Parameterization of single scattering properties of mid-latitude cirrus clouds for fast radiative transfer models using particle mixtures

A new parameterization of single scattering properties has been developed for mid latitude cirrus clouds, to be used in weather prediction and global circulation models. Ice clouds are treated as a mixture of ice crystals of different habits. Bulk optical properties of ice crystals are parametrized as function of the effective dimension (De) of measured particle size distributions that are representative of mid latitude cirrus clouds. De is itself parametrized as function of temperature and ice water content. The paper describes the results obtained with a stand-alone version of the radiation routine of the COSMO model and an initial test with the full forecast model on a complex meteorological situation over Europe

The MAMA Algorithm for Fast Computations of Upwelling Far- and Mid-Infrared Radiances in the Presence of Clouds

A methodology for the computation of spectrally resolved upwelling radiances in the presence of atmospheric diffusive layers is presented. The algorithm, called MAMA (Martinazzo–Maestri), provides fast simulations over the whole longwave spectrum, with high accuracy, particularly for optically thin scattering layers like cirrus clouds. The solution is obtained through a simplification of the multiple-scattering term in the general equation of the radiative transfer in a plane-parallel assumption. The scattering contribution is interpreted as a linear combination of the mean ambient radiances involved in the forward and back-scatter processes, which are multiplied by factors derived from the diffusive features of the layer. For this purpose, a fundamental property of the layer is introduced, named the 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. The paper discusses the solutions for mean upward and downward ambient radiances and their use in the simplification of the general radiative transfer equation for thermal infrared. To assess the algorithm performance, the results obtained with the MAMA code are compared with those derived with a discrete ordinate-based radiative transfer model for a large range of physical and optical properties of ice and liquid water clouds and for multiple atmospheric conditions. It is demonstrated that, for liquid water clouds, the MAMA code accuracy is mostly within 0.4 mW/(m2cm−1sr) with respect to the reference code both at far- and mid-infrared wavelengths. Ice cloud spectra are also accurately simulated at mid-infrared for all realistic cloud cases, which makes the MAMA code 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 an optical depth of 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. In summary, the MAMA method allows computations of cloudy sky high-resolution radiances over the full longwave spectrum (4–100 μm) in less than a second (for pre-computed gas optical depths and on a standard personal computer). The algorithm exploits the fundamental properties of the scattering layers, and the code can be easily updated in relation to new scattering properties

Fast radiative transfer in multiple scattering atmospheres at far and mid infrared wavelengths

Recognizing the value of Far Infrared Region (FIR) observations, in September 2019, the European Space Agency (ESA) selected FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) as the 9th Earth Explorer (EE9) (Palchetti et al., 2020) whose launch is foreseen in 2027. FORUM, dedicated to mapping Earth’s far-infrared emission globally, will produce an enormous quantity of new data, requiring the implementation of fast radiative transfer models applicable to the entire IR spectral region for an effective data exploitation and analysis. Full physics models (i.e. DISORT, Stamnes et al., 1988) rely on robust and accurate numerical methodologies to solve the radiance field in presence of multiple scattering events for specific scenarios. The complexity of the multiple scattering effects makes this class of models extremely time consuming and inappropriate for large dataset analysis. To save computational time, fast radiative transfer models adopt multiple strategies which might account for approximation of the physical problem, simplified numerical solutions, code parallelization, and the extensive use of parametrizations. In the first part of this study, we investigate the level of accuracy of the Chou’s approximation (Chou et al., 1999), a fast methodology, widely used in operative frameworks for its simplicity and easy implementation. The performance of this approximate solutions is evaluated with respect to a full-physics approach over a widespread collection of atmospheric scenarios using the goal NESR of FORUM as reference metric. The results show not negligible inaccuracies when the Far InfraRed (FIR) is considered (Martinazzo et al., 2021). In the second part the study, to reduce the bias of the Chou scaling method, a correction term is modelled and computed using the solution recently proposed by Tang (Tang et al. 2018). The Tang methodology, originally created to refine the Chou flux computations, is here adapted to simulations of radiance fields over the FIR spectral range, exploiting appropriate multiplicative coefficients. The range of validity of the new methodology is then evaluated, as already done for the Chou scheme, by comparing this fast solution against full physics simulations. The comparisons show an overall reduction of the radiance residuals overs most of the cloudy cases encountered in nature. In particular, the use of the Tang methodology with the new coefficients is accurate for the computation of radiance fields in presence of thin cirrus clouds which are one of the targets of the FORUM mission

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