Raymond T. Pierrehumbert's research while affiliated with University of Oxford and other places

The atmospheric dynamics of tidally locked hot Jupiters is characterized by strong equatorial winds. Understanding the interaction between global circulation and chemistry is crucial in atmospheric studies and interpreting observations. Two-dimensional (2D) photochemical transport models shed light on how the atmospheric composition depends on circulation. In this paper, we introduce the 2D photochemical (horizontal and vertical) transport model, VULCAN 2D, which improves on the pseudo-2D approaches by allowing for nonuniform zonal winds. We extensively validate our VULCAN 2D with analytical solutions and benchmark comparisons. Applications to HD 189733 b and HD 209458 b reveal a transition in mixing regimes: horizontal transport predominates below ∼0.1 mbar, while vertical mixing is more important at higher altitudes above 0.1 mbar. Motivated by the previously inferred carbon-rich atmosphere, we find that HD 209458 b with supersolar carbon-to-oxygen ratio (C/O) exhibits pronounced C 2 H 4 absorption on the morning limb but not on the evening limb, due to horizontal transport from the nightside. We discuss when a pseudo-2D approach is a valid assumption and its inherent limitations. Finally, we demonstrate the effect of horizontal transport in transmission observations and its impact on the morning−evening limb asymmetry with synthetic spectra, highlighting the need to consider global transport when interpreting exoplanet atmospheres.

Compositional convection is atmospheric mixing driven by density variations caused by compositional gradients. Previous studies have suggested that compositional gradients of atmospheric trace species within planetary atmospheres can impact convection and the final atmospheric temperature profile. In this work, we employ 3D convection-resolving simulations using Cloud Model 1 (CM1) to gain a fundamental understanding of how compositional variation influences convection and the final atmospheric state of exoplanet atmospheres. We perform 3D initial value problem simulations of noncondensing compositional convection for Earth-air, H 2 , and CO 2 atmospheres. Conventionally, atmospheric convection is assumed to mix the atmosphere to a final, marginally stable state defined by a unique temperature profile. However, when there is compositional variation within an atmosphere, a continuous family of stable end states is possible, differing in the final state composition profile. Our CM1 simulations are used to determine which of the family of possible compositional end states is selected. Leveraging the results from our CM1 simulations, we develop a dry convective adjustment scheme for use in general circulation models (GCMs). This scheme relies on an energy analysis to determine the final adjusted atmospheric state. Our convection scheme produces results that agree with our CM1 simulations and can easily be implemented in GCMs to improve modeling of compositional convection in exoplanet atmospheres.

Ultra-hot Jupiters (UHJs) are natural laboratories to study extreme physics in planetary atmospheres and their rich observational data sets are yet to be confronted with models with varying complexities at a population level. In this work, we update the general circulation model of Tan & Komacek (2019) to include a non-grey radiative transfer scheme and apply it to simulate the realistic thermal structures, phase-dependent spectra, and wavelength-dependent phase curves of UHJs. We performed grids of models over a large range of equilibrium temperatures and rotation periods for varying assumptions, showing that the fractional day–night brightness temperature differences remain almost constant or slightly increase with increasing equilibrium temperature from the visible to mid-infrared wavelengths. This differs from previous work primarily due to the increasing planetary rotation rate with increasing equilibrium temperature for fixed host star type. Radiative effects of varying atmospheric compositions become more significant in dayside brightness temperature in longer wavelengths. Data-model comparisons of dayside brightness temperatures and phase-curve amplitudes as a function of equilibrium temperature are in broad agreement. Observations show a large scatter compared to models even with a range of different assumptions, indicating significantly varying intrinsic properties in the hot Jupiter population. Our cloud-free models generally struggle to match all observations for individual targets with a single set of parameter choices, indicating the need for extra processes for understanding the heat transport of UHJs.

Hycean worlds are a proposed subset of sub-Neptune exoplanets with substantial water inventories, liquid surface oceans, and extended hydrogen-dominated atmospheres favorable for habitability. We aim to quantitatively define the inner edge of the Hycean habitable zone (HZ) using a 1D radiative-convective model. As a limiting case, we model a dry hydrogen–helium envelope above a surface ocean. For a 1 bar (10,100 bar) atmosphere, the hydrogen greenhouse effect alone sets the inner edge of the HZ at 0.216 au (0.58, 3.71 au) for a Sun-like G star and at 0.0364 au (0.110, 0.774 au) for an 3500 K M star. Introducing water vapor into the atmosphere, the runaway greenhouse instellation limit is greatly reduced due to the presence of superadiabatic layers where convection is inhibited. This moves the inner edge of the HZ from ≈1 au for a G star to 1.6 au (3.85 au) for a Hycean world with a H 2 –He inventory of 1 bar (10 bar). For an M star, the inner edge is equivalently moved from 0.17–0.28 au (0.54 au). Our results suggest that most of the current Hycean world observational targets are not likely to sustain a liquid water ocean. We present an analytical framework for interpreting our results, finding that the maximum possible outgoing longwave radiation scales approximately inversely with the dry mass inventory of the atmosphere. We discuss the possible limitations of our 1D modeling and recommend the use of 3D convection-resolving models to explore the robustness of superadiabatic layers.

