  General circulation models are often used to explore exoclimate parameter
spaces and classify atmospheric circulation regimes. Models are tuned to give
reasonable climate states for standard test cases, such as the Held-Suarez
test, and then used to simulate diverse exoclimates by varying input parameters
such as rotation rates, instellation, atmospheric optical properties,
frictional timescales and so on. In such studies, there is an implicit
assumption that the model which works reasonably well for the standard test
case will be credible at all points in an arbitrarily wide parameter space.
Here, we test this assumption using the open-source general circulation model
THOR to simulate atmospheric circulation on tidally locked Earth-like planets
with rotation periods of 0.1 to 100 days. We find that the model error, as
quantified by the ratio between physical and spurious numerical contributions
to the angular momentum balance, is extremely variable across this range of
rotation periods with some cases where numerical errors are the dominant
component. Increasing model grid resolution does improve errors but using a
higher-order numerical diffusion scheme can sometimes magnify errors for
finite-volume dynamical solvers. We further show that to minimize error and
make the angular momentum balance more physical within our model, the surface
friction timescale must be smaller than the rotational timescale.
