Fast monochromatic radiative transfer calculations for limb sounding

Satellite observations of atmospheric infrared spectra can be modeled accurately with line-by-line calculations, but these are too slow to be incorporated into operational retrieval schemes. However, a monochromatic calculation is still feasible if the line-by-line summation is replaced by pre-tabulated absorption coefficients, requiring a three-way optimization of storage space, accuracy and access time. Such a scheme is used for the operational processing of data from MIPAS, a limb-viewing interferometer. The tabulated data are compressed to a manageable size using singular value decomposition, although the reconstruction adds a small overhead. The number of monochromatic radiative transfer calculations is reduced by determining suitable quadrature points in the spectral domain, which reduces both processing time and data storage requirements. An important aspect of such optimizations is the control of the associated errors in the forward model calculation. The result is an acceleration of monochromatic forward model calculations by one or two orders of magnitude compared to a line-by-line calculation without any significant loss of accuracy, while obtaining data compression factors of 100 or more compared to a direct tabulation of absorption coefficients. © 2002 Elsevier Science Ltd. All rights reserved