Numerical performance of stability enhancing and speed increasing steps in radiative transfer solution methods

Methods for solving the radiative transfer problem, which is crucial for a number of sectors of industry, involve several numerical challenges. This paper gives a systematic presentation of the effect of the steps that are needed or possible to make any discrete ordinate radiative transfer solution method numerically efficient. This is done through studies of the numerical performance of the stability enhancing and speed increasing steps used in modern tools like Disort or Dort2002.Performance tests illustrate the effect of steps that are taken to improve the stability and speed. It is shown how the steps together give a stable solution procedure to a problem previously considered numerically intractable, and how they together decrease the computation time compared to a naive implementation with a factor 1000 in typical cases and far beyond in extreme cases. It is also shown that the speed increasing steps are not introduced at the cost of reduced accuracy. Further studies and developments, which can have a positive impact on computation time, are suggested.