ASSIMILATION OF GROUND-BASED GPS OBSERVATIONS IN THE CANADIAN REGIONAL ANALYSIS AND FORECAST SYSTEM

Microwave signals from Global Positioning System (GPS) satellites received at ground-based GPS receivers experience refraction as they pass through the atmosphere. This refraction reduces signal speed relative to vacuum, resulting in a delay in reception of the signal at the receiver. This delay is a nuisance parameter that must be estimated to obtain precise point positions with GPS. The delay due to the atmosphere directly above a location, called the zenith tropospheric delay (ZTD), is estimated quite accurately in the processing of data from ground-based GPS receivers with specialized geodetic software (e.g. GAMIT, GIPSY, Bernese). The ZTD is the time delay expressed in terms of excess path length (m or mm) and depends on integrated water vapour (IWV) and surface pressure. Given surface pressure and temperature measurements, ZTD observations from GPS sites provide an all-weather measure of IWV (Bevis et al, 1992). Inter-comparison studies (e.g. Deblonde et al, 2005) show that precipitable water (PW) derived from 2-week latency ZTD observations is of comparable accuracy to that obtained from radiosondes (error of ~2 mm). The Meteorological Research Branch (MRB) is evaluating ZTD observations for inclusion in the data assimilation and forecast systems of the Canadian Meteorological Centre (CMC). Since August 2004, near real time (NRT) observations from the US NOAA Environmental System Research Laboratory (ESRL) network of GPS receivers have been received at CMC and monitored by MRB. Assimilation experiments have been carried out with the observations and a modified version of the CMC regional three-dimensional variational (3D-Var) data assimilation and forecast system. Results of the monitoring will be presented and assimilation tests discussed.