Equilibrium atmospheres of a two-column radiative-convective model

Abstract

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.