  The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) project was
initiated to compare 3D climate models that are commonly used for predicting
theoretical climates of habitable zone extrasolar planets. One of the core
models studied as part of THAI is ExoCAM, an independently curated exoplanet
branch of the National Center for Atmospheric Research (NCAR) Community Earth
System Model (CESM) version 1.2.1. ExoCAM has been used for studying
atmospheres of terrestrial extrasolar planets around a variety of stars. To
accompany the THAI project and provide a primary reference, here we describe
ExoCAM and what makes it unique from standard configurations of CESM.
Furthermore, we also conduct a series of intramodel sensitivity tests of
relevant moist physical tuning parameters while using the THAI protocol as our
starting point. A common criticism of 3D climate models used for exoplanet
modeling is that cloud and convection routines often contain free parameters
that are tuned to the modern Earth, and thus may be a source of uncertainty in
evaluating exoplanet climates. Here, we explore sensitivities to numerous
configuration and parameter selections, including a recently updated radiation
scheme, a different cloud and convection physics package, different cloud and
precipitation tuning parameters, and a different sea ice albedo. Improvements
to our radiation scheme and the modification of cloud particle sizes have the
largest effect on global mean temperatures, with variations up to ~10 K,
highlighting the requirement for accurate radiative transfer and the importance
of cloud microphysics for simulating exoplanetary climates. However for the
vast majority of sensitivity tests, climate differences are small. For all
cases studied, intramodel differences do not bias general conclusions regarding
climate states and habitability.
