In the spirit of minimal modeling of complex systems, we develop an idealized
two-column model to investigate the climate of tidally locked terrestrial
planets with Earth-like atmospheres in the habitable zone of M-dwarf stars. The
model is able to approximate the fundamental features of the climate obtained
from three-dimensional (3D) atmospheric general circulation model (GCM)
simulations. One important reason for the two-column model's success is that it
reproduces the high cloud albedo of the GCM simulations, which reduces the
planet's temperature and delays the onset of a runaway greenhouse state. The
two-column model also clearly illustrates a secondary mechanism for determining
the climate: the nightside acts as a ``radiator fin'' through which infrared
energy can be lost to space easily. This radiator fin is maintained by a
temperature inversion and dry air on the nightside, and plays a similar role to
the subtropics on modern Earth. Since 1D radiative-convective models cannot
capture the effects of the cloud albedo and radiator fin, they are
systematically biased towards a narrower habitable zone. We also show that
cloud parameters are most important for determining the day--night thermal
emission contrast in the two-column model, which decreases and eventually
reverses as the stellar flux increases. This reversal is important because it
could be detected by future extrasolar planet characterization missions, which
would suggest that the planet has Earth-like water clouds and is potentially
habitable.Comment: The Astrophysical Journal (in press), 14 pages, 11 figures, 1 tabl
