Evaluation of Refractivity Profiles from CHAMP and SAC-C GPS Radio Occultation

Abstract

Since their launch, CHAMP and SAC-C have collected thousands of GPS radio occultations. In order to evaluate the refractivity derived from these radio occultation measurements by the Jet Propulsion Laboratory (JPL) using geometrical optics, we compare CHAMP and SAC-C refractivity with refractivity calculated from the Data Assimilation Office (DAO) 6-hour forecast global fields of temperature and humidity. We show that the differences between DAO and CHAMP refractivity present biases and standard deviation very similar to those of the differences between DAO and SAC-C refractivity. We also demonstrate a seasonal cycle in the standard deviation of the differences for altitudes below 12 km. Furthermore, the histograms of the differences for CHAMP and SAC-C are very similar, which is important for future assimilation and climate studies using GPS radio occultation from various missions. The skewed distributions usually translate into an apparent refractivity bias.