The impact of GPS radio occultation assimilation at the Met Office

Monitoring of global positioning system radio occultation (GPSRO) data was performed. By comparisons between coincident GRAS and FORMOSAT-3/COSMIC occultations and coincident GPSRO refractivity and Vaisala RS92 radiosonde-derived refractivity data, we show the consistency of GPSRO data from independent missions (and differing processing methods) and with independent in situ observation data. It was found that the bending angle and refractivity data from COSMIC is positively biased relative to that of GRAS by 0.2-0.5% in the stratosphere, which is where the GRAS refractivity agreed better with the RS92 radiosonde-derived refractivity than COSMIC. Forecast impact experiments using GPSRO refractivity and bending angle measurements were conducted (using 1D operators). It is shown that refractivity data (from FORMOSAT-3/COSMIC, GRAS, CHAMP and GRACE-A) provided a large positive impact, with the greatest improvements in the Southern Hemisphere extratropics, where r.m.s. fit to radiosondes improved by up to 10% at 250 hPa. A refractivity to bending angle assimilation comparison was made. We found that bending angle assimilation (1D operator) generally provides a greater positive impact than refractivity, with a notable improvement in the Northern Hemisphere extratropics, where a similar to 1% improvement in r.m.s. fit to radiosonde temperatures, geopotential heights and winds in the lower stratosphere was seen. (C) Crown Copyright 2010. Published by John Wiley & Sons, Ltd.