Effects of horizontal gradients on GPS radio occultation observation operators. II: A Fast Atmospheric Refractivity Gradient Operator (FARGO)

Previous works have suggested that strong atmospheric horizontal gradients need to be accounted for in the simulation of Global Positioning System radio occultation (GPS RO) refraction measurements. The contribution of such horizontal gradients to GPS RO observables can be calculated from a background with a multi‐plane two‐dimensional (2D) ray tracer. In part I we showed that the effects of horizontal gradients can be separated into tangent point drift (TPD) and horizontal gradients along the ray (HGR). We present here a fast observation operator called FARGO. FARGO incorporates both TPD and HGR effects but presents computation times 48 times faster than 2D ray tracing for calculating bending angle and more than 250 times faster for refractivity. TPD effects are simulated using slanted profiles while the HGR correction is applied in one occultation plane only and limited to the vicinity of the tangent point. When compared to observation operators which assume spherical symmetry, FARGO allows for reductions in the r.m.s. of the differences between observed and calculated bending angles and refractivities which are similar to those obtained with 2D ray tracing. Such reductions of the observed minus calculated refractivity can reach 80% for an occultation which occurred perpendicular to a tropospheric refractivity ridge (axis of high values) along a warm sector containing high water vapour amounts. We found that in about 5% of the cases, accounting for horizontal gradients via FARGO reduces the differences from real data by more than a half. The method used by FARGO to account for horizontal gradients can be extended for simulating refraction measurements collected by other instruments. Copyright © 2004 Royal Meteorological Society