The Relative Roles of Radiation and Convection in Equilibrium Atmospheres

Here we consider the changes to a conventional radiative-convective equilibrium (RCE) model that are needed when it is applied to atmospheres that depart significantly from present Earth conditions. Radiative transfer schemes such as the RRTM of Atmospheric and Environment Research, Inc. (AER), work well for present conditions. They can also cope with perturbations that, for example, increase greenhouse gas concentrations, provided such perturbations do not change surface pressure significantly. Here we relax this constraint to consider the relative importance of additional factors (such as pressure broadening, collision-induced absorption [CIA], convective deepening, and increased Rayleigh scatter) that come into play as atmospheric mass is increased.

We use an advanced RCE model that incorporates PRRTM. This is a planetary version of RRTM that has been adapted by AER to handle higher pressures of CO₂ (up to 11 bar). Our motivation was to explain the breakout of the Earth’s climate from its Snowball state, once volcanic emission of CO₂ had increased surface pressure to ≈1.4 bars. We find that while pressure broadening and collision-induced absorption are important, they are insufficient to explain the de-glaciation. Instead, the non-radiative factor of convective deepening, which dominates as surface pressure rises, provides the required additional warming.

As atmospheric mass increases further, the surface temperature is dominated by convective deepening, provided the surface radiation budget remains positive. With this assumption, our RCE model can reproduce the atmospheric temperature profile of Venus, despite its 90 bar surface pressure. However, we find an apparent paradox if we use the existing data on CO₂ absorption. There appear to be no experimental data corresponding to surface conditions on Venus, with the closest we found being for 36 bar and 470 K. However, even a generous extrapolation from this down to the surface fails to provide the necessary absorption coefficients in the ‘window’ regions. This implies the existence of CIA by other species, the next most abundant of which is N₂.