  General circulation models (GCMs) provide context for interpreting
multi-wavelength, multi-phase data of the atmospheres of tidally locked
exoplanets. In the current study, the non-hydrostatic THOR GCM is coupled with
the HELIOS radiative transfer solver for the first time, supported by an
equilibrium chemistry solver (FastChem), opacity calculator (HELIOS-K) and Mie
scattering code (LX-MIE). To accurately treat the scattering of radiation by
medium-sized to large aerosols/condensates, improved two-stream radiative
transfer is implemented within a GCM for the first time. Multiple scattering is
implemented using a Thomas algorithm formulation of the two-stream flux
solutions, which decreases the computational time by about 2 orders of
magnitude compared to the iterative method used in past versions of HELIOS. As
a case study, we present four GCMs of the hot Jupiter WASP-43b, where we
compare the temperature, velocity, entropy, and streamfunction, as well as the
synthetic spectra and phase curves, of runs using regular versus improved
two-stream radiative transfer and isothermal versus non-isothermal layers.
While the global climate is qualitatively robust, the synthetic spectra and
phase curves are sensitive to these details. A THOR+HELIOS WASP-43b GCM
(horizontal resolution of about 4 degrees on the sphere and with 40 radial
points) with multi-wavelength radiative transfer (30 k-table bins) running for
3000 Earth days (864,000 time steps) takes about 19-26 days to complete
depending on the type of GPU.
