Meta-modelling the climate of dry tide-locked rocky planets

Abstract

Context. Rocky planets hosted by close-in extrasolar systems are likely to be tidally locked in 1:1 spin-orbit resonance, a configuration where they exhibit a permanent dayside and nightside. Because of the resulting day-night temperature gradient, the climate and large-scale circulation of these planets are strongly determined by their atmospheric stability against collapse, which designates the runaway condensation of greenhouse gases on the nightside. Aims. To better constrain the surface conditions and climatic regime of rocky extrasolar planets located in the habitable zone of their host star, it is therefore crucial to elucidate the mechanisms that govern the day-night heat redistribution. Methods. As a first attempt to bridge the gap between multiple modelling approaches ranging from simplified analytical greenhouse models to sophisticated 3D general circulation models (GCMs), we developed a general circulation meta-model (GCMM) able to reproduce the closed-form solutions obtained in earlier studies, the numerical solutions obtained from GCM simulations, and solutions provided by intermediate models, assuming the slow rotator approximation. We used this approach to characterise the atmospheric stability of Earth-sized rocky planets with dry atmospheres containing CO2, and we benchmarked it against 3D GCM simulations using the THOR GCM. Results. We observe that the collapse pressure below which collapse occurs can vary by ~40% around the value predicted by analytical scaling laws depending on the mechanisms taken into account among radiative transfer, atmospheric dynamics, and turbulent diffusion. Particularly, we find (i) that the turbulent diffusion taking place in the dayside planetary boundary layer (PBL) globally tends to warm up the nightside surface hemisphere except in the transition zone between optically thin and optically thick regimes, (ii) that the PBL also significantly affects the day-night advection timescale, and (iii) that the slow rotator approximation holds from the moment that the normalised equatorial Rossby deformation radius is greater than 2.