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Cloud behaviour on tidally locked rocky planets from global high-resolution modelling

Nature Astronomy volume 7, pages 1070–1080 (2023)Cite this article

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Abstract

Determining the behaviour of convection and clouds is one of the biggest challenges in our understanding of exoplanetary climates. Given the lack of in situ observations, one of the most preferable approaches is to use cloud-resolving or cloud-permitting models (CPM). Here we present CPM simulations in a quasi-global domain with high spatial resolution (4 × 4 km2 grid) and explicit convection to study the cloud regime of 1:1 tidally locked rocky planets orbiting around low-mass stars. We show that the substellar region is covered by deep convective clouds and cloud albedo increases with increasing stellar flux. The CPM produces relatively lower cloud liquid water concentration, smaller cloud coverage, lower cloud albedo and deeper H2O spectral features than previous general circulation model simulations using empirical convection and cloud parameterizations. Furthermore, cloud streets—long bands of low-level clouds oriented nearly parallel to the direction of the mean boundary-layer winds—appear in the CPM and substantially affect energy balance and surface precipitation at a local level.

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Fig. 1: Maps of instantaneous vertically integrated cloud liquid and ice water amount in the SAM experiments with a horizontal resolution of 4 km.
Fig. 2: Time-mean vertically integrated cloud water amount (including both liquid and ice) simulated by the three models.
Fig. 3: Vertical profiles of cloud water content (ice plus liquid) in the small-domain simulations with different horizontal resolutions, 6.4, 1.6, 0.4 and 0.1 km.
Fig. 4: Stabilizing cloud feedback simulated by the three models.
Fig. 5: Cloud streets in the SAM experiments with a resolution of 4 km.
Fig. 6: The effects of cloud permitting on the observational characteristics of the planets.

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Data availability

All model output data used in this paper can be found in the public storage https://doi.org/10.5281/zenodo.7226615.

Code availability

SAM can be downloaded from http://rossby.msrc.sunysb.edu/~marat/SAM.html, ExoCAM can be download from https://github.com/storyofthewolf/ExoCAM, CAM3 can be downloaded from https://www.cesm.ucar.edu/models/atm-cam/ and the transmission spectra module PSG can be found on https://psg.gsfc.nasa.gov/index.php. Code modifications for SAM and CAM3 can be found in the public storage of Harvard Dataverse: https://doi.org/10.7910/DVN/EM1NPX.

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Acknowledgements

We thank M. F. Khairoutdinov for the release of the model SAM, E. T. Wolf for the release of the model ExoCAM and G. Villanueva for the release of the tool PSG. We are grateful for the helpful discussions with D. D. B. Koll, B. Yang, H. Yang, D. S. Abbot, N. Jeevanjee, C. Li and S. Fu. We thank J. Nie for his help in installing the model SAM on Tianhe-2. Z.F. was supported by the National Natural Science Foundation of China (NSFC) under grant no. 42175065, and J.Y. was supported by NSFC under grant nos. 42075046, 41888101 and 42161144011. In total, about 5 × 106 core hours have been used in the experiments. This corresponds to CO2 emission of ~8,000 kg, if we assume the power per core is 3.7 W and the carbon emission intensity is ~0.7 kg kWh−1.

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Author notes

  1. These authors contributed equally: Jun Yang, Yixiao Zhang.

Authors and Affiliations

  1. Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China

    Jun Yang, Yixiao Zhang, Zuntao Fu, Mingyu Yan, Xinyi Song, Mengyu Wei, Jiachen Liu & Feng Ding

  2. Previously at Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA

    Feng Ding

  3. Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA

    Zhihong Tan

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Contributions

J.Y. led this project. J.Y., Z.F. and Y.Z. designed the experiments. Y.Z. modified the model SAM and performed most of the SAM experiments. M.Y. carried out the six quasi-global cloud feedback experiments using SAM. J.Y. performed the CAM3 experiments. Y.Z. performed the ExoCAM experiments. X.S. calculated the observational characteristics. J.L. carried out the episodic deluge analyses. Y.Z., M.Y., X.S. and M.W. plotted the figures. F.D. and Z.T. contributed to the convection analyses. All authors discussed the results. J.Y. wrote the draft, and all authors improved the manuscript.

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Correspondence to Jun Yang.

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Nature Astronomy thanks Denis Sergeev and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Discussion, Supplementary Tables 1–4, Supplementary Figs 1–21 and 26 references.

Supplementary Video 1

Time variability of the vertically integrated cloud liquid and ice water amount (g m−2) in the simulation of TRAPPIST-1e with a horizontal resolution of 4 km. The time interval of each frame is 1 h.

Supplementary Video 2

Time variability of the vertically integrated cloud liquid and ice water amount (g m−2) in the simulation of K2-72e. This video is the same as Supplementary Video 1, but for the planet K2-72e.

Supplementary Video 3

Time variability of the vertically integrated cloud liquid and ice water amount (g m−2) in the simulation of TRAPPIST-1e. This video is the same as Supplementary Video 1, but the simulation uses a higher resolution of 2 km.

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Yang, J., Zhang, Y., Fu, Z. et al. Cloud behaviour on tidally locked rocky planets from global high-resolution modelling. Nat Astron 7, 1070–1080 (2023). https://doi.org/10.1038/s41550-023-02015-8

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