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Convection occurs ubiquitously on and in rotating geophysical and astrophysical bodies. Prior spherical shell studies have shown that the convection dynamics in polar regions can differ significantly from the lower latitude, equatorial dynamics. Yet most spherical shell convective scaling laws use globally-averaged quantities that erase latitudinal differences in the physics. Here we quantify those latitudinal differences by analysing spherical shell simulations in terms of their regionalized convective heat-transfer properties. This is done by measuring local Nusselt numbers in two specific, latitudinally separate, portions of the shell, the polar and the equatorial regions, $$Nu_p$$ and $$Nu_e$$ , respectively. In rotating spherical shells, convection first sets in outside the tangent cylinder such that equatorial heat transfer dominates at small and moderate supercriticalities. We show that the buoyancy forcing, parameterized by the Rayleigh number $Ra$ , must exceed the critical equatorial forcing by a factor of $${\approx }20$$ to trigger polar convection within the tangent cylinder. Once triggered, $$Nu_p$$ increases with $Ra$ much faster than does $$Nu_e$$ . The equatorial and polar heat fluxes then tend to become comparable at sufficiently high $Ra$ . Comparisons between the polar convection data and Cartesian numerical simulations reveal quantitative agreement between the two geometries in terms of heat transfer and averaged bulk temperature gradient. This agreement indicates that rotating spherical shell convection dynamics is accessible both through spherical simulations and via reduced investigatory pathways, be they theoretical, numerical or experimental.
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Convection in Thin Shells of Icy Satellites: Effects of Latitudinal Surface Temperature Variations
We use three‐dimensional numerical experiments of thin shell convection to explore what effects an expected latitudinal variation in solar insolation may have on a convection. We find that a global flow pattern of upwelling equatorial regions and downwelling polar regions, linked to higher and lower surface temperatures (Ts), respectively, is preferred. Due to the gradient inTs, boundary layer thicknesses vary from equatorial lows to polar highs, and polar oriented flow fields are established. AHadley cell‐type configuration with two hemispheric‐scale convective cells emerges with heat flow enhanced along the equator and suppressed poleward. The poleward transport pattern appears robust under a range of basal and mixed heating, isoviscous and temperature‐dependent viscosity, vigor of convection, and different degrees ofTsvariations. Our findings suggest that a latitudinal variation inTsis an important effect for convection within the thin ice shells of the outer satellites, becoming increasingly important as solar luminosity increases. VariableTsmodels predict lower heat flow and a more compressional regime near downwellings at higher latitudes, and higher heat flow and a more extensional regime near the equator. Within the ice shell, Hadley style flow could lead to large‐scale anisotropic ice properties that might be detectable with future seismic or electro‐magnetic observations.
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- Award ID(s):
- 1853856
- PAR ID:
- 10495086
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Planets
- Volume:
- 124
- Issue:
- 8
- ISSN:
- 2169-9097
- Page Range / eLocation ID:
- 2029 to 2053
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1029/2018JE005799
