Extension of radiative transfer code MOMO, matrix-operator model to the thermal infrared - Clear air validation by comparison to RTTOV and application to CALIPSO-IIR

- We present the extension of radiative transfer code MOMO, Matrix operator Model, to the thermal infrared.
- The emission of radiation by gases, aerosols and clouds is implemented in the matrix operator algorithm.
- A spectroscopy code named CGASA is integrated and simulates the water vapor continuum of absorption.
- The validation is done by comparisons to CALIPSO-IIR measurements and to simulations with RTTOV.

1-D radiative transfer code Matrix-Operator Model (MOMO), has been extended from [0.2−3.65μm] the band to the whole [0.2−100μm] spectrum. MOMO can now be used for the computation of a full range of radiation budgets (shortwave and longwave). This extension to the longwave part of the electromagnetic radiation required to consider radiative transfer processes that are features of the thermal infrared: the spectroscopy of the water vapor self- and foreign-continuum of absorption at 12μm and the emission of radiation by gases, aerosol, clouds and surface. MOMO׳s spectroscopy module, Coefficient of Gas Absorption (CGASA), has been developed for computation of gas extinction coefficients, considering continua and spectral line absorptions. The spectral dependences of gas emission/absorption coefficients and of Planck׳s function are treated using a k-distribution. The emission of radiation is implemented in the adding-doubling process of the matrix operator method using Schwarzschild׳s approach in the radiative transfer equation (a pure absorbing/emitting medium, namely without scattering). Within the layer, the Planck-function is assumed to have an exponential dependence on the optical-depth. In this paper, validation tests are presented for clear air case studies: comparisons to the analytical solution of a monochromatic Schwarzschild׳s case without scattering show an error of less than 0.07% for a realistic atmosphere with an optical depth and a blackbody temperature that decrease linearly with altitude. Comparisons to radiative transfer code RTTOV are presented for simulations of top of atmosphere brightness temperature for channels of the space-borne instrument MODIS. Results show an agreement varying from 0.1 K to less than 1 K depending on the channel. Finally MOMO results are compared to CALIPSO Infrared Imager Radiometer (IIR) measurements for clear air cases. A good agreement was found between computed and observed radiance: biases are smaller than 0.5 K and Root Mean Square Error (RMSE) varies between 0.4 K and 0.6 K depending on the channel. The extension of the code allows the utilization of MOMO as forward model for remote sensing algorithms in the full range spectrum. Another application is full range radiation budget computations (heating rates or forcings).