Short-Range Forecast Impact from Assimilation of GPS-IPW Observations into the Rapid Update Cycle

Integrated precipitable water (IPW) estimates derived from time delays in the arrival of global position- ing system (GPS) satellite signals are a relatively recent, high-frequency source of atmospheric moisture information,available for real-time data assimilation. Different experimental versions of the Rapid Update Cycle (RUC) have assimilated these observations,to assess GPS-IPW impact on moisture,forecasts. In these tests, GPS-IPW data have proven to be a useful real-time source of moisture information, leading to more accurate,short-range moisture,forecasts when,added,to other observations. A multiyear,experiment,with parallel (one with GPS-IPW processed 24 h after the fact, one without) 3-h cycles using the original 60-km RUC was run from,1999 to 2004 with verification of each cycle against rawinsonde,observations. This experiment,showed,a steady increase in the positive impact in short-range relative humidity,(RH) forecasts due to the GPS-IPW data as the number,of observing,sites increased,from 18 to almost 300 (as of 2004) across the United States and Canada. Positive impact from GPS-IPW on 850–700-hPa RH forecasts was also evident in 6- and 12-h forecasts. The impact of GPS-IPW data was also examined on forecasts from the more recent 20-km RUC, including a 1-h assimilation cycle and improved assimilation and physical parameter- izations, now using real-time GPS-IPW retrievals available 30 min after valid time. In a 3-month comparison during the March–May 2004 period, 20-km RUC cycles with and without assimilation of GPS-IPW were compared with IPW for 3-, 6-, 9-, and 12-h forecasts. Using this measure, assimilation of GPS-IPW data led to the strongest improvements,in the 3- and 6-h forecasts and smaller but still evident improvements,in 9- and 12-h forecasts. In a severe convective weather case, inclusion of GPS-IPW data improved forecasts of convective available potential energy, an important predictor of severe storm potential, and relative hu- midity. Positive impact from GPS-IPW assimilation was found to vary over season, geographical location, and time of day, apparently related to variations in vertical mixing. For example, GPS-IPW has a stronger effect on improving,RH forecasts at 850 hPa at nighttime,(than daytime) and in cooler seasons,(than warmer,seasons) when,surface moisture,observations,are less representative of conditions aloft. As a result of these studies, assimilation of GPS-IPW was added to the operational RUC run at NOAA/NCEP in June 2005 and to the operational North American,Mesoscale model,(also at NCEP) in June 2006 to improve,their accuracy,for short-range moisture,forecasts.