A Comparison of Water Vapor Derived from GPS Occultations and Global Weather Analyses

Despite its fundamental importance in radiative transfer, atmospheric dynamics and the hydrological cycle, atmospheric water is inadequately characterized particularly at a global scale. Occultation measurements from the Global Positioning System (GPS) should imporve upon thids situation. Individual occultations yield profiles of specific humidity accurate to -0.2 gkg providing sensitive measurements of lower and middle r’ tropospheric water vapor with global coverage in a unique, all-weather, limb-viewing geometry with several hundred meter to a kilometer vertical resolution. We have derived water vapor profiles from June 21 to July 4, 1995 using GPS occultation data combined with global temperature analyses from the European Center for Medium-range Weather Forecasts (ECMWF) and reanalyses from the National Center for Environmental Prediction (NCEP). The zonal structure of the profiles exhibits basic climatological features of tropospheric moisture. Specific humidity biases between the GPS results and the ECMWF global humidity analyses in the midto upper troposphere are -0.1 g k g or less. Occultation results below 6 km altitude are generally h e r than those of ECMWF with the bias typically increasing toward warmer temperatures. Near the height of the tradewind inversion, the ECMWF analyses are significantly moister than the occultation results, which is associated vertical smoothing and over-extension of the top of the boundary layer in the analyses. The latitudinal dependence of the bias structure exhibits similarities to biases observed between the ECMWF analyses and radiosondes and between ECMWF and SSMA total column water vapor. Overall the occultation results are drier than the NCEP reanalyses with a marked exception near the Inter-Tropical Convergence Zone (ITCZ). Comparisons with t e classical 1963-1973 moisture climatology of Peixoto and Oort reveals that the occultation results are significantly