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 as the surface dries, in agreement with previous numerical and observational studies. The primary factor contributing to this is the reduction in the temperature of the lifting condensation level implied by the Clausius-Clapeyron relationship. Additionally, the mass transport by atmospheric convection increases in drier conditions, consistent with prior work. Finally, inspection of the atmospheric circulation in the typical temperature-entropy space used for heat engine analysis implies that the temperature of convective tops is invariant under surface drying, although this result is sensitive to the planetary boundary layer parameterization. This could support the generalization of the Fixed Anvil Temperature hypothesis, which proposes that cloud top temperatures are constant as the surface temperature changes, to convection under varied surface humidity.