Integrated water vapor estimated by GPS compared to independent observations during MAP

Abstract

We analyze integrated water vapor (IWV) estimated by Global Positioning System (GPS) measurements at the Milano-Linate airport, Italy, throughout October 1999, during the Mesoscale Alpine Programme (MAP) Special Observing Period (SOP). IWV was obtained from zenith path delay parameters, estimated every two hours, during GPS data analysis. Different strategies for GPS data analysis (network configuration, parameter weighting, path delay models) were tested in order to achieve the best accuracy in IWV estimates. The results are compared to balloon sounding data and are analyzed with the help of other data, such as from numerical weather prediction models, that were available during the MAP campaign. We find mean and RMS errors of 0.5 kg/m<SUP>2</SUP> and 1.4 kg/m<SUP>2</SUP>, respectively, when compared tsounding data. The highest accuracy was found for GPS observations using data down to 5 - 10 degrees elevation angles in a network including far stations (more than 1000 km away). This analysis will be pursued for the study of episodes of strong convection leading to heavy precipitation encountered during the MAP SOP. Therefore, the GPS-derived IWV covering the MAP investigation area will be used.