A fast line-by-line method

Abstract

A new spectral line selection algorithm is developed. The algorithm makes it possible to decrease the number of spectral lines with the increase in altitude. In order for the computer code based on a line-by-line method to operate efficiently two line selections must be carried out. The first selection is rough. This enables the most weak lines not contributing to the optical thickness of the layer z 1– z 2 to be eliminated. The other lines are subjected to the second selection. In this case the maximum height up to which the line should be taken into account is determined. At the same time for each line at each altitude the maximum resonance frequency difference within the limits of which the contribution of the line to absorption should be taken into account is determined. The gain in the time of calculation of the integral transmittance may be of five times or greater. The calculation error of the integral transmittance is not larger than 0.5%.