Hurricanes on tidally locked terrestrial planets: fixed surface temperature experiments

Abstract

Aims. In this work, we study the presence of hurricanes on exoplanets. Tidally locked terrestrial planets around M dwarfs are the main targets of space missions looking to discover habitable exoplanets. The question of whether hurricanes can form on this kind of planet is important for determining their climate and habitability. Methods. Using a high-resolution global atmospheric circulation model, we investigated whether there are hurricanes on tidally locked terrestrial planets under fixed surface temperatures (TS). The relevant effects of the planetary rotation rate, surface temperature, and bulk atmospheric compositions were examined. Results. We find that hurricanes can form on the planets but not on all of them. For planets near the inner edge of the habitable zone of late M dwarfs, there are more numerous and stronger hurricanes on both day and night sides. For planets in the middle and outer ranges of the habitable zone, the possibility of hurricane formation is low or even close to zero, as has been suggested in recent studies. Earth-based hurricane theories are applicable to tidally locked planets only when the atmospheric compositions are similar to that of Earth. However, if the background atmosphere is lighter than H2O, hurricanes can hardly be produced because convection is always inhibited due to the effect of the mean molecular weight, similarly to the case of Saturn. These results have broad implications on the precipitation, ocean mixing, climate, and atmospheric characterization of tidally locked planets. Finally, A test with a coupled slab ocean and an Earth-like atmosphere in a tide-locked orbit of ten Earth days demonstrates that there are also hurricanes present in the experiment.

Tropical Cyclone Frequency

“…(The same is true, in principle, of simulations of TCs in past climates [Dandoy et al, 2021;Korty et al, 2012aKorty et al, , 2012bLawton et al, 2021;Q. Yan et al, 2015], or on other planets [M. Yan & Yang, 2020]. While the latter could ideally offer the possibility of observational verification, studies to date have not yet done such verification, presumably because the available observations are not adequate.…”

The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI). Part II: Moist Cases -- The Two Waterworlds

“…Indeed the planet's rotation period of 6.1 days and its radius and gravity being similar to that of Earth, together with a moderate instellation of ≈900 W m −2 and a low effective temperature of the host star (Table 1), all suggest relatively high occurrence of tropical cyclones, according to a combination of hurricane favorability metrics used by Komacek et al (2020). The simulated Hab 2 climates may provide more favorable conditions for hurricane formation than those of Hab 1, because of higher temperatures and a larger ice-free area of the ocean (Yan & Yang 2020). Additionally, the larger mean molecular mass of the background gas (44 g mol −1 in the Hab 2 case vs 28 g mol −1 in Hab 1) makes moist air parcels more buoyant, enhancing convection and thus the intensity of hurricanes.…”

“…Additionally, the larger mean molecular mass of the background gas (44 g mol −1 in the Hab 2 case vs 28 g mol −1 in Hab 1) makes moist air parcels more buoyant, enhancing convection and thus the intensity of hurricanes. Their typical radius, however, is expected to be smaller in the Hab 2 case relative to the Hab 1 case, because of the smaller atmospheric scale height, which is inversely proportional to the mean molecular mass of the atmosphere (Yan & Yang 2020). While observing individual cyclones on exoplanets is borderline impossible, their activity may increase the local water vapor content by up to 5 times (Yan & Yang 2020), which would potentially lay within detectable limits of the next generation telescopes (Komacek et al 2020).…”

“…Large areas of warm ocean also provide favourable conditions for tropical cyclogenesis, assuming TRAPPIST-1e is tidally locked and rotates relatively fast around its axis (Bin et al 2018;Yan & Yang 2020;Komacek et al 2020). Indeed the planet's rotation period of 6.1 days and its radius and gravity being similar to that of Earth, together with a moderate instellation of ≈900 W m −2 and a low effective temperature of the host star (Table 1), all suggest relatively high occurrence of tropical cyclones, according to a combination of hurricane favorability metrics used by Komacek et al (2020).…”

“…Their typical radius, however, is expected to be smaller in the Hab 2 case relative to the Hab 1 case, because of the smaller atmospheric scale height, which is inversely proportional to the mean molecular mass of the atmosphere (Yan & Yang 2020). While observing individual cyclones on exoplanets is borderline impossible, their activity may increase the local water vapor content by up to 5 times (Yan & Yang 2020), which would potentially lay within detectable limits of the next generation telescopes (Komacek et al 2020). Hurricanes and other weather systems can thus contribute to the time variability of clouds and humidity at the terminators, affecting the transmission spectrum (see Sec.…”