A numerical study on appearance of the runaway greenhouse state of a three-dimensional gray atmosphere

A numerical study on the runaway greenhouse state is performed by using a general circulation model (GCM) with simplified hydrologic and radiative processes. Except for the inclusion of three-dimensional atmospheric motion, the system utilized is basically equivalent to the one-dimensional radiative-convective equilibrium model of Nakajima et al, in which the runaway greenhouse state is defined. The results of integrations with various values of solar constant show that there exists an upper limit of the solar constant with which the atmosphere can reach a statistical equilibrium state. When the value of solar constant exceeds the limit, 1600 W m^-2, the atmosphere sets in a "thermally runaway" state. It is characterized by continuous increase of the amount of water vapor, continuous decrease of the outgoing longwave radiation, and continuous warming of the atmosphere and the ground surface. The upper-limit value of the solar constant obtained by the GCM experiments corresponds to the upper limit of outgoing longwave radiation determined by the one-dimensional model of Nakajima et al, with a fixed value of relative humidity, 60%, which is a typical value obtained by the GCM. The thermally runaway states realized in the GCM are caused by the radiation structure found by Nakajima et al. that prohibits the existence of thermal equilibrium states. The calculated values of the upper limit of radiation and water vapor content cannot be directly applied to describing real planetary atmospheres, since the model physical processes are quite simple-gray radiation scheme without clouds. However, because of this simplification, the GCM gives deeper insight into the structure of a runaway atmosphere.