Impact of CHAMP Radio Occultation Observations on Global Analysis and Forecasts in the Absence of AMSU Radiance Data

Challenging Minisatellite Payload (CHAMP) radio occultation (RO) observations during a two-week period are assimilated into global analyses using the National Center for Environmental Prediction (NCEP) three-dimensional variational (3D-Var) system with a recently improved observation operator for assimilating GPS bending angle data. The NCEP 3D-Var system used in this research is suboptimal since Advanced Microwave Unit (AMSU) radiances are not included in our experiments. Analyses with and without CHAMP observations are compared with each other and with collocated conventional radiosonde and dropsonde data, which are excluded from both experiments. Zonal mean temperature, humidity and surface pressure differences between the GPS analyses and NO-GPS analyses are examined. The GPS analyses in the Southern Hemisphere show higher temperatures than the NO-GPS analyses, particularly in the midand high latitudes. The GPS analyses show drier air in the lower troposphere and more moist air in the middle troposphere compared to the NO-GPS analyses. The surface pressure is slightly increased (maximum 0.8 hPa) in the Southern Hemisphere and decreased (maximum 0.25 hPa) Corresponding author: Xiaolei Zou, Department of Meteorology, Florida State University, 404, Love Bldg, Tallahassee, Florida, 32306-4520, USA. E-mail: [email protected] ( 2004, Meteorological Society of Japan in the Northern Hemisphere due to the inclusion of GPS observations. Compared with the collocated independent soundings, the large cold bias (as large as 2.5 K) in the NCEP Southern Hemisphere analyses produced without CHAMP observations is significantly reduced. On average, a 20% mean error reduction in the temperature analysis is obtained in the Southern Hemisphere when CHAMP data are included. Degradations in the surface pressure analysis found from previous the GPS/Meteorology data assimilation studies are greatly reduced. The differences between the surface pressure analysis errors with and without CHAMP data are less than 0.8G1.5 hPa. Comparisons of numerical forecasts initialized with analyses produced with and without CHAMP occultations display a small improvement in the forecasts in the tropics and the Southern Hemisphere associated with the use of the CHAMP ob-