The forward model: σ-IASI


σ-IASI is a monochromatic radiative transfer model designed for fast computation of spectral radiance and its derivatives (Jacobian) with respect to a given set of geophysical parameters. It represents a compromise between the accuracy of the exact line-by-line radiative model and the fastness of the hyper-fast radiative transfer model. This compromise is achieved by means of a look-up table of pre-computed pressure dependent monochromatic optical depths and an interpolation procedure. The optical depth look-up-table take into account also effects depending on the gas concentration, such as the line self-broadening of the water vapour.

Current version

σ-IASI has been developed in fotran90 and the current version runs on linux and windows platform with the Intel Compiler. The current version has been optimized (details in Carissimo et al. 2009) and it has new features of computation of Jacobian with respect to CO2 and H2O continua.

It has been also updated with respect to the spectrocopy and radiative transfer. The newest Optical depth look-up-table has been built starting from the version 11.3 of LBLRTM. In this newest version the line parameters are obtained from the compilation aer_v_2.1 developed by AER Inc. of Massachusetts, USA. This line compilation is derived from HITRAN2004 and includes updates up to 01/01/2007 (Gordon et al. 2007). CO2 continuum and line shapes have been developed based on the line coupling parameters from Hartmann's group (Niro et al. 2005).

The current version performs analytical derivatives of radiative transfer equation with respect the following parameters:

  • Atmospheric Temperature
  • Surface Emissivity
  • Cloud Emissivity
  • Surface Temperature
  • Cloud Temperature
  • H2O Atmospheric profile
  • CO2 Atmospheric profile
  • O3 Atmospheric profile
  • N2O Atmospheric profile
  • CO Atmospheric profile
  • CH4 Atmospheric profile
  • H2O self continuum coefficients
  • H2O foreign continuum coefficients
  • CO2 foreign continuum coefficients.
  • Figure Caption
    Residual (Observed-σ-IASI Calcliated) for one of the IASI spectra (blue line) compared with the ±1σ IASI radiometric noise (red line). The light grey rectangle indicated the spectral regions used for the physical retrieval (details in Carissimo et al. 2009). In this case CH4 and CO profiles are from climatology.

    Created and Maintained by:
    Last Update: February 24 2010.

     Guido Masiello
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