Radio occultation bending angle and impact parameter errors caused by horizontal refractive index gradients in the troposphere: A simulation study

Radio occultation (RO) bending angle and impact parameter values are derive d from a Doppler shift measurement, assuming spherical symmetry. The purpos e 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 throug h a three-dimensional refractive index field derived from a mesoscale model forecast, which has a horizontal grid of 12 km by 12 km, and includes wate r vapor. These have then been inverted, making the spherical symmetry assum ption. It is demonstrated that refractive index gradients perpendicular to the ray path can cause errors in both the bending angle and impact paramete r values, but the latter is the more significant. It is shown that the impa ct parameter value at the tangent point can differ by around similar to 100 m from the derived value. This can cause an effective bending angle error exceeding N 10% near the surface. A statistical analysis of the errors caus ed by horizontal gradients for simulations through 54 mesoscale forecasts, using fixed spacecraft trajectories and tangent point locations, is present ed. In general, the bending angle errors are found to be similar to 3% near the surface. A new set of analytical expressions for errors has been deriv ed. These are based on integrating the horizontal gradients along the ray p ath and are found to be in good agreement with the simulation results. The implications of this work for the assimilation of RO data into numerical we ather prediction models are discussed and areas of future work are outlined .