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The development of the Global Position System (GPS) satellite network provides new opportunities to
characterize atmospheric parameters using innovative techniques. The GPS Radio Occultation Technique
(GPS RO) is one of the most recent and promising atmospheric remote sensing technique applied to GPS
measurements. The GPS RO technique allows obtaining profiles of refractivity, temperature, pressure and
water vapor in the neutral atmosphere and electron density in the ionosphere. In the last years, other
missions confirmed the RO efficiency, like GPS/MET, COSMIC (Constellation Observing System for
Meteorology, Ionosphere, and Climate), Formosa Satellite Mission 3 and the last Radio Occultation
Sounder Antenna for the Atmosphere.
In this work, water vapor mixing ratio profiles retrieved from COSMIC observations are presented and
validated using ground based water vapor Raman lidar profiles. As far as we know, this is the first time
water vapor mixing ratio profiles provided by COSMIC are compared with a ground based Raman lidar.
COSMIC profiles used in this study are retrieved applying a one-dimensional variational method that make
use of ECMWF low resolution analysis data as a guess of atmospheric water vapor. Raman lidar
measurements of the water vapor mixing ratio profiles are provided by PEARL (Potenza EArlinet Raman
Lidar) system running at CIAO, located in Potenza, South Italy.
Performance of COSMIC retrieval are studied over a period of one year (2008) of systematic water vapor
Raman lidar measurements. A possible strategy for reducing the impact of the co-location mismatch
between satellite footprint and the lidar station is presented and the problem of the vertical resolution of
COSMIC profiles respect to the Raman lidar is also discussed.
The statistical analysis for the selected cases shows good performance of COSMIC in the identification of
the vertical gradients of the water vapor field, even though the average difference between the Raman lidar
and the COSMIC profiles suggests that caution should be taken in using COSMIC data as an absolute or
reference measurement of water vapor, in particular in the low and middle troposphere.
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