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The Optimal Estimation Method is used to retrieve temperature and water  vapor profiles from simulated radio occultation measurements in order to  assess how different retrieval schemes may affect the assimilation of this  data. High resolution ECMWF global fields are used by a state-of-the-art  radio occultation simulator to provide quasi-realistic bending angle and  refractivity profiles.  Both types of profiles are used in the  retrieval process to assess their advantages and disadvantages. The  impact of the GPS measurement is expressed as an improvement over  the a priori knowledge (taken from a 24h old analysis). Large  improvements are found for temperature in the upper troposphere and  lower stratosphere. Only very small improvements are found in the  lower troposphere, where water vapor is present. Water vapor  improvements are only significant between about 1 km to 7 km. No  pronounced difference is found between retrievals based upon bending  angles or refractivity.  Results are compared to idealized  retrievals, where the atmosphere is spherically symmetric and  instrument noise is not included.  Comparing idealized to  quasi-realistic calculations shows that the main impact of a ray  tracing algorithm can be expected for low latitude water vapor,  where the horizontal variability is high. We also address the effect  of altitude correlations in the temperature and water vapor.  Overall, we find that water vapor and temperature retrievals using  bending angle profiles are more CPU intensive than  refractivity profiles, but that they do not provide significantly  better results

atmospheric chemistry and physics

Retrieval of temperature and water vapor profiles from radio occultation refractivity and bending angle measurements using an Optimal Estimation approach: a simulation study