Keeping an "hot eyeball planet" wet

Consider such a tidally locked planet:

Many issues with tidally locked worlds have been discussed, but I'm not clear on how the water cycle for an otherwise Earth-like tidally locked world, would be sustainable.

Basic assumptions: Weather would vary very little since there would be no seasons and no day-night cycle. The planet would be scorching hot on one side and freezing cold on the other. A strip straddling the terminator, the midpoint between the dayside and the nightside, would have water within the temperature range for it to be liquid. Atmosphere is dense enough for the necessary heat distribution to have a reasonably broad habitable "ring" around the terminator.

Let's pretend the problems of solar winds slowly stripping the atmosphere and of solar flares are solved.

Instead of warm air rising at the equator and settling at the poles as it does on Earth, it rises on the dayside and settles past the terminator line. It carries with it moisture, from the evaporation of water on the dayside. Some water precipitates past the terminator line and, if topography is kind, runs from one side of the ring of life to the other, to repeat the cycle.

But what happens to the water that rains on the wrong side of a mountain or hill and is forced down, deeper into the nightside? Some water should end up frozen, trapped where sun won't ever come to melt it, removed from the water cycle.

What keeps my planet's water from irreversibly concentrating over time on the frigid wastes while the rest of the planet dries up?

posted over 4 years ago

aadv‭

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