Observations from the NOAA Global Systems Division (GSD) GPS-IPW network are received in near real-time at the Canadian Meteorological Centre (CMC) since August 2004. The GPS-IPW network consists of over 350 sites equipped with dual-frequency GPS receivers, located mainly over the United States. The primary GPS observation from these sites is the zenith tropospheric delay (ZTD), updated every 30-minutes. Collocated surface weather observations (pressure, temperature and relative humidity) are also available at most sites. The ZTD can be related to surface pressure and integrated atmospheric water vapour (IWV) above the GPS antenna. Collocated surface weather observations allow IWV to be determined from ZTD. Unlike other remote sensing platforms (e.g., ground and space-based radiometers), the accuracy of GPS IWV is largely unaffected by the presence of precipitation.

Studies have shown that assimilation of ground-based GPS observations can have a significant impact on forecasts of humidity and precipitation. This paper presents results of data impact experiments involving the addition of NOAA/GSD GPS-IPW network ZTD and surface weather observations to the CMC regional (North America) data assimilation system (RDAS). The RDAS consists of a three-dimensional variational (3D-Var) data assimilation with first-guess provided by the CMC Global Environmental Multiscale (GEM) regional forecast model. The impact on 48-hour regional GEM forecasts initialized with analyses from the RDAS is evaluated. Experiments are carried out for both summer and winter periods. The assimilation of GPS observations produces a generally positive impact on humidity forecasts in the mid-troposphere for the 0-24h range (based on verification with North American radiosondes), especially for the summer period in the southeast US. Verification of 24h precipitation accumulation forecasts with rain gauge measurements shows an overall positive impact over the entire 48h forecast period.