Initial results of combining GPS occultations with ECMWF global analyses within a 1DVar framework

We present results of combining occultation refractivity profiles from GPS/ MET with ECMWF global analyses in a 1DVar framework in order to separate th e wet and dry contributions to refractivity and assess their impact on the analyzed temperature, surface pressure and specific humidity fields. We fin d significant zonal mean temperature, surface pressure and humidity differe nces between the 1DVar solutions and the ECMWF analyses reflecting biases b etween the GPS refractivities and ECMWF analyses. Large profile-to-profile temperature discrepancies in the tropical lower stratosphere are due to wav es not represented in the analyses. The 1DVar solution is generally drier t han ECMWF particularly in the southern subtropics. Lack of moisture above 3 00 hPa in the present model caused the solution to make large adjustments i n low latitude surface pressure and tropospheric temperatures to increase u pper troposphere densities and compensate for the missing upper level moist ure. The discrepancies between the solution and the background and observat ional data sets represent roughly a 2-sigma level of agreement rather than the 1-sigma level desired in a 1DVar solution. Given the simplicity of our error covariances, our results are promising as a first step. In the future , the error covariances need to be refined and, in particular, to vary with location.