Radio occultation bending angle errors caused by horizontal gradients: a simulation study
HEALY, S.B.
Radio occultation (RO) bending angle and impact parameter values are derived from a Doppler shift measurement, assuming spherical symmetry. The purpose of this work is to illustrate the errors that arise when this assumption is not valid. Doppler shift values have been simulated for ray paths through a 3-D refractive index field derived from a mesoscale model forecast, with a Runge-Kutta-Fehlberg numerical solution of the ray equations. These have then been inverted, making the spherical symmetry assumption. The results indicate that the errors are significant. It is shown that the impact parameter value at the tangent point can differ by around approximately 100 m from the derived value, which is equivalent to a bending angle error of approximately 1-2%, calculated using a 'fast forward model'. It is also demonstrated that refractive index gradients perpendicular to the ray path cause errors in both the bending angle and impact parameter values. A set of analytical expressions for the errors has been derived, giving values in good agreement with the simulation results. The implications of this work for the assimilation of RO data into numerical weather prediction models are discussed and areas of future work are outlined.
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