Implementation of a ray-tracing operator for ground-based GPS Slant Delay observation modeling

[1] Implementation of an observation-modeling algorithm is necessary in order to make use of Slant Delay (SD) observations, processed from ground-based Global Positioning System (GPS) measurements, in Numerical Weather Prediction (NWP). This article introduces an algorithm for SD observation modeling. The algorithm is an extension of radio-occultation bending-angle observation modeling to ground-based GPS meteorology, and it is based on ray-tracing in a two-dimensional plane. The new algorithm is implemented in the framework of the High Resolution Limited Area Model (HIRLAM) in order to allow comparison with a reference model, that has been published earlier. Only insignificant differences are found between observation minus model background (OmB) statistics of the two implementations. In terms of computational efficiency, the new approach is found to be cheaper to apply in the NWP systems currently in use. However, the computational efficiency of the new implementation is shown to rapidly increase with increasing NWP model resolution.