anomalies with a range of up to 10 mm were found. GPS PW was compared with a modeled PW assuming water vapor saturation. This shows that GPS PW of stations located in the middle and high northern and southern latitudes is consistent with the temperature‐related saturation values of water vapor. In the tropics and subtropics the annual temperature variations are low. In these regions the variations in the PW can be dominated by other factors, including water vapor transport. At seasonal time scales the water vapor transport can be associated with atmospheric circulation such as monsoonal flow.