Changing Coriolis forceLet's do a thought experiment: What if Earth would rotate slower, such that the deflection of airflow would be reduced? With a 24 h day, tropical air can flow out to 30 degrees latitude before it is effectively stopped and moves eastward. As the day lengthens and rotation slows, the latitude to which it could reach increases. Eventually we can imagine a situation in which warm tropical air can flow all the way to the pole, cool there and sink to the surface, to move to the tropics close to the ground - we would have a single cell convection.Of course, once the rotation of the planet is too slow, the day/night temperature variation in longitude become as large as the irradiation-angle driven variations in latitude and the situation changes yet again, but at least the north/south circulation in the atmosphere of Venus seems to exemplify a single Hadley cell transport. If we on the other hand imagine speeding up the rotation such that days get shorter, the polewards flow would not even reach to 30 degrees latitude. What might conceivably happen then is that a pattern of multiple zones and belts emerges - in essence direct convection cells where the stable Hadley cell can fit in and meandering jet streams where and unstable Farrel cell would be. This seems to be exemplified by the gas giant atmospheres - with the obvious caveat that these do not show airflow over a solid surface, instead they have an unseen deep circulation. Nevertheless, this is what the simulation assumes.
Controlling the circulation modelBased on planet size and rotation speed, the simulation tries to determine a circulation model. The chosen model is announced at weather init as
but if the user does not agree with this choice, a different model can be selected. Currently available are (all numbers per Hemisphere):
An alternative EarthLet's imagine an alternative version of Earth that rotates at different speed - what would we see? The following config file sets the rotation period to 10 h, enough to produce a different weather pattern with five convection cells.
The resulting low cloud structure shows now two alleys of cyclones per hemisphere and a somewhat different subtropical ridge:
An interesting thing happens when we look at the average temperature of the planet - it turns out that in the simulation, a faster rotation of Earth leads to a higher temperature. This is very pronouced at 1.4 years (corresponding to the transition to a different circulation pattern) but already happens at a 15 h rotation period which has the same circulation pattern as the real Earth.
On some level this should not be overly surprising, as we have noted earlier that convective clouds tend to cool a planet as they form and increase albedo when irradiation is high but disappear and let IR radiation pass into space when irradiation is low. When the planet rotates faster, the total amount of radiation reaching every spot of the planet is not really changed, but the time is usually no longer sufficient to form convective clouds, so the apparent heating of the planet is chiefly driven by the lack of strong convective cloud development. Let us now consider another ingredient in weather and circulation as we know it - the existence of the low Troposphere, bounded above by a very stable Stratosphere.
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