Determining precipitationBack in the description of weather, we have seen that there are two types of clouds that generate precipitation - synoptic weather pattern clouds and convective clouds. A keyword precipitation_factor can be used to dial the overall level of precipitation. In addition the keywords precipitation_factor_convection, precipitation_threshold and cloud_factor_convection (all part of the weather block) can fine-tune the threshold of convective cloud cover at which rainfall typically arises (usually thunderstorm development), the amount of such rainfall as well as the ease with which convective clouds form at all.So basically the total amount of precipitation and the ratio of convective vs. synoptic precipitation are tunable inside the simulation, they do not emerge from it. The reason for this we have already seen - we would need a transport model for liquids to successfully predict precipitation and this is very complicated. What the simulation produces more reliably though is the spatial and temporal distribution of precipitation. Using some plausible reasoning, it might however be possible to get good estimates of what the precipitation parameters should be set to. We will use the moon Menya and the config file example26.cfg to see how this can be done.
MenyaMenya is a warm and moist tropical world with a dense atmosphere that is marginally breathable (but would cause health problems in the longer run) - since IR retention is enormous, the temperatures are totally different from Fenya.
Surface temperatures are fairly uniform as the atmosphere has good transport properties (note that the high transport coefficient necessitates to set the evolution timestep to 100 s rather than the default 1000 s to avoid numerical instability) - the temperature range is between 300 and 310 K. As the world is tidally locked and hence slowly rotating, there are no significant Coriolis forces and the circulation pattern is direct convection from tropics to poles, i.e. there are no synoptic clouds. What amount of convection would we expect on such a world? To what amount of cloud development would that lead? And how much rain would fall from these clouds?
Estimating parametersFirst, note that convection (i.e. the existence of updrafts of warm air) is only dependent on surface irradiation - which is much lower on Menya than on Earth (about 350 W/m2 vs. 1200 W/m2). The fact that Menya is warmer is irrelevant, because convection arises only when primarily the surface is heated and then acts as a mechanism to heat the air above, but Menya is warm because the air absorbs outgoing IR radiation and radiates it back - and this is a mechanism that directly heats the air, so to the degree that it operates, it requires no convection.However, there is 32 times more water in the Menya atmosphere than on Earth (the total column mass is about 80.000 kg vs. 10.000 kg, and the relative fraction of water in this column is four times more) - so there is reason to believe clouds can form more readily despite the weaker overall convection. The column mass is however irrelevant for this, because clouds form when the relative amount of water in air exceeds a threshold - so we can plausibly argue that cloud_factor_convection ought to be four - which somewhat offsets the weaker convection strength. It is tempting to assume that the large total amount of water implies that rainfalls must be copious, but this cannot be true. To be in a steady-state, the atmosphere must gain by evaporation any water that it loses as rain (and for a purely convective weather pattern, that must approximately be locally true). We have about 350 W/m2 to evaporate water, and for a moist, rainforest-covered world the Bowen ratio is about 0.1 which means that 90% of this energy end up evaporating water. Considering night and averaging over sun angles, only 25% of the irradiation are available when averaged over a longer time. We need 2230 kJ to vaporize a liter of water (because that is the latent heat), so gathering all the numbers we find that about 10 l/day evaporate in Earth tropical rainforests, and 3 l/day on tropical Menya - for Earth that translates to 300 mm of rain per months in tropical rainforest which isn't so bad, on Menya the number would be about 90 mm per month. Assuming that the threshold for thunderstorm formation does not change because there are no conditions which generate particular localized updrafts on Menya, we can then adjust the precipitation_factor_convection such that monthly precipitation reaches this value and from there get true precipitation patterns across the moon.
Properly recording precipitationThe simulation generates the current precipitation rate in mm per second in each evolution step. To get the daily (or monthly) total amount of precipitation (which is usually used in climate charts), it is necessary to record weather with a high sampling rate - the following declaration records tropical weather on Menya for 30 days with 1000 sampling points per day. This ensures that even short-lived thunderstorm precipitation is properly captured.
The recorded file is then numerically integrated with an external tool to get the total amount of precipitation during a month, and parameters are then adjusted such that the precipitation averages about 3 mm/ day.
The result shows thick cloudcover on Menya's dayside moving with the irradiation (thin contours) and embedded thunderstorm development (thick contours):
Rainfall is most copious in the tropics and absent in the polar region (which is expected, as they are in a region of stable high pressure systems because that is where the warm air sinks to the ground.
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