The vast majority of agri-food climate-based sustainability analyses use GWP100 as an impact assessment, usually in isolation; however, in recent years, discussions have criticised the ‘across-the-board’ application of GWP100 in Life Cycle Assessments (LCA), particularly of food systems which generate large amounts of methane (CH4) and considered whether reporting additional and/or alternative metrics may be more applicable to certain circumstances or research questions. This paper reports a sensitivity analysis using a pasture-based beef production system (a producer of high CH4 emissions) as an exemplar to compare various climate impact assessments: CO2-equivalents using GWP100 and GTP100, and ‘CO2-warming-equivalents’ using ‘GWP Star’, or GWP*. The inventory for this system was compiled using data from the UK Research and Innovation (UKRI) National Capability, the North Wyke Farm Platform, in Devon, SW England. LCAs can have an important bearing on: (i) policymakers’ decisions; (ii) farmer management decisions; (iii) consumers’ purchasing habits; and (iv) wider perceptions of whether certain activities can be considered ‘sustainable’ or not; it is, therefore, the responsibility of LCA practitioners and scientists to ensure that subjective decisions are tested as robustly as possible through appropriate sensitivity and uncertainty analyses. We demonstrate herein that the choice of climate impact assessment has dramatic effects on interpretation, with GWP100 and GTP100 producing substantially different results due to their different treatments of CH4 in the context of carbon dioxide (CO2) equivalents. Given its dynamic nature and previously proven strong correspondence with climate models, out of the three assessments covered, GWP* provides the most complete coverage of the temporal evolution of temperature change for different GHG emissions. We extend previous discussions on the limitations of static emission metrics and encourage LCA practitioners to consider due care and attention where additional information or dynamic approaches may prove superior, scientifically speaking, particularly in cases of decision support.

Hycean worlds are a proposed subset of sub-Neptune exoplanets with substantial water inventories, liquid surface oceans and extended hydrogen-dominated atmospheres that could be favourable for habitability. In this work, we aim to quantitatively define the inner edge of the Hycean habitable zone using a 1D radiative-convective model. As a limiting case, we model a dry hydrogen-helium envelope above a surface ocean. We find that 10 to 20 bars of atmosphere produces enough greenhouse effect to drive a liquid surface ocean supercritical when forced with current Earth-like instellation. Introducing water vapour into the atmosphere, we show the runaway greenhouse instellation limit is greatly reduced due to the presence of superadiabatic layers where convection is inhibited. This moves the inner edge of the habitable zone from $\approx$ 1 AU for a G-star to 1.6 AU (3.85 AU) for a Hycean world with a H$_2$-He inventory of 1 bar (10 bar). For an M-star, the inner edge is equivalently moved from 0.17 AU to 0.28 AU (0.54 AU). Our results suggest that most of the current Hycean world observational targets are not likely to sustain a liquid water ocean. We present an analytical framework for interpreting our results, finding that the maximum possible OLR scales approximately inversely with the dry mass inventory of the atmosphere. We discuss the possible limitations of our 1D modelling and recommend the use of 3D convection-resolving models to explore the robustness of superadiabatic layers.

The implications of the water vapor runaway greenhouse phenomenon for water-rich sub-Neptunes are developed. In particular, the nature of the postrunaway equilibration process for planets that have an extremely high water inventory is addressed. Crossing the threshold from subrunaway to superrunaway conditions leads to a transition from equilibrated states with cold, deep liquid oceans and deep interior ice-X phases to states with hot supercritical fluid interiors. There is a corresponding marked inflation of radius for a given mass, similar to the runaway greenhouse radius inflation effect noted earlier for terrestrial planets, but in the present case the inflation involves the entire interior of the planet. The calculation employs the AQUA equation-of-state database to simplify the internal structure calculation. Some speculations concerning the effect of H 2 admixture, silicate cores, and hot- versus cold-start evolution trajectories are offered. Observational implications are discussed though the search for the mass–radius signature of the phenomena considered is limited by degeneracies and by lack of data.

The gas giant HD 80606 b has a highly eccentric orbit (e ∼ 0.93). The variation due to the rapid shift of stellar irradiation provides a unique opportunity to probe the physical and chemical timescales and to study the interplay between climate dynamics and atmospheric chemistry. In this work, we present integrated models to study the atmospheric responses and the underlying physical and chemical mechanisms of HD 80606 b. We first run three-dimensional general circulation models (GCMs) to establish the atmospheric thermal and dynamical structures for different atmospheric metallicities and internal heat. Based on the GCM output, we then adopted a 1D time-dependent photochemical model to investigate the compositional variation along the eccentric orbit. The transition of the circulation patterns of HD 80606 b matched the dynamics regimes in previous works. Our photochemical models show that efficient vertical mixing leads to deep quench levels of the major carbon and nitrogen species and the quenching behavior does not change throughout the eccentric orbit. Instead, photolysis is the main driver of the time-dependent chemistry. While CH4 dominates over CO through most of the orbits, a transient state of [CO]/[CH4] > 1 after periastron is confirmed for all metallicity and internal heat cases. The upcoming JWST Cycle 1 GO program will be able to track this real-time CH4–CO conversion and infer the chemical timescale. Furthermore, sulfur species initiated by sudden heating and photochemical forcing exhibit both short-term and long-term cycles, opening an interesting avenue for detecting sulfur on exoplanets.

The implications of the water vapor runaway greenhouse phenomenon for water-rich subNeptunes are developed. In particular, the nature of the post-runaway equilibration process for planets that have an extremely high water inventory is addressed. Crossing the threshold from sub-runaway to super-runaway conditions leads to a transition from equilibrated states with cold deep liquid oceans and deep interior ice-X phases to states with hot supercritical fluid interiors. There is a corresponding marked inflation of radius for a given mass, similar to the runaway greenhouse radius inflation effect noted earlier for terrestrial planets, but in the present case the inflation involves the entire interior of the planet. The calculation employs the AQUA equation of state database to simplify the internal structure calculation. Some speculations concerning the effect of $\mathrm{H_2}$ admixture, silicate cores and hot vs. cold start evolution trajectories are offered. Observational implications are discussed, though the search for the mass-radius signature of the phenomena considered is limited by degeneracies and by lack of data.