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, 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?



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