Evaluate the Role That the Tri-Cellular Model of Atmospheric Circulation has in Dictating Global Climate

by f1sam (student)

The Tri-Cellular Model of Atmospheric Circulation shows how energy is redistributed across the globe and ensures there is not a surplus at the equator and deficit at the poles, which would be caused by the differential heating of the Earth’s surface by the Sun. The tri-cellular model is made up of three different air masses which control atmospheric movements and the redistribution of heat energy. The three air masses, starting from the equator, are called the Hadley cell, Ferrel cell and the polar cell.

Between the two Hadley Cells is an area of low pressure in equatorial latitudes known as the inter-tropical convergence zone (ITCZ). Here, at the equator, the sun is always high in the sky so this area receives a large amount of solar insolation so the ground heats rapidly in the day and there is a lot of surface evaporation and the air becomes extremely humid. As the hot air rises in convection currents an area of low pressure develops. This rising air cools and water vapour eventually condenses into cumuliform clouds and heavy rainfall results. Weather stations in the equatorial region record precipitation up to 200 days each year, making the equatorial and ITC zones the wettest on the planet. The equatorial region lacks a dry season and is constantly hot and humid but because the climate that has both rain and sun means that these areas are able to host one of the most diverse ecosystems on the planet in the form of rainforests. The Inter-Tropical Convergence Zone acts as a conveyor belt for hot, humid air that turns water vapour from higher latitudes into rainfall near the equator.

Also, the location of the ITCZ changes throughout the year and while it remains near the equator, the ITCZ over land goes farther north or south than the ITCZ over the oceans due to the variation in land temperatures. The location of the ITCZ can vary as much as 40° to 45° of latitude north or south of the equator based on the pattern of land and ocean.

Around 30°N/S latitude, air carried poleward descends in a high pressure area, known as the Sub Tropical High Pressure belt; found between the Hadley and Ferrel cells. Winds are formed when ...

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Around 30°N/S latitude, air carried poleward descends in a high pressure area, known as the Sub Tropical High Pressure belt; found between the Hadley and Ferrel cells. Winds are formed when air moves between different pressure belts, typically from an area of high pressure to an area of low pressure, due to density differences between the two air masses, so most winds originate close to the Sub Tropical High Pressure belt. Winds are generated between the subtropical high and the equatorial band of low pressure creating the trade winds. However due to the spinning of the earth these winds will not move straight from the differing areas of pressure. These will be diverted to either the left or right, depending on whether they are travelling North to South or South to North. This is the Coriolis Effect. Due to the Coriolis Effect, winds flow clockwise around a high-pressure area located in the northern hemisphere and counter-clockwise around one in the southern hemisphere. The resulting weather system is called an anticyclone and specifically a subtropical anticyclone in the Sub Tropical High Pressure Belt.

The Subtropical High has the opposite effect of the Inter Tropical Convergence Zone and results in the formation of deserts. Because the air pressure is high, little humid air flows into the region as air can only flow from areas of high pressure to areas of low pressure, not the other way around. The temperature is constantly hot because of the high angle of incidence of the sun but there is little rainfall because evaporated moisture is transported away from the desert towards the equator by surface winds, which are free of moisture as most of its moisture would have already been deposited at the equator. This results in much of the most of land in the High Pressure belts being arid or semi-arid desert.

In the Ferrel Cell part of the Global Atmospheric Circulation model the prevailing winds are westerlies but large low pressure circulation systems are common. Local wind directions can be quite variable, depending on the proximity of such systems, but the weather itself tends to move in a broadly west to east direction. This is why the weather experience in the UK, for example, is often much different from that in Russia. In the UK, the weather comes from the Atlantic Ocean; in Russia it comes from Central Europe.

Between the Polar and Ferrel Cells there are the Sub Polar Low Pressure zones where warm air from the high pressure areas, clashes with the colder air coming from the equator. The cold front will bulldoze under the warmer air forces it upwards. This makes the air cool and also causes frontal rain. The climate of this area is quite temperate but is also quite dominated by the frontal rain.

There are however, several factors, other than the Global Atmospheric Circulation Model, that effect global climate.