c. Problems Investigated in the Present Paper
While previous studies on the runaway greenhouse state have
generally used 1D equilibrium models, only Renno et al. (1994) have
succeeded in realizing a dynamic image of the original
concept. However, their study used a 1D model as well, and there
have been no studies to date that have used a three-dimensional (3D)
model. In the present study, the runaway greenhouse state will be
investigated in a 3D time evolution model, with special focus on the
points outlined below.
- 0. What is the feasibility of calculations with large values of solar constant?
-
Currently, there are no existing studies that calculate moist
atmospheric motion with increased values of the solar
constant. Therefore, it remains unclear whether calculations for
such cases are even possible using the atmospheric models that
have been designed to reproduce Earth's atmospheric circulation
under present conditions. Thus, this experiment is expected to
become a valuable performance test on the
GCM , in terms of evaluating its limitations.
- 1. Are the "runaway state" also realized in 3D?
-
In atmospheric circulation, the air is dry in the downdraft
regions of the Hadley cell, which means that radiation emissions
should be high in those regions. In such cases, heat that could
not be emitted in the equatorial region may be emitted from the
atmosphere in the subtropical regions, which may lead to the
realization of an equilibrium state for the system as a whole.
- 2. How does the equilibrium state change when solar constant is changed?
-
Even in cases where solar constant remains below the runaway limit
and an equilibrium state is reached, solar constant may still
affect the atmospheric structure. However, no studies have yet
addressed this theme. Thus, our results may provide clues for
understanding atmospheric structure in other planetary
atmospheres.
- 3. What is the atmospheric structure of a runaway greenhouse state?
-
If the runaway greenhouse state does indeed emerge in our 3D
model, the time evolution, temperature distribution, convection,
energy transport, and water vapor transport of the state will be
compared to those in the equilibrium state. The results should
provide indicators regarding the conditions of the Earth's
primitive atmosphere.
- 4. What determines the runaway limit (solar constant at which the runaway greenhouse state emerges)?
-
In 1D models, the incident solar radiation flux at which the
runaway greenhouse state appears is determined by the vertical
structure of the atmosphere. However, the value of the runaway
limit, and even the mechanisms that determine it, may differ
completely in 3D models. This is due to the presence of heat
transport and water vapor advection between the polar and
equatorial regions.
