Introduction

Exoplanets

• >4000 exoplanets discovered
• Most detected are gas giants (like Jupiter)
• More and more rocky planets are detected

... are awesome!

Terrestrial tidally locked planets

• Habitable ~ have surface liquid water 
• M-dwarf are relatively cool
• Potentially habitable planets around M-dwarfs have to orbit closer
• Strong tidal forces
• Planet can become tidally locked
• Orbital period = rotation period
• Permanent day-side and night-side

Climate of Earth-like tidally locked exoplanets

Climate of Earth-like tidally locked exoplanets

Climate of Earth-like tidally locked exoplanets

Challenges in convection modelling

• Wide scale separation between convection and planetary-scale circulation 
• Resolving convection is computationally expensive
• Convection is subgrid-scale and parameterised
• All existing parameterisations are tested against observations on Earth and still are a big uncertainty for future climate projections
• No in-situ measurements on exoplanets

Modelling framework

• Full deep-atmosphere non-hydrostatic Navier-Stokes equations
• Full suite of parameterisations
• Seamless modelling: the same model is used for global climate & local high-res modelling
• Adapted for exoplanet atmospheres

• Orbital parameters of Trappist-1e and Proxima Centauri b
• Planets are tidally locked to M-dwarf stars
• \(N_2\)-dominated atmosphere with traces of \(CO_2\) and \(H_2O\)
• Mean surface pressure of 1 bar
• Aquaplanet regime (a slab ocean)
• Two modes
  • Global coarse-resolution (\(2^\circ\times 2.5^\circ\)) model with parameterised convection
  • Regional high-resolution (4 km) model with explicit convection

Credit: NASA/JPL-Caltech

Credit: ESA

Trappist-1e

Proxima b

Representation of convection in our global setup

• Standard operational scheme in the UM
• Most sophisticated class of convection schemes
• Involves a lot of "tuning" parameters

• Convective adjustment to a reference state
• Only 2 free parameters
• Similar to Betts-Miller convection scheme

NoCnvPm

Adjust

MassFlux (Control)

• Convection parameterisation switched off

Mean global climate and its sensitivity to the convection parameterisation

Global simulation of Trappist-1e

Shown are:

• Surface temperature
• Cloud condensate
• Upper troposphere wind vectors

Surface temperature (\(T_s\))

• \(T_s\) day-night contrast larger on Proxima b than on Trappist-1e

Surface temperature (\(T_s\))

• \(T_s\) day-night contrast larger on Proxima b than on Trappist-1e

MassFlux

• \(T_s\) increases globally, especially in cold traps on the night side
• \(\Delta T_{s,dn}\) reduces

Adjust

Surface temperature (\(T_s\))

• \(T_s\) day-night contrast larger on Proxima b than on Trappist-1e

MassFlux

• \(T_s\) increases globally, especially in cold traps on the night side
• \(\Delta T_{s,dn}\) reduces

NoCnvPm

Adjust

• Small global-mean change
• Different response on Trappist1-e vs Proxima b

Surface temperature (\(T_s\)) and free troposphere winds

• \(T_s\) day-night contrast larger on Proxima b than on Trappist-1e

• \(T_s\) increases globally, especially in cold traps on the night side
• \(\Delta T_{s,dn}\) reduces

NoCnvPm

Adjust

MassFlux

• Small global-mean change
• Different response on Trappist1-e vs Proxima b

Circulation regime (eddy components)

• Planetary-scale equatorial Rossby wave-like pattern

MassFlux

Circulation regime (eddy components)

• Planetary-scale equatorial Rossby wave-like pattern

MassFlux

Circulation regime (eddy components)

• Planetary-scale equatorial Rossby wave-like pattern

MassFlux

• Almost a mirror-opposite of the MassFlux
• Dominated by extratropical baroclinic gyres

Adjust

Circulation regime (eddy components)

• Planetary-scale equatorial Rossby wave-like pattern

• Almost a mirror-opposite of the MassFlux
• Dominated by extratropical baroclinic gyres

NoCnvPm

Adjust

MassFlux

• Broadly similar response as in Adjust

Circulation regime (eddy components)

• Planetary-scale equatorial Rossby wave-like pattern

• Almost a mirror-opposite of the MassFlux
• Dominated by extratropical baroclinic gyres

