A comparison of water vapor derived from GPS occultations and global weather analyses

Despite its fundamental importance in radiative transfer, atmospheric dynam ics, and the hydrological cycle, atmospheric water is inadequately characte rized particularly at a global scale. Occultation measurements from the Glo bal Positioning System (GPS) should improve upon this situation. Individual occultations yield profiles of specific humidity accurate to 0.2 to 0.5 g/ kg providing sensitive measurements of lower and middle tropospheric water vapor with global coverage in a unique, all-weather, limb-viewing geometry with several hundred meters to a kilometer vertical resolution. We have der ived water vapor profiles from June 21 to July 4, 1995, using GPS occultati on data combined with global temperature analyses from the European Center for Medium-Range Weather Forecasts (ECMWF) and reanalyses from the National Centers for Environmental Prediction (NCEP). The zonal mean structure of t he profiles exhibits basic climatological features of tropospheric moisture . Specific humidity biases between the GPS results and the ECMWF global hum idity analyses in the middle to upper troposphere are similar to0.1 g/kg or less. Occultation results below 6 km altitude are generally drier than tho se of ECMWF with the bias generally increasing toward warmer temperatures. Near the height of the trade wind inversion, the ECMWF analyses are signifi cantly moister than the occultation results due to vertical smoothing and o verextension of the boundary layer top in the analyses. Overall, the occult ation results are drier than the NCEP reanalyses with a marked exception ne ar the Intertropical Convergence Zone (ITCZ) where occultation results are wetter by more than 10%. The occultation results are significantly wetter n ear the ITCZ and drier in the subtropics than the classical moisture climat ology of Peixoto and Oort. Similarities between the NCEP and the Peixoto an d Oort near-ITCZ differences suggest that a common analysis/model problem m ay be responsible. The generally wetter Peixoto and Oort results in the sub tropics are due in part to moist radiosonde biases. Discrepancies between t hese data sets are significant and limit our ability to resolve uncertainti es in moisture control and feedbacks in a changing climate.