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Description
A new methodology (called MAMA) for the computation of spectrally resolved upwelling radiances in the presence of atmospheric diffusive layers is presented. The algorithm is developed as a new analysis tool for application to interferometric data for the study of the atmospheric components. It offers fast and accurate radiance simulations of the Earth’s entire longwave emission spectrum, particularly excelling in scenarios involving optically thin scattering layers like cirrus clouds or aerosols. The solution is obtained through a simplification of the multiple scattering term in the general equation of the radiative transfer assuming the plane parallel approximation.
A new property of the layer is introduced, named angular back-scattering coefficient, which describes the fraction of radiation coming from a hemisphere and back-scattered into a specific direction (the observer in our case). This property, easily derived from the phase function of the particle size distribution, can be calculated from any generic single scattering properties database which allows for simple upgrades of the reference optical properties within the code. This work presents the solutions for mean upward and downward ambient radiance assuming a general plane-parallel geometry. To assess the algorithm performances, results obtained with the MAMA solution are compared with those derived with a reference discrete-ordinate radiative transfer model (DISORT) and with other scaling methodologies. The results highlight that, for liquid water clouds, the code accuracy is mostly within 0.4 mW/(m2cm−1) with respect to the reference code both at far and mid infrared wavelengths. The presence of ice clouds is accurately simulated at mid infrared for all realistic cloud cases, which makes this algorithm suitable for the analysis of any spectral measurements of current satellite infrared sounders. At far infrared, the MAMA accuracy is excellent when ice clouds with optical depth less than 2 are considered, which is particularly valuable since cirrus clouds are one of the main targets of the future mission FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) of the European Space Agency. Unusual cases which account for ice clouds with small particle sizes (effective radius mostly less than 15 um) and large optical depths (larger than 3) provides discrepancies that with respect the full physics code can be larger.
