  Condensible substances are nearly ubiquitous in planetary atmospheres. For
the most familiar case-water vapor in Earth's present climate-the condensible
gas is dilute, in the sense that its concentration is everywhere small relative
to the noncondensible background gases. A wide variety of important planetary
climate problems involve nondilute condensible substances. These include
planets near or undergoing a water vapor runaway and planets near the outer
edge of the conventional habitable zone, for which CO2 is the condensible.
Standard representations of convection in climate models rely on several
approximations appropriate only to the dilute limit, while nondilute convection
differs in fundamental ways from dilute convection. In this paper, a simple
parameterization of convection valid in the nondilute as well as dilute limits
is derived and used to discuss the basic character of nondilute convection. The
energy conservation properties of the scheme are discussed in detail and are
verified in radiative-convective simulations. As a further illustration of the
behavior of the scheme, results for a runaway greenhouse atmosphere for both
steady instellation and seasonally varying instellation corresponding to a
highly eccentric orbit are presented. The latter case illustrates that the high
thermal inertia associated with latent heat in nondilute atmospheres can damp
out the effects of even extreme seasonal forcing.
