GPS observations of daily variations in the atmospheric water vapor content

Estimates of the atmospheric Integrated Water Vapor (IWV) for 30 GPS sites in Europe, covering the period from January 1997 until August 1999 are anal yzed. For every month the diurnal cycle in the IWV is estimated by averagin g the data referred to that particular hour for that month. The average pea k-to-peak value of the diurnal cycle during the winter months is between 0. 6 mm and 1.5 mm, while during the summer months the peak-to-peak value is b etween 0.8 mm and 3.2 mm. The GPS results at the Onsala Space Observatory a re compared to those obtained from a co-located microwave radiometer and ne arby (distance 37 km) radiosonde launches. We find that the estimated diurn al variations are comparable to the uncertainties in the measurements and t he models used. However, a clear dependence between the peak-to-peak value and latitude of the observations is seen during the summer months. No such dependence is detected for the winter period. (C) 2001 Elsevier Science Ltd . All rights reserved.