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Moist convection in hydrogen atmospheres and the frequency of Saturn’s giant storms

Abstract

A giant storm erupted on Saturn in December 2010. It produced intense lightning and cloud disturbances and encircled the planet in six months. Six giant storms—also called Great White Spots—have been observed on Saturn since 1876, recurring every 20 to 30 years and alternating between mid-latitudes and the equator. Here we use thermodynamic arguments to demonstrate that the quasi-periodic occurrence of Saturn’s giant storms can be explained by a water-loading mechanism, in which moist convection is suppressed for decades owing to the relatively large molecular weight of water in a hydrogen–helium atmosphere. We find that the interaction between moist convection and radiative cooling in the troposphere above the cloud base produces an oscillation that leads to giant storm generation with a period of approximately 60 years for either mid-latitudes or the equator, provided the mixing ratio of water vapour in the troposphere exceeds 1.0%. We use a two-dimensional axisymmetric dynamic model and a top-cooling convective adjustment scheme to apply our conceptual model to Saturn. For a water vapour mixing ratio of 1.1%, simulated storms show a recurrence interval, ammonia depletion and tropospheric warming that are consistent with 2010 observations. Jupiter’s atmosphere is more depleted in water than Saturn, which may explain its lack of planet-encircling storms.

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Figure 1: Virtual temperature of moist adiabats.
Figure 2: Thermodynamic diagram for Saturn’s atmosphere.
Figure 3: Residual azimuthal wind and temperature anomaly after the geostrophic adjustment.
Figure 4: Time evolution of the ammonia vapour mixing ratio and the streamfunction.
Figure 5: Evolution of Saturn’s atmospheric temperature and minor constituents.

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Acknowledgements

This research was supported by the National Science Foundation, award number AST-1109299, and by the Cassini Project of NASA. We thank D. Yang for discussions on moist convection.

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Both authors participated equally in formulating the model and interpreting the final results. C.L. wrote the code for the numerical model and convective adjustment scheme.

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Correspondence to Cheng Li.

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The authors declare no competing financial interests.

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Li, C., Ingersoll, A. Moist convection in hydrogen atmospheres and the frequency of Saturn’s giant storms. Nature Geosci 8, 398–403 (2015). https://doi.org/10.1038/ngeo2405

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