What Makes the Windiest Exoplanets? – Exploring Atmospheric Dynamics

[Tazerfish]

TL;DR Summary

What non gas–giants have the fastest wind speeds?
What are the factors (like surface pressure, coriolis parameter, planetary radius) affecting wind speeds?
Where could I look to learn more about this?

I love weather and atmospheric physics! So when I recently read Dune, I couldn’t stop thinking about the “Coriolis Storm”, wondering if such a crazy storm could really exist, ripping around an Earth-like planet at 700 km/h.

For me, this became a worldbuilding exercise: What makes the windiest exoplanets?

I found this article discussing the fastest wind speeds we have observed in our solar system and beyond: https://www.forbes.com/sites/starts...-the-fastest-planetary-winds-in-the-universe/.
But as I was reading it, I realized I don’t understand the observed patterns at all.
Neptune, the furthest-out and coldest planet, has the highest wind speeds although temperature differences drive the winds.
Moreover, Venus, a planet that barely rotates at all, has wind speeds faster than Earth’s, exceeding even its planetary rotation speed.
That doesn't make any sense to me.

I’ve had a few introductory lectures on enviromental/atmospheric physics where I learned (among other things) about geostrophic flow, thermal wind, and global circulation.
However, real life is often so complicated that these simple models don’t get you far—at least when you only superficially understand them.
I struggle to apply the ideas here, instead asking the experts:

What factors produce high wind-speed planets ?
Where can I read up on exoplanet wind/dynamics ?

 

[sophiecentaur]

TL;DR Summary: What non gas–giants have the fastest wind speeds?
What are the factors (like surface pressure, coriolis parameter, planetary radius) affecting wind speeds?
Where could I look to learn more about this?

“Coriolis Storm”, wondering if such a crazy storm could really exist,

You could say that all cyclones / depressions on Earth are there due to the coriolis force. Isn't the great red spot on Jupiter due to the same thing?

 

[Tazerfish]

You could say that all cyclones / depressions on Earth are there due to the coriolis force. Isn't the great red spot on Jupiter due to the same thing?

Yes, the coriolis force is super ubiquitous on earth.
Sure, there are some small scale circulations like the sea breeze or mountain/valley winds that would happen much the same without the coriolis force, too small to be deflected much.
However, almost all large scale wind systems are "geostrophic flow" at higher altitudes, which means that the pressure gradient force is balanced by the coriolis force.


The Jetstream wouldn't exist without coriolis force. Cyclones wouldn't exist without coriolis force.
Winds would be much weaker and generally meridional, i.e. north to south or vice-versa, instead of east-west like today.


What I'm trying to figure out though, is what factors make the windiest planets.
Do you want fast rotation or slow rotation?
Should the planet be large or small?
Should it have a dense thick atmosphere?
What about clouds, do they help with stronger winds?

I'll add some hunches I have so far, attempting to answer my own question:

On earth, the latent heat released by water is essential in all our strongest storms, from tornados to hurricanes, so I assumed that the bone dry Arrakis should have less intense winds.
I guess that for our high wind speed planets we want a lot of water vapor or some other gas that stores a lot of latent heat.

There's also the question why Jupiter's great red spot can hang around so long, when earth's storms dissipate within a few days.
I think it's mostly because we have a surface.
The turbulent momentum transport and energy dissipation limits wind speeds over land, especially over mountainous/rough terrain.
Except for the Siberian high, you usually get much stronger pressure fluctuations and wind speeds over the oceans, there's just less in the way to slow the wind down.
So we probably want either a water world or a tectonically inactive planet where the mountains have already been ground down.
Then, wind can flow freely.

As for rotation, I'm really torn. It would intuitively seem that fast rotation is key. But in the equations for geostrophic flow or thermal wind, the coriolis parameter is actually in the denominator!
And in our solar system it doesn't seem like the fastest rotating planet Jupiter is winning the prize for fastest winds.
Moreover, Venus has surprisingly fast winds despite barely rotating at all.

 

[sophiecentaur]

what factors make the windiest planets.

Perhaps the specific chemistry of the atmosphere would make a difference. The energy involved in changes of state would affect the temperature distribution at different mean temperatures. If the (mean?) atmospheric temperature of Earth were above boiling point then there would be no / less precipitation. What changes of state are taking place on Jupiter, I wonder, to allow the long life of the red spot?

 

[Tazerfish]

The energy involved in changes of state would affect the temperature distribution at different mean temperatures.

You make a good point!
When dry air rises, it cools adiabatically at about 10C/1000m. Wet air on the other hand will release latent heat when water condenses, bringing that rate down, usually to about 5C per km.
On earth, the typical (tropospheric) vertical temperature gradient is about 6-7C per km, without water it would be closer to 10C.
Plus, I suspect it's also important for the horizontal heat transport. You can basically store heat in the state and release it somewhere else, so horizontal heat transport gets supercharged by water vapor.
I recall hearing that back in the Pleistocene-Eocine thermal maximum—50 million years ago, when the earth was an average 5–8 degrees warmer—the poles were not much colder than the equator.
A lot of that is probably the ice albedo effect. Snow is bright; when it melts the ground absorbs more heat.
Yet part of it is likely also improved horizontal heat transfer by air/water vapor.
Warmer air -> more water vapor/latent heat transfer -> better heat transfer.

That different temperature profile would then in turn influence winds.
The Jetstream, a high altitude wind band flowing around the earth, is driven by temperature differences between cold polar air and (sub-)tropical air.
The stronger the horizontal temperature gradient, the faster the Jetstream above.

