Operational Assimilation of GPS-IPW observations in the 13-km RUC at NCEP

A major revision was made to the Rapid Update Cycle (RUC) hourly assimilation cycle at NOAA/NCEP operations in June 2005. Changes included increasing the resolution to a 13-km grid (RUC13) and adding GPS-IPW observations. The introduction of GPS-IPW observations into the RUC13 represents an important asynoptic data source of moisture information for operational data assimilation. The NOAA GPS-Met network, currently with over 300 sites across the United States, provides continuous half hourly observations of integrated precipitable water (IPW) in near real time and under all weather conditions.

This paper will describe the design of the GPS-IPW assimilation, as follows:

- Revised moisture analysis in the RUC13, now with integrated assimilation of GPS-IPW, GOES-PW, and in situ observations, resulting in better moisture profiles.

- Quality control techniques to screen the effects of intermittent orbit busts resulting in problems with GPS, and the use of GPS data to identify rawinsonde moisture problems.

- Effect of the denser 2005 GPS-IPW network, as indicated by real-time monitoring of short-range forecast skill from RUC parallel versions with full, partial, and no GPS-IPW data.

Both the RUC20 and RUC13 analyses and forecasts with and without GPS-IPW data have been compared hourly to the GPS-IPW observations. Studies show that the GPS-IPW observations are comparable in quality to integrated (total column) RAOB observations, and that the use of hourly GPS-IPW data allows for the verification of many more analyses and forecasts from all cycles.

Investigation into the impact of the GPS-IPW data shows an improvement of the RUC20 analysis fit to the observations of approximately 1.5 mm. The RUC20 forecasts improvement (due to the inclusion of GPS-IPW) is around 1 mm for the 3-h forecast, with a slight impact persisting into the 6-h (~0.4 mm) and 9-h (~0.2 mm) forecasts. The paper will also present the same statistics for the RUC13.