Atmospheric circulation of tidally locked exoplanets: a suite of
  benchmark tests for dynamical solvers
  The complexity of atmospheric modelling and its inherent non-linearity,
together with the limited amount of data of exoplanets available, motivate
model intercomparisons and benchmark tests. In the geophysical community, the
Held-Suarez test is a standard benchmark for comparing dynamical core
simulations of the Earth's atmosphere with different solvers, based on
statistically-averaged flow quantities. In the present study, we perform
analogues of the Held-Suarez test for tidally-locked exoplanets with the
GFDL-Princeton Flexible Modeling System (FMS) by subjecting both the spectral
and finite difference dynamical cores to a suite of tests, including the
standard benchmark for Earth, a hypothetical tidally-locked Earth, a "shallow"
hot Jupiter model and a "deep" model of HD 209458b. We find qualitative and
quantitative agreement between the solvers for the Earth, tidally-locked Earth
and shallow hot Jupiter benchmarks, but the agreement is less than satisfactory
for the deep model of HD 209458b. Further investigation reveals that closer
agreement may be attained by arbitrarily adjusting the values of the horizontal
dissipation parameters in the two solvers, but it remains the case that the
magnitude of the horizontal dissipation is not easily specified from first
principles. Irrespective of radiative transfer or chemical composition
considerations, our study points to limitations in our ability to accurately
model hot Jupiter atmospheres with meteorological solvers at the level of ten
percent for the temperature field and several tens of percent for the velocity
field. Direct wind measurements should thus be particularly constraining for
the models. Our suite of benchmark tests also provides a reference point for
researchers wishing to adapt their codes to study the atmospheric circulation
regimes of tidally-locked Earths/Neptunes/Jupiters.
