Analysis of the annual cycle of the precipitable water vapour over Spain from 10-year homogenized series of GPS data

Abstract

This study reports a characterization of the precipitable water vapor (PWV) at ten sites over Spain from 10 years of hourly data from ground-based GPS receivers. The GPS-PWV data series turned out to be inhomogeneous due to the change in the calibration procedure of the variations of the antenna phase center in November 2006. Radiosonde data were used to homogenize the GPS data series and to assess the quality of the GPS measurements. The annual average value of PWV ranges from 14.5 to 20.0 mm, with an average of 18.3 ± 1.9 for the entire Spain. The highest values are registered at the sites on the coast, especially on the Mediterranean coast, and the lowest ones at inland sites. The PWV presents a clear annual cycle, with a minimum in winter and maximum at the end of the summer. However, the southwestern sites present a relative minimum in July. This minimum seems to be related with the presence of drier air masses in the atmospheric layers between 1 and 4 km altitude. The amplitude of the cycle ranged from 8.9 to 18.7 mm. The largest amplitudes are found at the Mediterranean coastal sites (approx. 15-19 mm) and the lowest ones at inland sites (approx. 9-10 mm). A harmonic analysis of the annual cycle showed that the 12-month period harmonic explains, on average, over 96 % of the variance. The average annual regime of PWV followed the cycle of the temperature, except for the relative minimum of PWV in July at the southwestern sites.