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The three experiments develop the framework for a model of atmospheric circulation on Earth. The equator-to-pole temperature gradient creates an energy imbalance which is equalized when air moves along pressure gradients and is affected by conservation of angular momentum as it travels meridionally. These constraints produce two structural regimes visible in the atmosphere: the Hadley Cell and mid-latitude eddies.

Hadley Cell

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The Hadley cell is a region of convective overturning from about 0° to 30° N and S: the northern and southern extent shifts seasonally. Warm air rises at the equator, moves towards the poles, sinks in the subtropics, and returns along the surface towards the equator. Zonal winds are generated due to conservation of angular momentum. In the upper atmosphere, the westerly subtropical jet forms at the poleward boundary of the cell. Towards the equator, the easterly trade winds can be observed, although their speed is reduced by surface friction. 

Poleward of 30° N and S, the Hadley cell breaks down. In a non-rotating Earth, the cell might extend all the way to the pole, but we observe that Coriolis deflection turns wind parallel to the equator at 30° where it sinks and returns to the equator. Temperature gradients poleward that cannot be equalized by convection lead to baroclinic instability. 

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Mid-latitude Eddies

Baroclinic instability leads to eddy formation. While there is a mid-latitude convective cell, its importance to heat transport is dwarfed by these eddies, which stir heat from the equator towards the pole. Eddy heat transport is horizontal, in contrast to the convective overturning at lower latitudes, and so varies less with height. The mid-latitude region dominated by weather systems extends from 30° to 60° N and S, where it gives way to the polar convective cell.

 

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The seasonal variation and heat transport capacity of the Hadley and mid-latitude regimes can be demonstrated by examining data from the atmosphere.

General Circulation in the Atmosphere

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The rotating table was spun up to 0.1 

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 and allowed to spin until a paper dot placed on the surface of the water appeared motionless to the overhead corotating camera, which was physically attached to the rotating table. The canister was filled with ice until full and then topped up with water.

The surface speed of the water was measured by tracking black paper dots in the Particle Tracker application. The speed on the tank floor was measured by tracking the purple pigment trails from granules of potassium permanganate using timecoded screenshots in the Particle Tracker.  Water speed shear between the top and bottom of the tank was measured with drops of blue food dye. 

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A metal canister of radius 18 

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 was placed in the center of a tank of radius 31

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. Again six HOBO temperature sensors were taped to the tank in the arrangement shown in the figure below. This arrangement differs from the first experiment: thermometers 1 through 5 were placed in a line radially from the tank center; thermometer 6 was placed between 3 and 4, on the bottom of the tank but 10

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 further in the clockwise direction. This arrangement was devised to capture thermal signatures of the eddies that were hypothesized to form. Thermometer 1 was placed at a height of 11 

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 and thermometer 5 at a height of 9 

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. The tank was filled with still water to a height of 14

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 with water .

The rotating table was spun up to 1 

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 and allowed to spin until a paper dot placed on the surface of the water appeared motionless to the overhead corotating camera. In this setup, the speed of the corotating camera was synced electronically with the table rotation speed through a computer interface. The canister was filled with 521.6

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 of ice and then topped up with water.

Surface speed, bottom speed and shear were measured as in the previous experiment: paper dots, permanganate granules and food dye, respectively. Two colors of food dye were used to illustrate eddy formation: blue dropped in the colder water near the canister and red in the warmer water near the outside edge.  

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Slow Rotation Experiment

Fast Rotation Experiment

Bibliography

 

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