Radio occultation bending angle and impact parameter errors caused by horizontal refractive index gradients in the troposphere: A simulation study
Radio occultation bending angle and impact parameter errors caused by horizontal refractive index gradients in the troposphere: A simulation study
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DOI：
10.1029/2001jd900050
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发表时间：
2001-06
影响因子：
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通讯作者：
S. Healy
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文献类型：
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作者：
S. Healy
无线电掩星（RO）弯曲角度和撞击参数值是从多普勒频移测量得出的，假设球对称。这项工作的目的是说明当该假设无效时会出现的错误。多普勒频移值已针对通过源自中尺度模型预测的三维折射率场的射线路径进行了模拟，该模型具有 12 公里 x 12 公里的水平网格，并包含水蒸气。然后将它们反转，做出球对称假设。结果表明，垂直于射线路径的折射率梯度会导致弯曲角度和冲击参数值的误差，但后者更为重要。结果表明，切点处的冲击参数值与导出值相差约 100 m。这可能会导致表面附近的有效弯曲角度误差超过 ∼ 10%。使用固定的航天器轨迹和切点位置，对通过 54 次中尺度预测进行模拟的水平梯度引起的误差进行了统计分析。一般来说，靠近表面的弯曲角度误差约为 3%。导出了一组新的误差分析表达式。这些基于沿着射线路径对水平梯度进行积分，并且发现与模拟结果非常一致。讨论了这项工作对将 RO 数据同化到数值天气预报模型中的影响，并概述了未来工作的领域。
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 three-dimensional refractive index field derived from a mesoscale model forecast, which has a horizontal grid of 12 km by 12 km, and includes water vapor. These have then been inverted, making the spherical symmetry assumption. It is demonstrated that refractive index gradients perpendicular to the ray path can cause errors in both the bending angle and impact parameter values, but the latter is the more significant. It is shown that the impact parameter value at the tangent point can differ by around ∼ 100 m from the derived value. This can cause an effective bending angle error exceeding ∼ 10% near the surface. A statistical analysis of the errors caused by horizontal gradients for simulations through 54 mesoscale forecasts, using fixed spacecraft trajectories and tangent point locations, is presented. In general, the bending angle errors are found to be ∼ 3% near the surface. A new set of analytical expressions for errors has been derived. These are based on integrating the horizontal gradients along the ray path 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 weather prediction models are discussed and areas of future work are outlined.