A simplified approach for generating GNSS radio occultation refractivity climatologies
The possibility of simplifying the retrieval scheme required to produce GNSS radio occultation refractivity climatologies is investigated. In a new, simplified retrieval approach, the main statistical analysis is performed in bending angle space and an estimate of the average bending angle profile is then propagated through an Abel transform. The average is composed of means and medians of ionospheric corrected bending angles up to 80 km. Above that, the observed profile is exponentially extrapolated to infinity using a fixed a priori scale height. The new approach circumvents the need to introduce a "statistical optimisation" processing step in which individual bending angle profiles are merged with a priori data, often taken from a climatology. This processing step can be complex, difficult to interpret, and is generally recognised as a potential source of structural uncertainty. The new scheme is compared with the more conventional approach of averaging individual refractivity profiles, produced with the implementation of statistical optimisation used in the EUMETSAT Radio Occultation Meteorology Satellite Application Facility (ROM SAF) operational processing. It is shown that the two GNSS radio occultation climatologies agree to within 0.1% from 5 km up to 35–40 km, for the three months January, February, and March 2011. During this time period, the new approach also produces slightly better agreement with ECMWF analyses between 40–50 km, which is encouraging. The possible limitations of the new approach caused by mean residual ionospheric errors and low observation numbers are discussed briefly, and areas for future work are suggested.