  The diversity of characteristics for the host of recently discovered
exoplanets opens up a great deal of fertile new territory for geophysical fluid
dynamics, particularly when the fluid flow is coupled to novel thermodynamics,
radiative transfer or chemistry. In this paper, we survey one of these new
areas-the climate dynamics of atmospheres with a non-dilute condensible
component, defined as the situation in which a condensible component of the
atmosphere makes up a substantial fraction of the atmospheric mass within some
layer. Non-dilute dynamics can occur for a wide range of condensibles,
generically applying near both the inner and the outer edges of the
conventional habitable zone and in connection with runaway greenhouse
phenomena. It also applies in a wide variety of other planetary circumstances.
We first present a number of analytical results developing some key features of
non-dilute atmospheres, and then show how some of these features are manifest
in simulations with a general circulation model adapted to handle non-dilute
atmospheres. We find that non-dilute atmospheres have weak horizontal
temperature gradients even for rapidly rotating planets, and that their
circulations are largely barotropic. The relative humidity of the condensible
component tends towards 100% as the atmosphere becomes more non-dilute, which
has important implications for runaway greenhouse thresholds. Non-dilute
atmospheres exhibit a number of interesting organized convection features, for
which there is not yet any adequate theoretical understanding.

Earth and Planetary Astrophysics;Atmospheric and Oceanic Physics