  Radiative-transfer (RT) is a fundamental part of modelling exoplanet
atmospheres with general circulation models (GCMs). An accurate RT scheme is
required for estimates of the atmospheric energy transport and for gaining
physical insight from model spectra. We implement three RT schemes for Exo-FMS:
semi-grey, non-grey `picket fence', and real gas with correlated-k. We
benchmark the Exo-FMS GCM using these RT schemes to hot Jupiter simulation
results from the literature. We perform a HD 209458b-like simulation with the
three schemes and compare their results. These simulations are then
post-processed to compare their observable differences. The semi-grey scheme
results show qualitative agreement with previous studies in line with
variations seen between GCM models. The real gas model reproduces well the
temperature and dynamical structures from other studies. After post-processing
our non-grey picket fence scheme compares very favourably with the real gas
model, producing similar transmission spectra, emission spectra and phase curve
behaviours. Exo-FMS is able to reliably reproduce the essential features of
contemporary GCM models in the hot gas giant regime. Our results suggest the
picket fence approach offers a simple way to improve upon RT realism beyond
semi-grey schemes.
