Equilibrium atmospheres of a two-column radiative-convective model

Interaction between steady, large-scale atmospheric circulations and a radi ative-convective environment is considered. As a model tool, we use a two-c olumn radiative-convective model with an explicit hydrological cycle that u ses clear-sky conditions in the radiation calculation. A flow field is calc ulated by the linearized, hydrostatic equations of motion in a non-rotating frame of reference. Mechanical damping is represented by vertical diffusio n of momentum and surface drag. The flow advects heat and moisture, and the reby modifies the local radiative-convective equilibrium. A dynamically pas sive ocean mixed layer is situated below the model atmosphere. All externally specified parameters are identical in the two columns, imply ing that local radiative-convective equilibrium is a steady solution. For w eak mechanical damping (or small column length), the local equilibrium is g enerally unstable due to a positive feedback between large-scale subsidence and infrared cooling, which operates via advective drying. A circulating e quilibrium, in which the air ascends in one column and descends in the othe r, is attained. Due to a reduced content of clear-sky water vapour, which i s the major infrared absorber in the model, the circulating equilibrium can emit the absorbed solar radiation at a significantly lower surface tempera ture than the corresponding local equilibrium. In the limit of a nearly inviscid atmosphere, the intensity of the large-sc ale circulation is controlled chiefly by the mid-tropospheric radiative coo ling in the downdraught column. In this regime, we find two distinct equili bria with circulation that are distinguished by the features of the downdra ught column: one branch with deep convection but where the integrated conve ctive heating vanishes due to evaporation of precipitation; and one branch with shallow (or no) convection where the surface boundary layer is disconn ected from the free atmosphere.