  We are adapting the Global Circulation Model (GCM) of the UK Met Office, the
so--called Unified Model (UM), for the study of hot Jupiters. In this work we
demonstrate the successful adaptation of the most sophisticated dynamical core,
the component of the GCM which solves the equations of motion for the
atmosphere, available within the UM, ENDGame (Even Newer Dynamics for General
atmospheric modelling of the environment). Within the same numerical scheme
ENDGame supports solution to the dynamical equations under varying degrees of
simplification. We present results from a simple, shallow (in atmospheric
domain) hot Jupiter model (SHJ), and a more realistic (with a deeper
atmosphere) HD 209458b test case. For both test cases we find that the
large--scale, time--averaged (over the 1200 days prescribed test period),
dynamical state of the atmosphere is relatively insensitive to the level of
simplification of the dynamical equations. However, problems exist when
attempting to reproduce the results for these test cases derived from other
models. For the SHJ case the lower (and upper) boundary intersects the dominant
dynamical features of the atmosphere meaning the results are heavily dependent
on the boundary conditions. For the HD 209458b test case, when using the more
complete dynamical models, the atmosphere is still clearly evolving after 1200
days, and in a transient state. Solving the complete (deep atmosphere and
non--hydrostatic) dynamical equations allows exchange between the vertical and
horizontal momentum of the atmosphere, via Coriolis and metric terms.
Subsequently, interaction between the upper atmosphere and the deeper more
slowly evolving (radiatively inactive) atmosphere significantly alters the
results, and acts over timescales longer than 1200 days.
