GPS integrated precipitable water (GPS-IPW) data have been assimilated into several versions of the developmental Rapid Update Cycle (RUC) run at the Forecast Systems Laboratory (FSL) since the 60km RUC in 1997. Ongoing verification of the 60-km 3-h RUC cycle continues through the present day, a rich database for long term statistics. Increasing impact over time in the relative humidity (RH) field can be clearly seen, as the number of GPS observations assimilated has increased from less than 20 to over 200 over the United States. Impact at 700 hPa has increased from near 1% to around 7% over the 6 years of the study, with 850 hPa and 500mb hPa also showing improvement in the 3h RH forecast due to GPS-IPW of 5% and 4% respectively in the latest statistics.

The 20km RUC backup run at FSL routinely assimilates GPS data as well, one of the few differences between it and the official NCEP version. Comparing the behavior of the two systems has led to several observations on not only the utility of ingesting GPS data, but improvements into how it is being used. Not surprisingly, the RUC analysis including the GPS-IPW data matches the GPS observations more closely than the analysis without GPS, however, the 3 hour forecasts also show this same result, indicating that the information has been carried forward into the forecast in a positive manner. Both versions of the RUC converge at both 0000 and 1200 UTC as the rawindsonde data is ingested.

Real time comparisons of the two versions of RUC can be viewed at http://waylon.fsl.noaa.gov/cgi-bin/ruc20/ruc20.cgi

In our paper and at the conference we will present short term impact as in a case study, and longer term impact from our ongoing verification, both from the 60km and the 20km versions of the RUC.