Effects of horizontal gradients on GPS radio occultation observation operators. I: Ray tracing

Global Positioning System (GPS) radio occultation (RO) bending angle and refractivity data provide information about the vertical structure of temperature and humidity in the earth's atmosphere. Assuming spherical symmetry in a GPS RO observation operator creates errors when calculating GPS RO observables from a background. These errors find their source in the presence of horizontal gradients of temperature and humidity. Tangent point drift (TPD) causes occulting rays to scan the atmosphere in various occultation planes. Horizontal gradients along the ray (HGR) cause the bending of each ray to be different from calculations that assume spherical symmetry. Using simulations, we show that HGR‐induced r.m.s. errors are larger than TPD‐induced r.m.s. errors in the lower troposphere only. TPD‐induced errors are more important in the stratosphere. We account for TPD and HGR using a multi‐plane two‐dimensional (2D) ray‐tracing observation operator. We show that bending angle and refractivity can be calculated more accurately with this 2D operator than an operator that assumes spherical symmetry. We use 6335 CHAMP and SAC‐C occultations as a validation. These occultations were processed by the Jet Propulsion Laboratory using geometrical optics and canonical transform methods. Our results indicate that analyses and 6‐hour forecasts can provide sufficiently accurate horizontal gradient information to the 2D operator. We show that the effects due to HGR can be satisfactorily simulated using ray tracing in one plane only. The effects due to TPD can be accounted for by considering a slanted profile which follows the tangent point trajectory. These results have important implications for the development of fast observation operators for data assimilation in numerical weather prediction. Copyright © 2004 Royal Meteorological Society