Efficient line-by-line calculation of absorption coefficients to high numerical accuracy

Modeling radiative transfer at high resolution is computationally expensive due to the need to evaluate spectral absorption coefficients on a line-by-line basis. A simple yet efficient method is presented here for calculating these coefficients to high numerical accuracy. For each spectral line retained in the model, this method evaluates a contribution to absorption over the entire frequency range of interest (i.e., the far wings of each line are not truncated). The accuracy achieved is independent of the size of the sampling interval. The level of accuracy may be varied continuously over many orders of magnitude, thereby permitting the user to trade accuracy for speed and rendering it easy to perform convergence studies that quantify the numerical error. The method is applicable to any analytic model of the spectral line shape. (C) 1997 Elsevier Science Ltd.