The Use of RAOB data for the Validation of a New Method for the Derivation of the Water Vapor Content in the Atmosphere from GPS Radio Occultation Data
Benedetto, C. ; Vespe, F. ; et al.
Jan - 2003

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type: Conference Proceedings

Abstract
It was proposed a new method to derive the water vapor content in the atmosphere by using GPS Radio Occultation (GPS RO) without external information.. In particular with the proposed approach, the dry refractivity profiles are obtained fitting the the values of the outer troposphere layers (from h=h250K up to the stratopause) using the Hopfield dry atmosphere model. In this model the pressure P0 and temperature T0 for a radius R= Rearth, are the parameters to estimate. So the humidity profiles are extracted subtracting the dry refractivity given by the Hopfield model from the total values retrieved by GPS RO. The first results achieved applying such approach revealed to be promising, but a more extensive calibration and validation campaigns must be performed. In the present work we show the results of such calibration and validation activities. For the calibration we have used the radiowinsondes observations (RAOB) Ðnamely, temperature, relative humidity and total pressure- to reconstruct the total refractivity profiles. The proposed method has been applied to these refractivity profiles just in the same way made for GPS RO. In non linear least square fit of the dry profiles of the outer layers we have estimated not only P0 and T0, but the degree of the polytropic power as well, that usually is set = 4 in the standard Hopfield model. We have divided the globe in regions 30¡ x 30¡ wide. So, we obtained 72 cells and, for each of these, we have analyzed 10 RAOB observations at least, spanning over a whole year, both in the midday and in the midnight. The comparisons of the derived wet profiles with those of the RAOB are quite satisfying. The results confirm the validity of the proposed method. The work, furthermore, has outlined that the results are significantly improved if we adopt values of the degree of th polytropic slightly lower than 4 and it slightly varies with the period of the year. So we have corrected the Hopfield model in which the degree of the polytropic depends on latitude, longitude and the period of time.

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