| CRTM Radiative Transfer Schemes |
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Advanced Doubling and Adding |
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Advanced doubling-adding ( ADA) method is an improved version of doubling-adding (DA) method and computes the multiple scattering much fast (Liu and Weng, 2006, JAS). The doubling-adding method has not been widely used for operational retrievals or data assimilation due to its huge demand on computational resource and is often for accurate and detailed radiative transfer calculations in the field of research and education. The significant advantage of the ADA is its analytical expression replacing the most complicated thermal source terms in the doubling-adding method. It is running about 60 times faster than tradictional DA with overall accuracy of less than 0.01K. This radiative transfer solver is now a place-holder algorithm for CRTM.
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Successive Order of Interaction |
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The successive order of interaction (SOI) radiative transfer model is developed by Heidinger et al., (2005, JAM) . The method takes some advantages from the truncated doubling method for a single layer and the successive order of interaction for the vertical integration. The truncation in doubling procedure is a good approximation for the infrared and microwave spectra. The technique is flexible and useful for the radiative transfer calculations. For the integration over the vertical layers, SOI method takes the similarities between successive orders of scatterings (SOS) and SOI. The solution is obtained when the increment to the solution is within predefined threshold. This is a candidate algorithm for CRTM solver |
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Delta 4 stream |
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A polarized Delta-4-stream model has been developed by Liou et al. (2005, JAS) for for thermal and microwave radiative transfer. It is an extension of four-stream approximation which is originally for flux computations (Liou 1992). The model can handle the polarization from clouds in an analytic form. Delta truncation is applied to reduce phase function to 4 expansion terms. The optical depth and single scattering albedo as well as the expansion coefficients are rescaled to take account for any strong forward scattering. The delta 4-stream method computes transmission and reflection for each layer, analytically. The layer source function is approximately obtained by the layer emissivity multiplied with the Planck function of a median temperature of the layer. Finally, adding method is used to integrate the surface and the vertical layers. This is a candidate algorithm for CRTM solver
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Discrete Ordinate Tangent Linear |
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Discrete ordinate tangent linear radiative transfer (DOTLRT) has been developed by Voronovich et al. (2004, IEEE). Different from traditional discrete ordinate method (DISORT), DOTLRT symmetries matrices in the solution of the radiative transfer equation. It makes the solution numerically stable. A remarkable benefit using the symmetric matrix is that the derivative of eigenvalues and eigenvectors can be calculated from the derivative of the elements of the matrix. The model computes the layer transmission and reflection matrices using matrix formulations. The layer source function is approximately obtained from the reflection and transmission in the same manor as that in Four-stream model. A stack procedure is deployed to integrate the surface and vertical layers. This is a candidate algorithm for CRTM solver
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