arXiv subjects

Evelyn Macdonald

Publications and source records attributed to Evelyn Macdonald.

6 recordsLinked to original sources

arXiv2026

Topography-Induced Stationary Waves and the Onset of Nightside Warming on Rocky Planets around M-dwarf Stars

Among potentially habitable worlds, rocky planets orbiting M dwarfs offer the most favorable prospects for atmospheric characterization, yet their climates may differ substantially from those of Earth analogs. In the tidally locked limit, the nightside's tendency to radiatively cool and potentially trap volatiles as permanent ice introduces a strong dependence of habitability on the planet's surface and atmospheric boundary conditions. We perform a suite of synchronously rotating experiments spanning a wide range of topographic and orographic realizations with different mean elevations and landmass distributions. Across a grid of $p_{\mathrm{N2}} = 0.5$-$8~\mathrm{bar}$ and $F_{\star} = 1200$-$1700~\mathrm{W\,m^{-2}}$, we find that surface relief breaks the flow symmetry, replacing the circumpolar vortices with mechanically forced stationary waves. Steep orography produces standing Rossby gyres that strengthen the cross-terminator jet and align vertical uplift with the day--night boundary. These new circulation regimes enhance moisture transport, increasing the infrared optical depth and promoting additional nightside cloud formation, which produces a stronger cloud-greenhouse feedback and lower the critical fluxes required for global planetary deglaciation. Broad, elevated plateaus drive a similarly fragmented but slightly weaker circulation, yielding less effective moisture transport. These results show that the relief and spatial distribution of landmasses, parameters unconstrained for most exoplanets, can exert strong controls on the climatic bifurcations of tidally locked M-dwarf exoplanets.

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arXiv2024

Climate Transition to Temperate Nightside at High Atmosphere Mass

Our recent work shows how M-Earth climates and transmission spectra depend on the amount of ice-free ocean on the planet's dayside and the mass of N2 in its atmosphere. M-Earths with more ice-free ocean and thicker atmospheres are hotter and more humid, and have larger water vapour features in their transmission spectra. In this paper, we describe a climate transition in high-pN2 simulations from the traditional ``eyeball" M-Earth climate, in which only the substellar region is temperate, to a ``temperate nightside" regime in which both the dayside and the nightside are entirely ice-free. Between these two states, there is a ``transition" regime with partial nightside ice cover. We use 3D climate simulations to describe the climate transition from frozen to deglaciated nightsides. We attribute this transition to increased advection and heat transport by water vapour in thicker atmospheres. We find that the nightside transitions smoothly back and forth between frozen and ice-free when the instellation or pCO2 is perturbed, with no hysteresis. We also find an analogous transition in colder planets: those with thin atmospheres can have a dayside hotspot when the instellation is low, whereas those with more massive atmospheres are more likely to be in the ``snowball" regime, featuring a completely frozen dayside, due to the increased advection of heat away from the substellar point. We show how both of these climate transitions are sensitive to instellation, land cover, and atmosphere mass. We generate synthetic transmission spectra and phase curves for the range of climates in our simulations.

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arXiv2024

Water Vapour Transit Ambiguities for Habitable M-Earths

We have shown in a recent study, using 3D climate simulations, that dayside land cover has a substantial impact on the climate of a synchronously rotating temperate rocky planet such as Proxima Centauri b. Building on that result, we generate synthetic transit spectra from our simulations to assess the impact of these land-induced climate uncertainties on water vapour transit signals. We find that distinct climate regimes will likely be very difficult to differentiate in transit spectra, even under the more favourable conditions of smaller planets orbiting ultracool dwarfs. Further, we show that additional climate ambiguities arise when both land cover and atmosphere mass are unknown, as is likely to be the case for transiting planets. While water vapour may be detectable under favourable conditions, it may be nearly impossible to infer a rocky planet's surface conditions or climate state from its transit spectrum due to the interdependent effects of land cover and atmosphere mass on surface temperature, humidity, and terminator cloud cover.