NoCnvPm

Adjust

MassFlux

• Broadly similar response as in Adjust

• Different shape of latent heating on the day side
• Change in baroclinicity

Clouds and precipitation

MassFlux

Clouds and precipitation

MassFlux

Clouds and precipitation

MassFlux

Clouds and precipitation

• Large reduction of clouds on the day side
• More clouds in polar regions of Trappist-1e

Adjust

MassFlux

Clouds and precipitation

• Large reduction of clouds on the day side
• More clouds in polar regions of Trappist-1e

NoCnvPm

Adjust

• Day side: closer to MassFlux
• Night side: closer to Adjust

MassFlux

Summary of global simulations: the effects of convection parameterisation

• The global climate of TL exoplanets is altered (and not just the day side!)
• The effect is planet-dependent
• Caution should be taken when interpreting results of GCMs with one parameterisation or another.

Extra

Circulation regime

What's happening in the Trappist-1e case?

• Different shape of latent heating on the day side
• Change in baroclinicity

Differences in circulation regimes

• Leconte+ (2013)
• Haqq-Misra+ (2017)

Energy redistribution

• Using the horizontal divergence of the MSE flux, \(\int_0^{z_{top}} \rho \nabla\cdot\left(\vec u (c_p T + g z + L_v q) \right)dz\)
• The largest difference is in the moist component (\(Lq\))
• Adjust results in a more intense energy transport

Vertical profiles of temperature and humidity

Vertical cross-sections of heat fluxes at \(90^\circ E\)

High-resolution simulations

"global" (MassFlux)

"HighRes"

Nested grid setup

Spatial variability: top-of-atmosphere outgoing LW radiation

Spatial variability: top-of-atmosphere outgoing LW radiation

Spatial variability: top-of-atmosphere outgoing LW radiation

Spatial variability: top-of-atmosphere outgoing LW radiation

Histograms of instantaneous TOA OLR

Spatial variability: top-of-atmosphere outgoing LW radiation

Histograms of instantaneous TOA OLR

Vertical cloud structure (averaged profiles)

Vertical cloud structure (averaged profiles)

Vertical cloud structure (averaged profiles)

Latent heating (averaged profiles)

Extra

Global impact of resolved convection

• Caveat: one-way nesting of the model
• How to assess the impact of HighRes simulations on the global climate?

• Caveat: one-way nesting of the model
• How to assess the impact of HighRes simulations on the global climate?

1. Run an ensemble of global experiments with perturbed convection parameters

1. Run an ensemble of global experiments with perturbed convection parameters
2. Integrate a metric of convection strength over the substellar region

• Caveat: one-way nesting of the model
• How to assess the impact of HighRes simulations on the global climate?

1. Run an ensemble of global experiments with perturbed convection parameters
2. Integrate a metric of convection strength over the substellar region
3. Correlate with a metric of day-night heat redistribution

• Caveat: one-way nesting of the model
• How to assess the impact of HighRes simulations on the global climate?

1. Run an ensemble of global experiments with perturbed convection parameters
2. Integrate a metric of convection strength over the substellar region
3. Correlate with a metric of day-night heat redistribution
4. Extrapolate for the convection metric from HighRes simulations

• Caveat: one-way nesting of the model
• How to assess the impact of HighRes simulations on the global climate?

1. Run an ensemble of global experiments with perturbed convection parameters
2. Integrate a metric of convection strength over the substellar region
3. Correlate with a metric of day-night heat redistribution
4. Extrapolate for the convection metric from HighRes simulations

• Caveat: one-way nesting of the model
• How to assess the impact of HighRes simulations on the global climate?

1. Run an ensemble of global experiments with perturbed convection parameters
2. Integrate a metric of convection strength over the substellar region
3. Correlate with a metric of day-night heat redistribution
4. Extrapolate for the convection metric from HighRes simulations

• Caveat: one-way nesting of the model
• How to assess the impact of HighRes simulations on the global climate?

Summary

• Representation of convection is important for the global climate of exoplanets
• Using a simpler adjustment scheme leads to
  • on the day side: reduction of clouds, decrease in albedo (by ~60%)
  • on the night side: warming of 20-30 K
  • reduction in the day-night temperature contrast
  • change in the global circulation
• The sensitivity to convection scheme is planet-dependent
• The choice of convection scheme does not push the simulated climate out of habitable state
• A potential global convection-permitting simulation may enhance the day-night temperature contrast