Dear ARTS developers, I'm trying to understand some changes to clear-sky simulations when moving to ARTS v2.4 and have noticed that there seems to be a difference in how a cutoff frequency for absorption lines is handled. I've put a basic example illustrating this at https://github.com/stuartfox/arts-cutoff-problem, which has the controlfiles I've used at ARTS v2.4 and ARTS v2.3.1277 and the input data I used.
The example performs a clear-sky simulation using only water vapour absorption lines read from an ARTSCAT-3 format file (which was originally derived from the HITRAN-style file provided by AER). Note that Ive used a slightly silly cutoff frequency of 20GHz around each line to highlight the problem; it shows small (but non-negligible) differences in brightness temperature around the main water vapour lines, and at v2.4 the output also has a couple of very large negative brightness temperatures which leads me to think there may be a bug in that version. If I remove the line cutoff (by changing line 27 of the v2.4 controlfile to cutoff_option="None" and line 25 of the v2.3 controlfile to cutoff=-1) then I get very good agreement between the two versions. Is this a problem with ARTS or am I doing something wrong with my setup? Thanks, Stuart