  Liquid water is one of the most important materials affecting the climate and
habitability of a terrestrial planet. Liquid water vaporizes entirely when
planets receive insolation above a certain value, which is called the runaway
greenhouse threshold. This threshold forms the inner most limit of the
habitable zone. Here, we investigate the effect of the distribution of surface
water on the runaway greenhouse threshold for Earth-sized planets using a
three-dimensional dynamic atmosphere model. We considered a 1-bar atmosphere
whose composition is similar to the current Earth's atmosphere with a zonally
uniform distribution of surface water. As previous studies have already shown,
we also recognized two climate regimes: the land planet regime, which has dry
low latitude and wet high latitude region, and the aqua planet regime, which is
globally wet. We showed that each regime is controlled by the width of the
Hadley circulation, the amount of surface water, and the planetary topography.
We found that the runaway greenhouse threshold varies continuously with the
surface water distribution from about 130% (an aqua planet) to 180% (the
extreme case of a land planet) of the present insolation at Earth's orbit. Our
results indicate that the inner edge of the habitable zone is not a single
sharp boundary, but a border whose location varies depending on planetary
surface condition, such as the amount of surface water. Since land planets have
wider habitable zones and less cloud cover, land planets would be good targets
for future observations investigating planetary habitability.
