Modelling of atmospheric mid-infrared radiative transfer: the AMIL2DA algorithm intercomparison experiment

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Abstract

When retrieving atmospheric parameters from radiance spectra, the forward modelling of radiative transfer through the Earth's atmosphere plays a key role, since inappropriate modelling directly maps on to the retrieved state parameters. In the context of pre-launch activities of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) experiment, which is a high resolution limb emission sounder for measurement of atmospheric composition and temperature, five scientific groups intercompared their forward models within the framework of the Advanced MIPAS Level 2 Data Analysis (AMIL2DA) project. These forward models have been developed, or, in certain respects, adapted in order to be used as part of the groups’ MIPAS data processing. The following functionalities have been assessed: the calculation of line strengths including non-local thermodynamic equilibrium, the evaluation of the spectral line shape, application of chi-factors and semi-empirical continua, the interpolation of pre-tabulated absorption cross sections in pressure and temperature, line coupling, atmospheric ray tracing, the integration of the radiative transfer equation through an inhomogeneous atmosphere, the convolution of monochromatic spectra with an instrument line shape function, and the integration of the incoming radiances over the instrument field of view.

Introduction

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) [1], [2], [3] on the Envisat Earth observation satellite developed by the European Space Agency (ESA) is a mid-infrared limb emission sounder which provides vertical profiles of atmospheric species relevant to several inter-linked problems in ozone chemistry and global change. Routine data analysis under ESA responsibility covers only six the so-called key species (H2O, O3, N2O, CH4, HNO3, NO2) as well as pressure and temperature. The MIPAS data, however, contain much more information on further atmospheric trace species of very high scientific value. Thus, exploitation of MIPAS data with respect to (a) the retrieval of many more species relevant to atmospheric research such as the complete nitrogen family, chlorine source gases and reservoirs, greenhouse gases, ozone precursors, aerosols, etc., and (b) the retrieval of key species abundance profiles by methods of higher sophistication than affordable under near real time processing constraints are of high interest for better understanding of ozone chemistry and global change. In order to support the development and characterization of methods and tools for this extended analysis of MIPAS data, the project AMIL2DA (Advanced MIPAS Level 2 Data Analysis) has been initiated.

When atmospheric parameters are retrieved from atmospheric radiance spectra, accurate forward modelling of radiative transfer through the Earth's atmosphere is of particular importance, since forward modelling errors directly map on to the retrieved quantities. Intercomparison studies are a standard approach to assess the reliability of radiative transfer models [4], [5], [6]. Hence, a major component of the AMIL2DA project is the comparison of the radiative transfer (forward) models used by each group for their retrievals.

Section snippets

Theory

Disregarding scattering, which is justified in many applications of infrared remote sensing, the monochromatic radiative transfer equation [7] can be written asLν=L0,ντν(lobs,0)+l=0lobsJν(T(l))∂τν(lobs,l)∂ldl,where Lν is spectral radiance at wavenumber ν at the location of the observer lobs,L0 is background radiance (assumed zero in this intercomparison experiment), l is the path coordinate, τν(l1,l2) is the spectral transmittance between two locations l1 and l2, and J is the source function,

The models

Five groups participated in this intercomparison experiment, each with their own line-by-line radiative transfer model: The Karlsruhe Optimized and Precise Radiative transfer code (KOPRA) [13], [14] of the Forschungszentrum Karlsruhe, the Reference Forward Model (RFM) of Oxford University (http://www.atm.ox.ac.uk/RFM/), a direct integration radiative transfer code by DLR (MIRART, Modular Infrared Atmospheric Radiative Transfer [15]), the Optimized Forward Model (OFM) by IFAC (formerly IROE) [16]

The intercomparison experiment

In order to detect forward model deficiencies, a cross comparison exercise was carried out to mutually validate radiative transfer codes. The idea of the setup of the intercomparison exercise was to start from simple settings proving the basic functionalities of the radiative transfer codes, and then to proceed to more complex and realistic scenarios. In order to avoid any masking of differences in the basic functionalities of the codes by additional sophistication introduced by realistic

Conclusion

The intercomparison experiment was quite enlightening for all participants. The overall interconsistency of spectra is good, except for test cases where functionalities were required which were beyond the specification of some of the codes, e.g. line coupling, non-local thermodynamic equilibrium, etc. Some differences in details are caused by different implementations of the Voigt line shape and related approximations. Different approaches to calculate the line intensity as a function of

Acknowledgements

AMIL2DA is a Shared Cost Action within the RTD generic activities of the 5th FP EESD Programme of the European Commission, Project EVG1-CT-1999-00015. M.L.-P. has been partially supported by PNE under contract PNE-017/2000-C. P. Varanasi gave access to spectroscopic cross-section data prior to their publication in HITRAN. Furthermore thanks go to J.-M. Flaud and G. Schwarz, who provided helpful comments to the manuscript.

References (53)

  • A.K Hui et al.

    Rapid computation of the Voigt and complex error functions

    JQSRT

    (1978)
  • F Schreier

    The Voigt and complex error functiona comparison of computational methods

    JQSRT

    (1992)
  • K.V Chance et al.

    The Smithsonian astrophysical observatory database 1992

    JQSRT

    (1994)
  • N Jacquinet-Husson et al.

    The 1997 spectroscopic GEISA databank

    JQSRT

    (1999)
  • H.M Pickett et al.

    Submillimeter, millimeter, and microwave spectral line catalog

    JQSRT

    (1998)
  • P Varanasi et al.

    Thermal infrared absorption coefficients of CFC-12 at atmospheric conditions

    JQSRT

    (1994)
  • M López-Puertas et al.

    Non-local thermodynamic equilibrium limb radiances for the MIPAS instrument on ENVISAT-1

    JQSRT

    (1998)
  • M Endemann et al.

    Envisat's high-resolution limb sounderMIPAS

    ESA Bull

    (1993)
  • Endemann M, Gare P, Smith D, Hoerning K, Fladt B, Gessner R. MIPAS design overview and current development status. In:...
  • Fischer H, Oelhaf H. Remote sensing of vertical profiles of atmospheric trace constituents with MIPAS limb emission...
  • Fischer H, Anderson GP, von Clarmann T, Clough SA, Coffey MT, Goldman A, Kneizys FX. Intercomparison of transmittance...
  • Sherlock VJ. Results from the first UKMO IASI radiative transfer model intercomparison. Technical Report, Numerical...
  • L Garand et al.

    Intercomparison of radiative transfer codes applied to HIRS and AMSU channels

  • S Chandrasekhar

    Radiative transfer

    (1960)
  • B Edlen

    The refractive index of air

    Metrologia

    (1966)
  • A.R Curtis

    A statistical model for water-vapour absorption

    Q J R Meteorol Soc

    (1952)
  • Cited by (0)

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