Preprints
https://doi.org/10.5194/gmd-2020-424
https://doi.org/10.5194/gmd-2020-424

Submitted as: model description paper 15 Feb 2021

Submitted as: model description paper | 15 Feb 2021

Review status: this preprint is currently under review for the journal GMD.

A new gas absorption optical depth parameterisation for RTTOV v13

James Hocking1, Jérôme Vidot2, Pascal Brunel2, Pascale Roquet2, Bruna Silveira2, Emma Turner1, and Cristina Lupu3 James Hocking et al.
  • 1Met Office, Fitzroy Road, Exeter, EX1 2PB, UK
  • 2CNRM, Université de Toulouse, Météo-France, CNRS, Lannion, France
  • 3ECMWF, Shinfield Park, Reading, UK

Abstract. This paper describes a new gas optical depth parameterisation implemented in the most recent release, version 13, of the radiative transfer model RTTOV (Radiative Transfer for TOVS). RTTOV is a fast, one-dimensional radiative transfer model for simulating top-of-atmosphere visible, infrared and microwave radiances observed by downward-viewing space-borne passive sensors. A key component of the model is the fast parameterisation of absorption by the various gases in the atmosphere. The existing parameterisation in RTTOV has been extended over many years to allow for additional variable gases in RTTOV simulations and to account for solar radiation and better support geostationary sensors by extending the validity to higher zenith angles. However, there are limitations inherent in the current approach which make it difficult to develop it further, for example by adding new variable gases. We describe a new parameterisation that can be applied across the whole spectrum, allows for a wide range of zenith angles in support of solar radiation and geostationary sensors, and for which it will be easier to add new variable gases in support of user requirements. Comparisons against line-by-line radiative transfer simulations, and against observations in the ECMWF operational system yield promising results, suggesting that the new parameterisation generally compares well with the old one in terms of accuracy. Further validation is planned, including testing in operational numerical weather prediction data assimilation systems.

James Hocking et al.

Status: open (until 12 Apr 2021)

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James Hocking et al.

James Hocking et al.

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Short summary
RTTOV is a fast radiative transfer model for simulating passive satellite-based observations at visible, infra-red and microwave wavelengths. A core part of the model is a parameterisation of the absorption of radiation by the various gases present in the atmosphere. We present a new parameterisation that performs well compared to the existing one in terms of accuracy and can be developed further more easily. The new parameterisation is implemented in the latest release, RTTOV v13.0.