Response of atmospheric convection to surface drying: new insights from isentropic analysis

Laurel Regibeau-Rockett , Morgan O'Neill

There is strong evidence that the atmospheric moisture content of several solar system planets, including Earth, has varied over their lifetimes. A growing body of work also documents a range of atmospheric water vapor content on exoplanets. An improved understanding of the coupling between atmospheric moisture availability and convection could yield greater intuition about the past and current states of planetary atmospheres, including Earth's atmosphere. In this work, we investigate the changing heat engine behavior of localized radiative-convective equilibrium convection in a suite of moist-to-nearly-dry numerical simulations. Each simulation has a constant surface relative humidity, with values ranging from saturated to nearly dry surface conditions. We observe a deepening of the planetary boundary layer and a corresponding lifting of the cloud base under surface drying, in agreement with previous numerical and observational studies. The primary factor contributing to this is the reduction in the lifting condensation level temperature implied by the Clausius-Clapeyron relationship. Additionally, a diagnostic of the overall mass transport by atmospheric convection increases in drier conditions, consistent with prior work. This mainly results from an increase in planetary boundary layer convective mass transport. In contrast, free-tropospheric convective mass transport decreases, in agreement with previous studies. Finally, we find that surface evaporation is associated with less irreversible entropy production under surface drying and transitions from a spontaneous process to a non-spontaneous process. This occurs because near-surface air is more humid than the surface in the drier experiments, whereas in moister conditions the boundary layer is drier than the surface.