The use of refractivity retrieved by radio occultation technique for the derivation of atmospheric water vapor content
Vespe, F. ; Benedetto, C. ; Pacione, R.
Jan - 2004

journal : Physics and Chemistry of the Earth,

Issue : 2-3
type: Article Journal

Abstract
Abstract The use of the GPS radio occultation technique (GPS RO) for retrieving physical and chemical parameters of the Earths atmosphere has been deeply and widely investigated over the last decade. The technique has proved to be particularly precise in retrieving temperature profiles with high vertical resolution (<1 km). Temperature profiles are obtained for the dry part of atmosphere solving for the system of two equations (Smith and Weintraub equation for refractivity and the hydrostatic equilibrium relationship) with two unknowns (hydrostatic pressure P and temperature T ). The system cannot be solved for the lower troposphere because water vapor pressure is not negligible. This makes it necessary to include some additional external information, such as humidity computed from meteorological analyses available from the European Center for Medium-Range Weather Forecasting (ECMWF) or the National Center for Environmental Prediction (NCEP), or zenith troposphere delays estimated from GPS ground station measurements [IAG2001 Scientific Assembly (2001); J. Geophys. Res. 102(D21) (2001) 27,221]. This investigates the possibility of retrieving humidity using the refractivity profiles derived from GPS RO data only. In particular with the proposed method, the dry pressure profiles are obtained fitting the refractivity of the outer troposphere layers (from h=h 250 K up to the stratopause) using the Hopfield dry atmosphere model [J. Geophys. Res. 74(18) (1969) 4487], in which ground pressure and temperature are the parameters to be estimated. In the second step humidity profiles are extracted by subtracting the dry refractivity given by the estimated Hopfield model from the values (wet + dry) retrieved by GPS RO. The refractivity profiles used in the work were retrieved from CHAMP GPS RO data [Geophys. Res. Lett. 28 (2001) 3263]. The humidity profiles obtained with the proposed technique have been compared and validated with profiles retrieved from radwinsonde observations (RAOB) over eight sites. The results are promising in that the new method seems to retrieve profiles comparable to those obtained using the method currently in use (TAO 11(1) (2000) 53).

keywords : Atmospheric remote sensing,Pressure,Radio occultation,Temperature,Water vapor