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arXiv2022

ExoPlaSim: Extending the Planet Simulator for Exoplanets

The discovery of a large number of terrestrial exoplanets in the habitable zones of their stars, many of which are qualitatively different from Earth, has led to a growing need for fast and flexible 3D climate models, which could model such planets and explore multiple possible climate states and surface conditions. We respond to that need by creating ExoPlaSim, a modified version of the Planet Simulator (PlaSim) that is designed to be applicable to synchronously rotating terrestrial planets, planets orbiting stars with non-solar spectra, and planets with non-Earth-like surface pressures. In this paper we describe our modifications, present validation tests of ExoPlaSim's performance against other GCMs, and demonstrate its utility by performing two simple experiments involving hundreds of models. We find that ExoPlaSim agrees qualitatively with more-sophisticated GCMs such as ExoCAM, LMDG, and ROCKE-3D, falling within the ensemble distribution on multiple measures. The model is fast enough that it enables large parameter surveys with hundreds to thousands of models, potentially enabling the efficient use of a 3D climate model in retrievals of future exoplanet observations. We describe our efforts to make ExoPlaSim accessible to non-modellers, including observers, non-computational theorists, students, and educators through a new Python API and streamlined installation through pip, along with online documentation.

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arXiv2021

Climate uncertainties caused by unknown land distribution on habitable M-Earths

A planet's surface conditions can significantly impact its climate and habitability. In this study, we use the 3D general circulation model ExoPlaSim to systematically vary dayside land cover on a synchronously rotating, temperate rocky planet under two extreme and opposite continent configurations, in which either all of the land or all of the ocean is centred at the substellar point. We identify water vapour and sea ice as competing drivers of climate, and we isolate land-dependent regimes under which one or the other dominates. We find that the amount and configuration of land can change the planet's globally averaged surface temperature by up to 20K, and its atmospheric water vapour content by several orders of magnitude. The most discrepant models have partial dayside land cover with opposite continent configurations. Since transit spectroscopy may permit observations of M-dwarf planets' atmospheres, but not their surfaces, these land-related climate differences likely represent a limiting uncertainty in a given planet's climate, even if its atmospheric composition is known. Our results are robust to variations in atmospheric CO2 concentration, stellar temperature, and instellation.

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arXiv2020

A Large Repository of 3D Climate Model Outputs for Community Analysis and Postprocessing

As the number of known exoplanets has climbed into the thousands, efforts by theorists to understand the diversity of climates that may exist on terrestrial planets in the habitable zone have also accelerated. These efforts have ranged from analytical, to simple 0-D, 1-D, and 2-D models, to highly-sophisticated 3D global climate models (GCMs) adapted from Earth climate and weather models. The advantage of the latter is that fewer physical processes are reduced to simple parameterizations and empirical fits, and may instead be represented by physically-motivated algorithms. However, many such models are difficult to use, and take a long time to reach a converged state relative to simpler models, thereby limiting the amount of parameter space that can be explored. We use PlaSim, a 3D climate model of intermediate complexity, to bridge this gap, allowing us to produce hundreds to thousands of model outputs that have reached energy balance equilibrium at the surface and top of the atmosphere. We are making our model outputs available to the community in a permanent Dataverse repository (https://dataverse.scholarsportal.info/dataverse/kmenou). A subset of our model outputs can be used directly with external spectral postprocessing tools, and we have used them with petitRADTRANS and SBDART in order to create synthetic observables representative of fully-3D climates. Another natural use of this repository will be to use more-sophisticated GCMs to cross-check and verify PlaSim's results, and to explore in more detail those regions of the exoplanet parameter space identified in our PlaSim results as being of particular interest. We will continue to add models to this repository in the future, including more than 1000 models in the short- to medium-term future, expanding the diversity of climates represented therein.

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