Structure of the Atmosphere in Radiative–Convective Equilibrium

Abstract

To investigate water vapor transport in an atmosphere in radiative?convective equilibrium, a simplified dynamical convection model (DCM) was constructed that explicitly models moist convection and longwave radiation in a gray atmosphere. In the subsidence region of the equilibrium, atmosphere is predicted by the DCM, dynamical heating, and radiative cooling balance. Subsidence that satisfies local thermodynamical balance includes detrainment from adjacent cumulus updraft at all levels in the free troposphere, from high levels with a small absolute humidity to low levels with a large absolute humidity. In this subsidence region, absolute humidity increases downward, but relative humidity is approximately constant with height. This contrasts sharply with results from a cumulus chimney model (CCM) that limits detrainment to near the tropopause and produces drying in the free troposphere. To demonstrate the accuracy of the transport mechanism implied by the DCM, results from a kinematic circulation model (KCM) were examined. The DCM and the KCM both produced an atmosphere far moister than predicted by the CCM. The feature of the detrainment at all levels of the free troposphere under a normal atmospheric situation does not depend on the radiation schemes used in the models. Furthermore, an analytic solution of the humidity fields, obtained using a few additional assumptions on atmospheric properties, agrees with the humidity fields in the DCM and KCM. The relative humidity in the subsidence region in the free troposphere has a mostly uniform vertical profile and the mean value in the horizontal is independent of the horizontal scale. Water vapor transport moistens the atmosphere, preventing the excess drying that occurs in the CCM.