Global Warming of the Atmosphere in Radiative–Convective Equilibrium

Abstract

Many studies of global warming have commonly reported positive warming feedback by water vapor, exhibiting relative humidity in the atmosphere unchanged for different warming conditions. However, this is not self-evident, since water vapor content in the atmosphere may be significantly affected by atmospheric convections, such as cumulus convection, which involve strong vertical motions of air. To find an explanation, global warming experiments were run in this study that included atmospheres at radiative?convective equilibrium with differing amounts of a noncondensable greenhouse gas. The models used were the dynamical convection model (DCM) and kinematic circulation model (KCM). When the noncondensable greenhouse gas is increased in the models, the free atmosphere in both the DCM and KCM show similar increases in air temperature and water vapor content. Changes in temperature and water vapor occur such that the relative humidity remains mostly constant. As Iwasa et al. show, water vapor distribution is controlled by a net circulation that is driven by radiative cooling. It is not convectively forced. Relative humidity is unchanged because the net circulation that increases temperature leaves the subsidence flow pattern similar. The DCM reveals a new aspect of global warming. The vertical temperature profile in the dry convective boundary layer (CBL) becomes dry adiabatic, a lapse rate larger than the moist adiabatic lapse rate in the free troposphere. Both the depth of the CBL and tropospheric temperatures increase. The development of the CBL accompanies an additional temperature increase in the bottom atmosphere and at the surface, in contrast to temperature profiles predicted from models without such CBL structures.