  Remote observing of exoplanetary atmospheres is now possible, offering us
access to circulation regimes unlike any of the familiar Solar System cases.
Atmospheric circulation models are being developed to study these new regimes
but model validations and intercomparisons are needed to establish their
consistency and accuracy. To this end, we present a simple Earth-like
validation of the pseudo-spectral solver of meteorological equations called
IGCM (Intermediate General Circulation Model), based on Newtonian relaxation to
a prescribed latitudinal profile of equilibrium temperatures. We then describe
a straightforward and idealized model extension to the atmospheric flow on a
hot Jupiter with the same IGCM solver. This shallow, three-dimensional hot
Jupiter model is based on Newtonian relaxation to a permanent day-night pattern
of equilibrium temperatures and the absence of surface drag. The baroclinic
regime of the Earth's lower atmosphere is contrasted with the more barotropic
regime of the simulated hot Jupiter flow. For plausible conditions at the 0.1-1
bar pressure level on HD 209458b, the simulated flow is characterized by
unsteadiness, subsonic wind speeds, a zonally-perturbed superrotating
equatorial jet and large scale polar vortices. Violation of the Rayleigh-Kuo
inflexion point criterion on the flanks of the accelerating equatorial jet
indicates that barotropic (horizontal shear) instabilities may be important
dynamical features of the simulated flow. Similarities and differences with
previously published simulated hot Jupiter flows are briefly noted.
