Using GPS-IPW in a 4-D data assimilation system

The NOAA Forecast Systems Laboratory (FSL) has been continuously calculatin g integrated atmospheric precipitable water (IPW) from GPS signal delays si nce 1994. Using rapid orbit information, these data have the accuracy requi red for use in a numerical weather prediction model through data assimilati on. Parallel cycles with and without GPS-IPW data have been running at FSL since November 1997 using the 60-km Rapid Update Cycle (RUC). Verification of the analysis and the 3, 6, and 12-h forecasts against rawinsondes has be en ongoing throughout the experiment. Results from these statistics show a consistent improvement in short-range forecasts of relative humidity when t he GPS data are included. Precipitation verification has also been calculat ed for this experiment, and results show that GPS data also improve these f orecasts. Recently, the average number of available GPS observations jumped from 18 to 56, and results for November-December 1999 show that the previo us slight positive signal is now amplified to a stronger positive impact on the short-range moisture forecasts.