Modeling the impact of the Earth's oblateness on the retrieval of temperature and pressure profiles from limb sounding

Abstract

Limb sounding observations of the Earth's atmosphere has recently been carried out by the GPS/MET experiment on board the American Microlab-1 satellite launched in April 1995 (e.g. Ware et al. , 1996). Utilizing real orbit ephemerides from the GPS/MET experiment, radio occultations have been simulated using a 3D ray tracing code, taking into account the non-spherical shape of the atmosphere due to the oblateness of the Earth. Subsequent inversion of the synthetic data shows that neglect of the Earth's oblateness in the retrieval of temperature profiles may cause a bias at altitudes below 40 km. From this altitude the temperature bias increases downwards, and can, in extreme cases, become as large as 3 K at 10 km altitude and 6 K at the ground. The size of the bias depends on the occultation geometry and the geographic latitude of the limb zone. A method to correct for the bias is proposed using an assumption of local spherical symmetry tangential to the ellipsoid. After a simple correction, temperature profiles are retrieved within 0.25 K (~0.1%) accuracy at the ground. It is shown that the orientation of the occultation plane has to be taken into account in the correction procedure if pressure profiles are to be retrieved to better than 0.4% accuracy.