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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 4, issue 6 | Copyright

Special issue: Observing atmosphere and climate with occultation techniques...

Atmos. Meas. Tech., 4, 1021-1026, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 09 Jun 2011

Research article | 09 Jun 2011

Processing of GRAS/METOP radio occultation data recorded in closed-loop and raw-sampling modes

M. E. Gorbunov1,2, K. B. Lauritsen2, H.-H. Benzon2, G. B. Larsen2, S. Syndergaard2, and M. B. Sørensen2 M. E. Gorbunov et al.
  • 1A. M. Obukhov Institute for Atmospheric Physics, Russian Academy of Sciences, Moscow, Russia
  • 2Danish Meteorological Institute, Copenhagen, Denmark

Abstract. Instrument GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) on-board of the Metop-A satellite was activated on 27 October 2006. Currently, Metop-A is a fully operational satellite with GRAS providing from 650–700 occultations per day. We describe our processing of GRAS data based on the modification of our OCC software, which was modified to become capable of reading and processing GRAS data. We perform a statistical comparison of bending angles and refractivities derived from GRAS data with those derived from ECMWF analyses. We conclude that GRAS data have error characteristics close to those of COSMIC data. In the height range 10–30 km, the systematic refractivity difference GRAS–ECMWF is of the order of 0.1–0.2 %, and the standard deviation is 0.3–0.6 %. In the lower troposphere GRAS refractivity and bending angle indicate a negative bias, which reaches its maximum value in the tropics. In particular the retrieved refractivity is biased by up to 2.5 %. The negative bias pattern is similar to that found in the statistical validation of COSMIC data. This makes it probable that the bias should not be attributed to the instrument design or hardware.

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