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Abstract Congestus clouds, characterized by their vertical extent into the middle troposphere, are widespread in tropical regions and play an important role in Earth's climate system. However, fundamental questions regarding their formation and prevalence remain unanswered. Here, we endeavor to answer how congestus cloud tops form by detraining preferentially at altitudes between 5 and 6 km and why this detraining outflow is invigorated by drier mid‐tropospheric conditions. We construct a clear‐sky radiative‐convective framework of congestus cloud‐top formation that is grounded in the discovery of an important spectroscopic property of water vapor. In this mass‐ and energy‐conserving framework, convective detrainment maximizes at a height of 5 and 6 km due to a swift decline in radiative cooling in clear‐sky regions. This decline is, in turn, a consequence of water vapor spectroscopy: more specifically, a drop in the number of strong absorption lines in the water vapor rotation band. In a simple spectral model, we link this spectroscopic property to the shape of the rotation band, which can be approximated as the product of a power law and a sine wave representing the band's deviation from statistical log‐linearity. The characteristic “C”‐shaped relative humidity profile in the tropics further strengthens the outflow in drier mid‐level conditions by amplifying vertical decreases in the clear‐sky cooling rate. Essential to this process are strong RH gradients, which are most pronounced under the driest conditions and induce a vertical decrease in the optical depth lapse rate across the mid‐troposphere. 

Abstract Earth's tropics are characterized by quasi‐steady precipitation with small oscillations about a mean value, which has led to the hypothesis that moist convection is in a state of quasi‐equilibrium (QE). In contrast, very warm simulations of Earth's tropical convection are characterized by relaxation‐oscillator‐like (RO) precipitation, with short‐lived convective storms and torrential rainfall forming and dissipating at regular intervals with little to no precipitation in between. We develop a model of moist convection by combining a zero‐buoyancy model of bulk‐plume convection with a QE heat engine model, and we use it to show that QE is violated at high surface temperatures. We hypothesize that the RO state emerges when the equilibrium condition of the convective heat engine is violated, that is, when the heating rate times a thermodynamic efficiency exceeds the rate at which work can be performed. We test our hypothesis against one‐ and three‐dimensional numerical simulations and find that it accurately predicts the onset of RO convection. The proposed mechanism for RO emergence from QE breakdown is agnostic of the condensable, and can be applied to any planetary atmosphere undergoing moist convection. To date, RO states have only been demonstrated in three‐dimensional convection‐resolving simulations, which has made it seem that the physics of the RO state requires simulations that can explicitly resolve the three‐dimensional interaction of cloudy plumes and their environment. We demonstrate that RO states also exist in single‐column simulations of radiative‐convective equilibrium with parameterized convection, albeit in a different surface temperature range and with much longer storm‐free intervals. 

Abstract This work is a direct continuation of McKinney et al., who attempted to create a planet with Earth-like temperatures and physical properties but with precipitation and circulation patterns that were Titan-like. McKinney et al. attempted to do so by changing only three basic planetary parameters: the ratio of dry land to ocean on the surface, the rotation period, and the volatility of the condensable. Each of these parameters is varied from an Earth-like value to a Titan-like one to analyze the climate transition between these two planetary archetypes. In this work, we expand on McKinney et al. by including a seasonal cycle and increasing the number of diagnostic criteria for determining Titan-like dynamics. The simulations use Earth-like obliquity and an Earth-like solar constant. We find that the presence of a dry land strip extending to at least 55°N/S is most effective at creating Titan-like climatic conditions on an otherwise Earth-like planet, such as high-latitude summer precipitation maxima and a low-humidity equator. In contrast, slow rotation and high atmospheric vapor abundance have minimal climatic impacts despite being characteristic features of Titan. Our experiments show that it is not difficult to produce distinctly Titan-like features in an Earth-like GCM with minimal changes to its fundamental parameters. This suggests that Earth-like planets could have a large range of global climate states throughout their history just through changes in topography. Similarly, Titan may have experienced more Earth-like climate states in periods where its tropics were wetter.