GPS Measurements of Atmospheric Water Vapour in a Low-Latitude Region

Abstract: Signals at radio frequencies are distorted while travelling from a constellation of GPS satellites through the atmosphere. These signals can be used to estimate the amount of integrated precipitable water vapour (IPWV). There are advantages over other conventional techniques in terms of temporal and spatial resolution. The GPS technique is also operational during all weather conditions and is potentially useful for weather forecasting and climate study. The low latitude region whih contains high amounts of water vapour is the focus of this study. Data from fourteen GPS sites - included in the International GNSS Servisce (IGS) - located between 20oS and20oN in latitude and between 70oE and 170oE in longitude have been analyzed. The data span is from 1998 to 2004. Comparisons with radiosonde data and data from two numerical weather models (ECMWF and NCEP/NCAR/DOE) were performed. Results of long-term trends, diurnal, and semi-diurnal variability in the IPWV are presented. There is an overll agreement in estimated trends of IPWV between the different techniques. Both increasing and decreasing trends are detectd in the region. Diurnal singals in the IPWV are largest in the continental interior, whereas semi-diurnal signals are relatively more important for sites located on oceanic island. Significant differences between wet and dry seasons are seen for the diurnal signal at the sites located closest to the equator.