Equilibrium atmospheres of a two‐column radiative‐convective model

Interaction between steady, large‐scale atmospheric circulations and a radiative‐convective environment is considered. As a model tool, we use a two‐column radiative‐convective model with an explicit hydrological cycle that uses clear‐sky conditions in the radiation calculation. A flow field is calculated by the linearized, hydrostatic equations of motion in a non‐rotating frame of reference. Mechanical damping is represented by vertical diffusion of momentum and surface drag. the flow advects heat and moisture, and thereby modifies the local radiative‐convective equilibrium. A dynamically passive ocean mixed layer is situated below the model atmosphere. All externally specified parameters are identical in the two columns, implying that local radiative‐convective equilibrium is a steady solution. For weak mechanical damping (or small column length), the local equilibrium is generally unstable due to a positive feedback between large‐scale subsidence and infrared cooling, which operates via advective drying. A circulating equilibrium, in which the air ascends in one column and descends in the other, is attained. Due to a reduced content of clear‐sky water vapour, which is the major infrared absorber in the model, the circulating equilibrium can emit the absorbed solar radiation at a significantly lower surface temperature than the corresponding local equilibrium. In the limit of a nearly inviscid atmosphere, the intensity of the large‐scale circulation is controlled chiefly by the mid‐tropospheric radiative cooling in the downdraught column. In this regime, we find two distinct equilibria with circulation that are distinguished by the features of the downdraught column: one branch with deep convection but where the integrated convective heating vanishes due to evaporation of precipitation; and one branch with shallow (or no) convection where the surface boundary layer is disconnected from the free atmosphere.