An optimized line by line code for plume signature calculations - I: Model and data

An optimized line-by-line fortran code for the calculation of aircraft infrared plume signature has been developed an implemented. In this model, the absorption coefficient is separated into two components. The first, which varies quickly with wavenumber, results from the addition of nearby individual line contributions with Voigt line-shapes. The second term, which accounts for both line-wings and collision-induced absorption processes, has a simple dependence on pressure and varies slowly with wavelength. It is pre-calculated and tabulated versus wavenumber and temperature and its treatment involves very small CPU times. For an efficient calculation of the contributions of nearby lines, where most of the computational effort lies, a number of procedures have been developed. The first is a quick evaluation of the Voigt line shape through interpolations within tabulated values. The second is the use of multiple frequency grids of increasing steps on which the various parts of the lines are projected successively starting from line center. The third optimization is a sort of the lines accounted for in the calculations which rejects those of negligible contribution. Finally, more CPU time can be saved by using a proper cut-off distance in the line wings and/or stopping the calculations in spectral regions that are optically very thick. One of the characteristics of our approach, when compared to previous ones, is that all approximations made to quicken calculations lead to errors that are controlled through precision criteria which are chosen by the user.