So back when the poles weren't very cold, they would have had a much weaker Jetstream. We actually see something similar with climate change.
Not every part of the earth warms at the same rate; the north pole in particular warms much faster, reducing the temperature difference and weakening the Jetstream.
Afaik this isn't 100% empirically confirmed yet, but the theory makes sense.
Fun Fact: A weaker Jetstream is more prone to large meanders and producing weather that *just won't move*.


As for Jupiter, I think the longevity of the great red spot is mostly due to a lack of friction/dissipation.
While the velocities are high, there isn't much velocity difference between adjacent air.
It's like the Jetstream. You wouldn't even know you were in it!
Everything moves almost laminarly in the same direction, no turbulence, no wind gusts, no energy dissipation.
Maybe I have a completely wrong idea of this.
Tropical cyclones ultimately convect warm air from the surface up, and they do so pretty chaotically and turbulently.
Because the ground layer has very different horizontal velocities (it's slowed by the ground), the mixing/swapping with the higher air produces a lot of momentum transfer, i.e. friction slowing the storm down.

If you had no momentum transfer to the ground, a tropical cyclone could just persist indefinitely, or until the inside has lost it's temperature difference with the outside through radiation. It was a warm spot before, but now it's radiated away it's excess heat. For the heat content of the air, that will take about a week. Then again, if you have to cool down the underlying ocean, then that could take hundreds of times longer.


Sorry, that was way too long ^^

 

[Tazerfish]

I feel like meteorologists with a good theoretical understanding might be able to help me out.

Atmospheric dynamics of other planets doesn't seem to be a big research field, or maybe I just can't find the paper/books on it.

EDIT:I just saw that there's a sci-fi worldbuilding section in this forum. After I've managed to nail down what specifically I want to know—the current question is incredibly broad—I might post there :)

I hope I wasn't too incoherent, and thanks for the help Sophie!

PS: I thought a bit about earth being dry at 100C. We're used to water boiling at 100C, but in a pressure cooker you can go up to 120C with liquid water, on Everest it boils before 90C.
When we start boiling the oceans, the pressure increases.This can go on for a long time— the oceans are huge—so that evaporating them all would produce hundreds of bars/atmospheres of pressure from the water vapor. The mass isn't going anywhere so the pressure on the bottom of the ocean wouldn't even change much, there's just a slight decrease as some of what was contained in the oceans now spreads over the land.
For water to be gaseous at the surface, at 300bar, the temperature there would have to be more than 350C.
We'd be (not) living on Venus 2.0.

 

[sophiecentaur]

PS: I thought a bit about earth being dry at 100C.

Yes. That would be at standard pressure. With an atmosphere of 100% saturation, the density would be different so another temperature would apply, I guess. But the whole thing would have to be much more complicated than that - as it already is in our atmosphere. But I reckon there would have to be a temperature (sitting nearer the Sun) where this could happen.
Just wondering about Venus . . . . .

 

[Tazerfish]

Yes. That would be at standard pressure. With an atmosphere of 100% saturation, the density would be different so another temperature would apply, I guess. But the whole thing would have to be much more complicated than that - as it already is in our atmosphere. But I reckon there would have to be a temperature (sitting nearer the Sun) where this could happen.
Just wondering about Venus . . . . .

Venus is very weird.
I found this article claiming that solar tides and some sort of atmospheric waves were responsible for Venus' winds moving a mind boggling 60 times faster than the planets surface!
https://www.smithsonianmag.com/smart-news/forces-behind-venus-super-rotating-atmosphere-180974762/

I have to admit, I don't get how.


One more fun/scary fact: The earth's ocean will actually boil away in a billion years or so as the sun expands and becomes more luminous.
I think a few people were even worried about this as "runaway greenhouse effect" even today.
Water vapor is a greenhouse gas—the most important one in terms of W/m^2 actually.
But because it is a function of temperature, it's not modelled as an input/parameter to the earth system; instead, it's treated as a feedback.
If that feedback was strong enough, then a slight increase in temperature would produce more water to evaporate, which would increase the temperature, increasing evaporation, increasing temperature.... ad absurdum.

Thankfully, there isn't enough fossil fuel in the world to kick us into such a runaway greenhouse scenario. The outgoing Longwave radiation just rises too quickly with temperature, unable to be overcome by the water vapor feedback.

Nonetheless, it will happen in the future!
Just give the sun a couple hundred million years ;)

 

[Tazerfish]

I've actually found a book that seems to give a good knowledge base to answer my questions: https://www.jstor.org/stable/j.ctt183gz90
Comparative climatology of terrestrial planets.
I found bits of it on atmospheric dynamics online ...
# Atmospheric circulation of terrestrial planets
By
Adam P. Showman - Uni Arizona
Robin D. Wordsworth - Uni Chicago
Timothy M. Merlis - Princeton Uni
Yohai Kaspi - Weizmann Institute of science

That chapter seemed very relevant and reasonably accessible.


Moreover, in looking up Venus' superrotating winds I learned that there are General Circulation Models (GCMs) that people have used to simulate all sort of planetary climates.
They're like weather forecasting models.
But what I'm getting from all of this: It's only a couple decades away from the bleeding edge. It's really niche.
Most of the people reading that stuff are probably graduate students or beyond.

I'm kind of out of my depth. Sadly, it's really hard to observe exoplanets, nevermind their atmospheric circulations. So I can't turn to empirical observations either.

Journey over destination, eh?
I did learn a lot of things on the way, but I think this is as deep as I'm willing to go.