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.