Global CirculationNow, to understand the rationale of some of the following simulation options, we need to take a deeper look at the way how Earth's atmosphere manages to transport thermal energy from the tropics polewards. The reality of the process is very complicated, so by necessity the following is highly idealized.Again the presentation largely follows Meteorology for Scientists and Engineers by Roland B. Stull We can start by looking at irradiation patterns. Remember that a world with an Earth-like axis tilt, they are such that the tropics receive much more radiation than the poles. In the presence of an atmosphere, this implies that air is warmed predominantly in the tropics and - since warm air expands and develops buoyancy - rises from there. By the same logic, air cools over the polar region, gets more dense and loses buoyancy and sinks towards the surface. All of this could rise to a circulation pattern in which warm air from the tropics flows at high altitude polewards, cools and sinks to the surface and the resulting cool air moves back to the equator. However, such a circulation pattern cannot exist on Earth.
The Hadley and Polar cellDue to Earth's rotation, there is a Coriolis force acting on every movement. On the northern hemisphere, this corresponds to a rightward deflection of airflow. So any airmass rising from the tropics and trying to flow north experiences a progressive eastward deflection, till eventually the northward motion ends. The airflow is unable to reach the pole and cools at the latitude it is till it eventually sinks to the surface. The region where this sink occurs is known as the subtropical ridge and is centered at around 30 deg in latitude.Thus surface winds converge at the equator (or rather, the region of the planet that is currently receiving the maximal amount of light - which migrates with the seasons) and rise there in a pattern of daily thunderstorm development. This is known as the Intertropical Convergence Zone (ITCZ), and the removal of air aloft creates a rather stable surface low pressure region and leads to copious cloud development and rainfall. At 30 deg latitude, the sinking air conversely leads to a stable surface high pressure region and hence very little cloud development. The whole circulation pattern is able to bring thermal energy from the tropics to about 30 deg latitude and is known as a Hadley Cell. In principle the same (albeit driven by less radiative energy) happens at the poles - warm air rises and migrates aloft poleward, sinks there to the surface and moves southward along the surface till it reaches again warmer zones, creating a fairly stable surface high pressure region at the poles and a low pressure zone at around 60 degrees latitude. This is known as a Polar Cell. Now - what transports energy between Hadley and Polar cell? The Ferrel cell and jet streamsWhat would need to happen is for warm air to move on the ground poleward from 30 deg to 60 deg latitude and then rise there while cold air flows aloft toward the tropics. This would smoothly connect with the circulation pattern of Hadley and Polar cell and at the same time move thermal energy in the required direction, and the pattern is known as Ferrel Cell.However, the idea that warm air stays close to the ground while it flows over progressively colder terrain has a flaw - this air would develop buoyancy and rise, and this makes the Ferrel cell unstable as a circulation pattern - it can't transport appreciable amounts of heat anywhere. As a consequence, a progressively sharper temperature gradient would develop between warm air at 30 deg latitude and cold air at 60 deg latitude, somehow meeting at a kind of front. Since warm air is less dense, the warm air column towers higher in the atmosphere - at the same altitude is has higher pressure than the cold air column - and air aloft would move along this pressure gradient poleward. This is known as the thermal wind. But again Coriolis force deflects this thermal wind eastward, leading to a massive high-altitude airflow, the jetstream. This airflow likewise is unstable - it can develop various wave patterns (baroclinic and barotropic waves) so that in essence it meanders between the subtropical ridge and the polar front (again, this is rather schematic - there can be a polar and a weaker subtropical jet stream, both of them meandering). It is this meandering pattern which can transport thermal energy from the warm to the cold zones (and actually the efficiency of the jetstream heat transport by far exceeds that of the direct convection cells). The upper atmosphere waves of the jetstream in turn cause air to flow out quickly of some region, which leads to a large-scale rising of air from the surface and creates a pattern of surface low pressure regions, the so-called cyclones.
Circulation patterns in the simulationThe simulation uses a model of a global circulation to generate plausible weather patterns. For instance, it creates (and moves with the axis tilt) the location of the subtropical ridge as a region in which convective cloud formation is suppressed and it generates and evolves cyclones in the region of jet stream meanders. As we have seen before, this provides a crude description of seasons.Any relevant parameters to change the weather generated by the circulation are specified as part of the weather block as follows:
The ridge_migration_factor determines how much the ITCZ and the subtropical ridges are seasonally moving. If the factor is 1, the ITCZ is always moved to the latitude at which the Sun is in the zenith at noon, if the factor is 0 it always remains at the equator. The real ITCZ of Earth is complicated - in the Amazonas bassin it remains stationary while in Asia it shows seasonal migration. By default the value is set to 0.5 to simuate some inertia of the weather system as it tries to follow the maximum irradiation. The ridge_suppression_factor indicates how strong the surface high pressure region at the ridge is and hence of much convective clouds are suppressed. If the value is 1, clouds are not suppressed at all, if the value is zero no convective clouds can appear no matter the energy radiated onto the surface. By default the value is 0.1, indicating strong suppression of cloud formation. Finally the parameter circulation_model allows to directly change the model that is used - see the next tutorial for details. The circulation in the simulation is not actually used to transport heat, the transport model that is applied is based on a long-time average of the actual transport, and the strength of transport is user-controlled, not simulated in detail. The particular circulation observed on Earth seems dependent on a number of factors though:
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