Tropospheric watervapor imaging by combination of spaceborne and ground-based GNSS sounding data

The excess path delay of Global Navigation Satellite System (GNSS) signals caused by the neutral atmosphere can be measured with high accuracy (about 1 mm) given the exact position of the spaceborne transmitter and the ground-based receiver (and a standard correction for dispersive ionospheric effects). The excess path delay can be decomposed into a (larger) 'hydrostatic' component caused by the induced polarization of all atmospheric constituents and (smaller) 'wet' component introduced by the orientation polarization due to the polar water molecules. The hydrostatic delay in zenith direction can be determined accurately, given surface pressure measurements. It can be mapped onto an arbitrary ray path employing an appropriate mapping function. We present a new mapping function based on geometric considerations. The remaining wet delay is proportional to slant integrated water vapor (SIWV). Vertical water vapor density profiles can be obtained by GNSS based active limb sounding employing the radio occultation technique. This method measures the excess phase path of GNSS-transmitted radio waves caused by refraction during passage through the earth's neutral atmosphere and ionosphere in limb sounding geometry. Tomographic (height resolving) imaging becomes feasible when data from the two different information sources are combined. We developed an imaging technique for tropospheric water vapor by combining SIWV data with occultation-derived profile information using optimal estimation. The algorithm was tested using simulated data derived from atmospheric analyses of the European Center for Medium-range Weather Forecasts (ECMWF). The retrieval results show the ability of recovering two-dimensional water vapor density information. In areas with high absolute humidity the relative error is generally smaller than 10 percent up to a height of about 5 km. (author)