Feedback loopsIn the discussion of precipitation and the Ice Age, we briefly touched upon the concept of a feedback mechanism, in this case the precipitation (and hence snow amount) being reduced in a cold atmosphere by the diminished capacity of chilly air to hold water vapour.This is an example for a negative feedback mechanism - as the atmosphere cools, progressively less snow is available to reduce the albedo of so far snow-free parts of a planet, and so the precipitation as a function of atmosphere temperature serves to some degree to counter further cooling to a state in which the whole planet is snow-covered. In contrast, in the tutorial on global warming, the water vapour content of the atmosphere served as a positive feedback loop - the more water the atmosphere can hold, the more IR radiation it can capture, the warmer the atmosphere grows, the more water it can hold... A positive feedback loop may lead to a runaway development that ends very far from the initial situation, such as on Venus where all the water of the planet is vaporized and generates an enormous Greenhouse effect. The simulation contains some (limited) control over such atmospheric feedback processes.
Precipitation and cloud formationAs mentioned before, the precipitation feedback with atmosphere temperature is on by default, but it can be de-activated. There is an option to set cloud formation feedback with atmosphere temperature, although it is far from clear that this should be done.Precipitation measures the absolute amount of water that is released from the atmosphere, but clouds form when the amount of water relative to the maximal amount of water the atmosphere can hold exceeds unity - in other words, in colder air one also needs less water to form a cloud. There may still be merit in introducing some feedback though, as the amount of water that can absorb or reflect radiation of course depends on the total rather than the relative value, and furthermore secondary effects in cloud formation may matter. For that reason, the strength of the feedback relative to the Clausius-Clapeyron relation is set for all cloud types (except convective clouds) by parameters as follows (the example depicts the default settings):
Thus, setting a parameter to e.g. 0.4 will make 60% of the cloud amount independent on temperature, the other 40% will be determined by the Clausius-Clapeyron relation (i.e. in colder air there will be less clouds).
Forest firesAn example for a negative feedback loop are forest fires - they become increasingly probable when temperatures increase, yet emit smoke that serves as condensation nuclei for cloud formation and hence increase albedo.To simulate forest fires however, the material file has to be modified to indicate what terrain types are combustible. Here's an example for such a declaration with the fire_factor a multiplier for how much smoke the terrain can generate.
Inside the config file, the following set of parameters is relevant:
Here, fire_cloud_factor determines how much cloud fraction can be generated by a fully active fire inside a single surface element (this is multiplied by fire_factor from the material file). fire_min_T specifies the onset temperature in K for fires, fire_max_T the temperature at which fires burn with full strength and generate the cloud level specified by fire_cloud_factor. Finally smoke_stratosphere_transfer selects the amount of smoke that acts to create high-level clouds in a non-local way - the simulation does an average of the smoke level across the planet, then multiplies with the specified factor and imposes this as additional high level clouds across the whole planet.
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