Comparison of ECMWF analyses with GPS radio occultations from CHAMP

Abstract. A climatological validation of the 6-hourly operational ECMWF troposphere and lower stratosphere temperatures as well as geopotential heights between 1000 and 10 hPa is performed using the 2001–2007 (80 months from May 2001 to December 2007) CHAMP radio occultation data. Generally there is a good agreement between ECMWF and CHAMP temperatures averaged over 300–10 hPa for all years/seasons with global annual mean biases (standard deviations) less than 0.3 (1.7) K. Regional and temporal discrepancies occur within the polar vortex mainly on the Southern Hemisphere and the tropical tropopause region. Global annual mean biases (standard deviations) of geopotential heights between 300 and 10 hPa are in the range of −30 up to +5 (30–50) geopotential meter. Larger deviations from the mean values are also observed in the tropics and polar zones. Both, the biases and standard deviations between CHAMP and ECMWF temperatures and geopotential heights differ significantly before and after February and December 2006, i.e. the dates when ECMWF increased the number of model levels from L60 to L91 (1 February 2006) and where ECMWF became one of the first weather centers assimilating radio occultation data (since 12 December 2006), mainly from the COSMIC mission. At ECMWF the CHAMP data were only assimilated until 4 February 2007, e.g. both data sets are mostly independent from each other during the time period considered here.