Short-range Forecast Impact From Assimilation of GPS-IPW Observations Into The Rapid Update Cycle

Abstract

Integrated precipitable water ( IPW) estimates derived from time delays in the arrival of global positioning 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 parameterizations, 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 9and 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 humidity. 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.