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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/amt-2020-147
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-147
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 12 May 2020

Submitted as: research article | 12 May 2020

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This preprint is currently under review for the journal AMT.

Generalized Canonical Transform method for radio occultation sounding with improved retrieval in the presence of horizontal gradients

Michael Gorbunov1,2, Gottfried Kirchengast3, and Kent B. Lauritsen4 Michael Gorbunov et al.
  • 1A.M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzevsky per. 3, 119017, Moscow, Russia
  • 2Hydrometcenter of Russia, Bol. Prechistensky per. 11-13, 123242, Moscow, Russia
  • 3Wegener Center for Climate and Global Change (WEGC) and Institute for Geophysics, Astrophysics, and Meteorology/Institute of Physics, University of Graz, Brandhofgasse 5, 8010, Graz, Austria
  • 4Danish Meteorological Institute, 2100, Copenhagen, Denmark

Abstract. By now, a series of advanced Wave Optical (WO) approaches to the processing of Radio Occultation (RO) observations are widely used. In particular, the Canonical Transform (CT) method and its further developments need to be mentioned. The latter include the Full Spectrum Inversion (FSI) method, the Geometric Optical (GO) Phase Matching (PM) method, and the general approach based on the Fourier Integral Operators (FIOs), also referred to as the CT type 2 (CT2) method. The general idea of these methods is the application of a canonical transform that changes the coordinates in the phase space from time and Doppler frequency to impact parameter and bending angle. For the spherically symmetric atmosphere, the impact parameter, being invariant for each ray, is a unique coordinate of the ray manifold. Therefore, the derivative of the phase of the wave field in the transformed space is directly linked to the bending angle, as a single-valued function of the impact parameter. However, in the presence of horizontal gradients, this approach may not work. Here we introduce a further generalization of the CT methods in order to reduce the errors due to horizontal gradients. We describe, in particular, the modified CT2 method denoted CT2A, which complements the former with one more affine transform: a new coordinate that is a linear combination of the impact parameter and bending angle. The linear combination coefficient is a tunable parameter. We derive the explicit formulas for the CT2A and develop the updated numerical algorithm. For testing the method, we performed statistical analyses based on COSMIC RO retrievals and (collocated) ECMWF analysis profiles. We demonstrate that it is possible to find a reasonably optimal value of the new tunable CT2A parameter that mitigates systematic errors in the lower troposphere and allows the practical realization of the improved capability to cope with horizontal gradients.

Michael Gorbunov et al.

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Michael Gorbunov et al.

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Short summary
By now, the Canonical Transform (CT) approach to the processing of Radio Occultation (RO) observations is widely used. For the spherically symmetric atmosphere, the applicability of this method can be strictly proven. However, in the presence of horizontal gradients, this approach may not work. Here we introduce a generalization of the CT method in order to reduce the errors due to horizontal gradients.
By now, the Canonical Transform (CT) approach to the processing of Radio Occultation (RO)...
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