Atmospheric transmittance and radiance calculations using line-by-line computer models

A description is given of the basic elements of a line-by-line computer model with particular reference to a new transmittance/radiance code GENLN2 currently being developed. Calculations may be performed for a multi-layered atmosphere of mixed gases and for different viewing geometries. The main problems associated with line-by-line calculations are concerned with line shape and continuum absorption, the treatment of lines outside the given spectral range, and the way in which spectral sampling is performed. These are considered together with methods of improving speed of computation in general. Results and intercomparisons from different applications of the technique are presented.