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Background

The setting for a story I am creating is an earth-like planet near jovian surface area, existing in a universe where the force of gravity is appreciably weaker than in our own, such that a planet with the surface area around that of Jupiter will have a surface gravity of 1G. This planet has 24 hour days, an axial tilt of 23 degrees, and an atmosphere analogous to Earth's. The planet is in all ways an Earth analogue. The only difference is scale. Or is it?

How Many Cells

I understand that the number of convection cells within the atmosphere is determined by the rotational rate of the planet, and I get that it has something to do with the Coriolis effect, but I'm too much of a non-scientist know grasp much more than that. The faster the planet rotates, the more heavily trajectories will bend relative to the surface of the planet. If a planet is larger than Earth, but has the same rotation rate, its surface travels faster than the same latitudes on Earth. My question is whether these factors frustrate the application of Earth's wind system to this world.

How To Determine It

I'm looking for a way to determine definitively how many convection cells to expect on a world of a given surface area and rotation rate, if there is one. I don't see why the difference in the gravitational constant would have any pertinent effect, but I'm definitely not a scientist who studies this kind of thing. Mainly, I need some way of finding out at what latitudes each zone begins and ends. This setting is intended to be a reasonably realistic fantasy world, just so that it is scientifically impeccable from the audience-perspective of age-of-sail equivalent societies.

That's all I have. Winds guide you, fellow travelers.

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  • $\begingroup$ Did you try to estimate the equatorial bulge of this planet? Could it be possible that 24h day is way too fast rotation? $\endgroup$
    – fraxinus
    Oct 10, 2021 at 16:36
  • $\begingroup$ @fraxinus I used this online calculator, artificial-gravity.com/sw/SpinCalc, to calculate this, and it seems to state that the centrifugal acceleration at the equator of this planet is sufficiently less than the natural gravity, though the downward pull of gravity at that location might be noticeably less (0.96G instead of 1G). The tangental velocity at the equator does look a little intense, 18,301 km/h compared to Earth's 1,689 km, and that could cause some extreme winds there, but that fits the setting fine, as the lands south of the equator are unknown and unexplored. $\endgroup$ Nov 5, 2021 at 18:13

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I think this difficult question to answer, but sounds like an interesting world building premise. There seems to be quite a bit of debate on the details of how Jupiter's atmosphere works.

People have been able to do laboratory scale lab experiments by rotating shapes in water and get the banding, and other features. Computationally there is a paper I skimmed "Zonal flow regimes in rotating anelastic spherical shells: An application to giant planets" by Gastine et al. that does numerical simulations. So from a fluid mechanics and physics eventually you could probably try to come up with some number of bands as an answer.

The problem the scientists have trying to model Jupiter seems to be the related to understanding what is driving the convective flow of the atmosphere, is it shallow, or is it deep, and what kind of heat transfer is driving the flow. So in the modeling they seem to divide the problem two categories. Or in their words, "the influence of density stratification on the transition between the rotation-dominated and the buoyancy-dominated zonal flow regimes."

With your light gravity, perhaps you would have less buoyancy forces and thus be more rotation dominated? That might lead to more bands jetting east or west which could be convenient from a story telling point of view. But I am not sure of the "height" of the atmosphere in your case. With less gravity it seems like that could be an issue. Also in general with such a large surface area it seems like you might need to be careful about the ratio of water to land which might drive the weather. I suspect turbulence could be a big issue.

I suppose you could have vertical layering in the atmosphere and for your purposes of travel that could also help divide into regions. That could be convenient.

Google for instance has been studying stratospheric balloons and controlling their navigation by controlling the altitude of the balloon since the winds at high altitudes can vary their direction at different heights.

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