NEMESIS Q&A

This page collate answers to some of the questions that have arisen from users of NEMESIS. It's incomplete, but hopefully of use.

What does IPZEN do?

IPZEN is a flag telling NEMESIS how to interpret your emission angles - are they at the bottom of the atmosphere (0, default), the 1-bar pressure level (1), or at the top of the atmosphere (2)? When images or spectra have been navigated to determine the emission angles, the sensible choice would be to use the 1-bar level, as this would have been used by the software determining the emission angles. However, the default is IPZEN=0. Confusingly, the emission angle is not dependent on the REF file at all (i.e., it's not at the bottom of the whole atmopshere). Instead, it is at the later specified by LAYHT in the SET file. This is then written to the 'nadir' line in the 'atm' block of the PAT file (a NEMESIS intermediate). This bottom layer is assigned the index 1, so the emission angle is therefore defined at this LAYHT location. So if you change LAYHT with IPZEN=0, you'll find that this is also subtly changing the emission angle of the paths through your atmosphere.

So how should I be choosing LAYHT (and hence TSURFACE)?

I tend to select an arbitrary point somewhere higher than the bottom of my REF file, but beneath the levels covered by the contribution functions of the spectral region of interest. There's no point in setting LAYHT to be really deep if you have a REF file that spans a massive range of pressure levels. Also, since the heights (for gas giants) are calculated by hydrostatic equilibrium, the bottom height of the PRF file can differ from the bottom height in the REF file by a fair bit depending on temperature and latitude (which affects gravity). Hence, it’s usually best to set LAYHT to be comfortably more than the bottom height in the .ref file to guard against it accidentally becoming less than than the bottom height in the .prf file, which leads to odd layer values being calculated.