Preliminary impact studies using Global Positioning System radio occultation profiles at NCEP

Following the successful launch of the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellites in April 2006, NCEP's Environmental Modeling Center (EMC) is planning to use the COSMIC data in its next-generation Global Data Assimilation System. In preparation for the assimilation of GPS radio occultation (RO) data from COSMIC and other missions, NCEP/EMC has developed the infrastructure necessary to use profiles of refractivity and bending angle in an operational framework. In both forward operators, horizontal gradients of refractivity have been neglected and each operator has been tuned with its corresponding quality control checks and error characterization. In this paper, the benefits of the assimilation of profiles of GPS RO on top of the current observations being regularly used in operations are analyzed. In addition, differences between the assimilation of bending angle and refractivity are discussed. To avoid unrealistic increments within the higher model layers, experiments not using GPS RO observations above 30 km are also performed. This stratospheric data assimilation problem was present in earlier experiments with GPS RO data at NCEP/EMC and impacted the forecast in the lower-atmospheric levels as well as the stratosphere. Some characteristics of the assimilation of profiles of bending angle are also discussed. Data from the Challenging Minisatellite Payload (CHAMP) satellite are available in non-real time at NOAA and have been used to perform the experiments examined herein.