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I have been working on a tidally locked planet for some time now. During this time I have worked under the assumption that the substellar point would be arid because of the constant bombardment of heat given by the sun. However, the more I read about such planets the more it seems to me that the suggestion is that the substellar point is tropical.

Would that large continent over the substellar point be tropical rainforest at its centre or would the constant evaporation prevent large plants from growing there?

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  • $\begingroup$ Depends on how hot it gets, depends on how large the large continent is, and so on. Stars can be hotter or cooler, nearer or farther away. Large continents can be huge (like Eurasia) or not quite so huge (like South America). $\endgroup$
    – AlexP
    Commented Aug 23, 2023 at 9:36
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    $\begingroup$ It can be colder then the North Pole or hotter than Mercury. "Tidally locked" doesn't mean a planet with some specific climate. It is a particular case of orbital mechanics, climate is much more complex. And no one really knows how it would work in reality. $\endgroup$
    – D'Monlord
    Commented Aug 23, 2023 at 10:21
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    – Community Bot
    Commented Aug 23, 2023 at 10:55

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Assuming it is the right temp for rain there should be a rainforest and quite a big one.

This also assumes there is enough water but not so much its just an ocean.

Something similar creates real rainforests on earth, See a Hadley cells

enter image description here

Rising air is where most rain comes from. As warm air moves along the ground it picks up moisture. As that warm moist air rises the potential to hold water drops because the air cools and its pressure drops as it rises. So moisture in the air condenses out as the rising air becomes supersaturated as saturation potential drops. Its basically a ring shaped Hadley cell. this should be the place were the most rain falls.

Cloud cover will be an issue, since these clouds will shield the ground below it, the warmest point on the planet should be constantly moving, meaning you may actually get something like weather with clear days and storms near the calculated point.

The other consideration is erosion, the erosional forces in the area will be huge, unless the planet has active tectonics or its not perfectly locked this area should erode until it is uniformly flat. So there actually may be a shallow sea, so more like a mangrove forest, but only if the planet has been locked for a LONG time.

The only problem you will have is is there enough moisture left for rain in the first place since most of your moisture should freeze out on the cold side. But if you have enough water to get rain, this is where it should be falling.

It is worth noting a tidally locked planet still rotates so Coriolis effects still occur. It just rotates once per year so the effects are not very strong. This will help cause disruption to the air flow and means the rainforest will not be round but more football shaped.

Useful take on the problem. https://worldbuildingpasta.blogspot.com/2020/12/an-apple-pie-from-scratch-part-ivd.html

How circulation works on a tidally locked planet.

Ditto

https://www.pnas.org/doi/10.1073/pnas.2022705118

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Probably yes, but...

I asked myself the same question. For example, this paper says it depends on the size of the continent:

On a tidally locked planet, water vapor is transported toward the substellar point, where precipitation is maximum, from a surrounding evaporative ring by strong boundary layer convergence. Once near the substellar point, strong surface heating induces deep convection that forces water vapor upward, resulting in it being precipitated out (Boutle et al. 2017). We have found that this process is insensitive to the introduction of a substellar continent, meaning that the center of a continent can be very wet, as rain always preferentially falls near the substellar point.

The introduction of a substellar land mass serves to reduce the availability of moisture at the substellar point, which decreases both the water vapor greenhouse effect and the cloud radiative effect.

Precipitation and clouds (model)

I'm not sure but probably higher average surface temperatures will lead to even more rainfall. As for evaporation, it seems to be a problem of the surrounding lands, some sort of our subtropics. Also, they are likely to be hotter than the substellar point.

Topography is also important: if you eventually build up high enough mountains around your substellar point they could potentially cool and "stop" clouds (rain shadow)

PS: sorry if my english isn't clear:)

The Influence of a Substellar Continent on the Climate of a Tidally Locked Exoplanet

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Being the point receiving more energy, it would be also the point where the air would constantly go up and flow out towards the sides. The ensuing atmospheric circulation would make it very difficult for any rain to fall here, because there would be no time for allowing condensation and precipitation.

Additionally, the relative small area with respect to the surroundings would mean that the winds would blow stronger, again sucking and pushing away quickly whatever humidity evaporates in the air.

It's possible that there would be a ring of precipitation rich areas around the substellar point, if the other circumstances are right. But to determine that some simulation would be needed.

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  • $\begingroup$ actually rising air is where most rain comes from, the potential to hold water drops as the the air rises and cools, so moisture in the air condenses out as the rising air becomes supersaturated as saturation potential drops. water is picked up from the air moving inwards along the ground. Its basically a ring shaped Hadley cell. $\endgroup$
    – John
    Commented Aug 23, 2023 at 14:43
  • $\begingroup$ @John, he's right about this. The stellar side would heat the air, increasing its moisture capacity. If it had oceans, then you would get heavy rains where the outflow of air slowed down. Any forest with even vaguely terrestrial plants would have all of their moisture sucked out of them. $\endgroup$
    – Robert Rapplean
    Commented Aug 23, 2023 at 15:35
  • $\begingroup$ @RobertRapplean except that's not how it works, upwelling air is were the moisture drops out, warm air rises causing it to drop its water load. where you think there should be rain are actually were deserts are on earth. check your assumptions against reality. On earth the most evaporation happens at the equator but also the highest rainfall. $\endgroup$
    – John
    Commented Aug 23, 2023 at 17:35
  • $\begingroup$ @John, You're thinking of a spinning planet. Hadley cells are a Coriolis effect thing. The only example of a tide-locked planet with an atmosphere is Venus, where the solar heating resulted in a full-planet perpetual hurricane that actually abraded the surface to counter-spin. $\endgroup$
    – Robert Rapplean
    Commented Aug 23, 2023 at 20:24
  • $\begingroup$ @RobertRapplean No, Hadley cells are caused by temperature difference, they are broken into 3 different cells that circle the globe latitudinally because of the spin of the planet. Also a tidally locked planet is still spinning it is just spinning once per year. And yes Venus has a massive double layer Hadley cell, 1 per hemisphere, if Venus were cold enough for rain it would have equatorial rainfall. . But also Venus is NOT tidally locked it is only close, it does not have a permanent solar point. $\endgroup$
    – John
    Commented Aug 23, 2023 at 20:50

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