Evaluation of a 3D Radio Occultation Observation Operator

In the current numerical weather data assimilation systems, the radio occultation data are available in the form of bending angles or refractivity and the forward observation operators are one dimensional and implemented under the assumption of spherical symmetry neglecting the horizontal gradients of the atmospheric refractivity. In this study, we propose to implement and evaluate a 3D observation operator that considers the horizontal characteristics of the atmospheric model and includes a ray tracing model. We will evaluate the 3D operator against the available Spire radio occultation observations and outputs of the standard 1D operator and derive statistics to search for regimes where accuracy improvements are notable. This will allow us to prepare and implement a hybrid observation operator thus alleviating the computational costs associated with the use of the 3D operator itself.