Climates of tidally locked exo-terrestrial planets with a global cloud-resolving model

Based on the current state of observation for exoplanets, we focus on terrestrial planets around M-type stars. Such terrestrial planets within the habitable zone around M-type stars are expected to be in the tidally locked state, resulting in the synchronous rotation which means that they should have permanent day-side and night-side.Climates of tidally locked exo-terrestrial planets have been investigated using GCMs. Previous studies have gradually revealed the climatic characteristics of tidally locked exo-terrestrial planets. For a tidally locked exo-terrestrial planet, the cloud stabilizing feedback has been considered to maintain surface water because of a difference in the distribution of insolation, causing permanent day-night sides (Yang et al., 2013). The dynamics regime for tidally-locked terrestrial planets is divided into three regimes, depending on the equatorial Rossby deformation radius and the Rhine length: the rapid rotator, the Rhines rotator and the slow rotator regimes (Haqq-Misra et al., 2018). The inner edges of the habitable zones around M-type stars have been estimated using a 3-D GCM with self-consistent relationship between stellar parameters and the planetary rotational and orbital period (Kopparapu et al., 2016, 2017). For the highly anticipated TRAPPIST-1 system, an intercomparison project among GCM models, named THAI project, is already underway, and our understanding is expected to advance significantly (Fauchez et al., 2021, Turbet et al., 2022, Sergeev et al., 2022).

Clouds pose significant uncertainties in models for exoplanetary atmosphere. Traditionally, conventical GCMs with low resolution have used cumulus parameterization and large-scale condensation schemes to evaluate cloud-related processes. These treatments cannot explicitly resolve sub-scale physical phenomena, such as cloud formation processes. In recent years, climate experiments on tidally locked terrestrial planets have begun using several convection-resolving and cloud-resolving models, marking the start of more detailed investigations into the climate of tidally locked planets (Lefévre et al., 2021; Yang et al., 2023; Sergeev et al., 2024). Here, we introduce NICAM(Non-hydrostatic icosahedral atmosphere model), known as a global cloud-resolving model (GCRM; e.g., Satoh et al., 2019). Our model can explicitly resolve cloud distribution and the vertical moisture transport of water vapor. We performed climate simulation with ~10 km horizontal mesh for TRAPPIST1-e case. The set of experiments conducted was based on planetary parameters from Agol et al. (2021) and those specified by the THAI project, considering four orbital periods ranging from 5 to 10 days, and three planetary cases: an Earth-sized planet, TRAPPIST1-e, and Proxima Centauri b. The assumed planet is an aqua planet configuration with 50 m and 1 m of the mixed layer. The simulated period is 15 and 2 years to reach an equilibrium state, respectively. Our simulation is the highest resolution global simulation with GCRM for exo-terrestrial planets to investigate characteristics of a potential habitable climate.

In the presentation, we will show the results of systematically conducted climate experiments using GCRM, focusing in particular on analyzing the impact of clouds on the global climate in the energy budget. Furthermore, we examine how atmospheric circulation patterns vary with planetary radius and orbital period. Our results help quantify the role of clouds in the global climate and enable a detailed examination of each climate state through intercomparison of the experiments. Such a cloud-resolving model will open a new era of climate studies and our understanding of habitability.