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. Global Navigation Satellite System (GNSS) radio occultation (RO) data enablethe retrieval of near-vertical profiles of atmospheric parameters likebending angle, refractivity, pressure, and temperature. The retrieval stepfrom bending angle to refractivity, however, involves an Abel integral withan upper limit of infinity. RO data are practically limited to altitudes belowabout 80 km and the observed bending angle profiles show decreasingsignal-to-noise ratio with increasing altitude. Some kind of high-altitudebackground data are therefore needed in order to perform this retrieval step(this approach is known as high-altitude initialization). Any bias in thebackground data will affect all RO data products beyond bending angle. Areduction of the influence of the background is therefore desirable – inparticular for climate applications. Recently a new approach for the production of GNSS radio occultationclimatologies has been proposed. The idea is to perform the averaging ofindividual profiles in bending angle space and then propagate the meanbending angle profiles through the Abel transform. Climatological products ofrefractivity, density, pressure, and temperature are directly retrieved fromthe mean bending angles. The averaging of a large number of profiles suppresses noise in the data,enabling observed bending angle data to be used up to 80 km withoutthe need for a priori information. Some background information for the Abelintegral is still necessary above 80 km. This work is a follow-up study, having the focus on the comparison of theaverage profile inversion climatologies (API) from the two processing centersWEGC and DMI, which study monthly COSMIC (Constellation Observing System forMeteorology, Ionosphere, and Climate) data from January to March 2011. Theimpact of different backgrounds above 80 km is tested, and differentimplementations of the Abel integral are investigated. Results are comparedfor the climatological products with ECMWF analyses, MIPAS, and SABERdata. It is shown that different implementations of the Abel integral have littleimpact on the API climatologies. On the other hand, different extrapolationsof the bending angle profile above 80 km play a key role in theresulting monthly mean refractivities above 35 km in altitude. Below thatrespective altitude the API climatologies show a good agreement between thetwo processing centers WEGC and DMI. Due to the downward propagation withinthe retrieval, effects of the high-altitude initialization lead todifferences in dry-temperature climatologies down to 20 km in altitude. When applying an exponential extrapolation to the bending angles above80 km at both centers, the dry-temperature climatologies agree amongWEGC, DMI, ECMWF analysis, and MIPAS up to 35 km in altitude within±0.5 K and up to 40 km in altitude within ±1 K. Weconclude that the API retrieval is a valid approach up to the lowerstratosphere. It is a computationally efficient alternative method forproducing dry atmospheric RO climatologies.

Comparison study of COSMIC RO dry-air climatologies based on average profile inversion