Operational Assimilation of GPS-IPW Observations In The 13-km RUC at NCEP

Abstract

GPS integrated precipitable water (GPS-IPW) observations are a relatively recent asynoptic data source of moisture information for data assimilation. Short-range numerical weather forecasts suffer from inadequate observational definition of the threedimensional moisture field due to its high spatial and temporal variability. Generally, there have been three observational sources for atmospheric moisture: rawinsondes, surface, and satellite (not available in cloudy areas below cloud top). Estimates of IPW from GPS signal time delays can complement these moisture observations. GPS-IPW using zenith total delay provides only a vertically integrated value, by definition, but with at least hourly resolution and in all weather conditions, including those with cloud and even precipitation, conditions when observations are most important for forecasts of the atmospheric moisture. The NOAA Earth System Research Laboratory (ESRL) Global Systems Division (GSD) has developed, over the past several years, a GPSIPW network, which now produces high-accuracy, half-hourly, near-real-time measurements at more than 300 stations in the U.S. as of June 2005 (Fig. 1, Gutman et al. 2003). This data is now, for the first time, being assimilated into a real-time operational model coming out of the National Centers for Environmental Prediction (NCEP), the 13-km version of the Rapid Update Cycle (RUC13) implemented operationally in June 2005. GPS-IPW data have been assimilated into several developmental versions of the Rapid Update Cycle (RUC) since the 60-km RUC (RUC60) in 1997. Verification of the 3-h RUC60 cycle with assimilated GPS-IPW from 1997 through 2004 provides a rich database for long-term statistics. Increasing positive impact on short-range relative humidity (RH) forecasts has been evident (shown in section 4 of this paper) as the number of GPS observations assimilated has increased from less than 20 to almost 300 over the United States during the last seven years. In this paper, we present the most recent results from a series of GPS-IPW data impact studies performed at GSD with the 13-km Rapid Update Cycle data assimilation and numerical forecast system. Also, a multi-year parallel cycle using the RUC60 with earlier results presented by Benjamin et al. (1998), Smith et al. (2000), and Gutman and Benjamin (2001) has been continued with results through 2004 presented. Statistics from comparisons of 20-km RUC (RUC20) runs with and without GPS data are also included.