Atmospheric Convection Plays a Key Role in the Climate of Tidally Locked Terrestrial Exoplanets: Insights from High-resolution Simulations

Abstract

Using a 3D general circulation model (GCM), we investigate the sensitivity of the climate of tidally locked Earthlike exoplanets, Trappist-1e and Proxima Centauri b, to the choice of a convection parameterization. Compared to a mass-flux convection parameterization, a simplified convection adjustment parameterization leads to a >60% decrease of the cloud albedo, increasing the mean dayside temperature by . The representation of convection also affects the atmospheric conditions of the night side, via a change in planetary-scale wave patterns. As a result, using the convection adjustment scheme makes the nightside cold traps warmer by 17–36 K for the planets in our simulations. The day–night thermal contrast is sensitive to the representation of convection in 3D GCM simulations, so caution should be taken when interpreting emission phase curves. The choice of convection treatment, however, does not alter the simulated climate enough to result in a departure from habitable conditions, at least for the atmospheric composition and planetary parameters used in our study. The near-surface conditions both in the Trappist-1e and Proxima b cases remain temperate, allowing for an active water cycle. We further advance our analysis using high-resolution model experiments, in which atmospheric convection is simulated explicitly. Our results suggest that in a hypothetical global convection-permitting simulation, the surface temperature contrast would be higher than in the coarse-resolution simulations with parameterized convection. In other words, models with parameterized convection may overestimate the inter-hemispheric heat redistribution efficiency.

Implications of different stellar spectra for the climate of tidally locked Earth-like exoplanets

“…These regions are similar in magnitude in cases T1:T1e and ProC:T1e, but case Sun:T1e does not have a substantial region of the planetary surface that may sustain liquid water, and it may be considered less habitable as a result. All simulations have similar nearsurface winds, showing a convergence towards the sub-stellar point, because solar forcing gives rise to a region of intense convection, as discussed in Boutle et al (2017) and Sergeev et al (2020).…”

“…In this work we use the Met Office general circulation model (GCM), the UM, which has been adapted to a range of exoplanet applications and used for a large number of studies covering hot Jupiters (Mayne et al 2014a(Mayne et al , 2017Amundsen et al 2016;Helling et al 2016;Tremblin et al 2017;Drummond et al 2018aDrummond et al ,b, 2020Lines et al 2018aLines et al ,b, 2019Sainsbury-Martinez et al 2019;Debras et al 2019Debras et al , 2020, mini-Neptunes/Super Earths (Drummond et al 2018c;Mayne et al 2019) and terrestrial planets (Mayne et al 2014b;Boutle et al 2017Boutle et al , 2020Lewis et al 2018;Fauchez et al 2020;Yates et al 2020;Joshi et al 2020;Sergeev et al 2020). For this work, we follow a similar configuration to that of Boutle et al (2017) and Lewis et al (2018), based on the Global Atmosphere 7.0 configuration (Walters et al 2019).…”

“…This also leads to cloud formation, with the zonal jet transporting moisture (and cloud) horizontally high up in the troposphere. Subsidence and further condensation occur on the nightside (Yang & Abbot 2014;Boutle et al 2017;Lewis et al 2018;Sergeev et al 2020). To explore this for our simulations, Fig.…”

“…Boutle et al (2017) showed that for a simulation of Proxima Centauri b, vigorous convection over the sub-stellar point acted to transport heat and moisture vertically to the altitude of the zonal jet. Recently, Sergeev et al (2020) have explored the differences obtained when employing various treatments and parametrisations of convection within 3D simulations of a tidally locked terrestrial exoplanet, and performing highresolution convection-permitting simulations free from such approximated treatments. Sergeev et al (2020) showed that important differences in the vertical and horizontal transport of heat and moisture exist between coarse-resolution, employing convection parametrisations, and high-resolution simulations with explicit convection.…”

“…Recently, Sergeev et al (2020) have explored the differences obtained when employing various treatments and parametrisations of convection within 3D simulations of a tidally locked terrestrial exoplanet, and performing highresolution convection-permitting simulations free from such approximated treatments. Sergeev et al (2020) showed that important differences in the vertical and horizontal transport of heat and moisture exist between coarse-resolution, employing convection parametrisations, and high-resolution simulations with explicit convection. However, these studies have not yet been extended to explore the impact of differing stellar spectra on the behaviour of the convective transport, cloud coverage, and day-night transport.…”