Assimilation of Global Positioning System radio occultation data in the ECMWF ERA–Interim reanalysis

Abstract This article presents results of the assimilation of Global Positioning System (GPS) radio occultation (RO) bending‐angle data from CHAMP, FORMOSAT‐3/COSMIC and MetOp‐A GRAS in the European Centre for Medium‐range Weather Forecasts (ECMWF) global reanalysis ERA–Interim. We find that GPSRO data present the highest daily assimilation percentage rate among all the various types of observations, suggesting that these data are readily usable by today's reanalysis systems. Over time, except when additional GPSRO data are introduced, the ERA–Interim short‐term forecasts (background) are found to be stable compared with GPSRO data, and so are the ERA–Interim temperatures compared with radiosondes. This suggests that the GPSRO data are potentially as stable as the verification used here, at least to the extent that the possible quality variations in GPSRO data assimilated in ERA–Interim appear invisible when compared with radiosondes. We observe very good consistency between data from all six COSMIC receivers. Small differences between COSMIC and CHAMP (GRAS) data are observed in the lower troposphere (stratosphere). The mean effect of adding GPSRO data in ERA–Interim is to reduce temperature biases with respect to radiosondes in the ERA–Interim background by warming the tropopause region and lower stratosphere by about 0.1–0.2 K in all hemispheres. We also find hints of a drying effect in the mean water‐vapour content in the Tropics in ERA–Interim when GPSRO data are introduced. The fit to upper tropospheric wind observations in the Southern Hemisphere and the Tropics is also improved when GPSRO data are present. Overall the GPSRO data act as references via variational bias correction to correct satellite radiances. Removing the GPSRO data from the ERA–Interim system leaves the latter more prone to fitting the warmly biased aircraft data. All these effects mostly became apparent when large amounts of GPSRO data started being assimilated, at the end of 2006, with the introduction of COSMIC data. Copyright © 2010 Royal Meteorological Society