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The two key characteristics of the Hadley cell (from the equator to about 30°) are that there is large-scale overturning circulation, and that wind speeds increase in the pole-ward direction. The next goal was to simulate this in a laboratory environment to study the heat transport associated with Hadley circulation.
Laboratory Experiment
Set-Up
A key characteristic of Hadley circulation is that it is only maintained when velocity is slow. As velocity of air becomes too fast, eddies start to form. In order to simulate Hadley circulation, the rotation rate of a tank of water must be very slow (about 1 revolution per minute); the precise speed was 62 seconds per rotation. The tank was 44 cm in diameter and was filled with 29°C water to a depth of 9 cm. At the center of the tank was a canister, 13 cm in diameter, filled with ice, which was meant to represent one of the Earth’s poles. More specifically, the canister was filled with 415.7 grams of ice, and enough melted ice so that the water level in the canister was also at 9 cm. There were eight temperature sensors total (two sets of four sensors, at 90° apart): 2 on the side of the canister, 2 on the base of the tank at about 2 cm from the edge of the canister, 2 on the base of the tank at 10 cm from the edge of the canister, and 2 on the outer rim of the tank.
Before starting the experiment, the tank of warm water was rotated at about 1 rpm until reaching solid body rotation, which took about 20 minutes. Upon reaching solid body rotation, the ice was placed into the canister, at which point Hadley-like circulation began.
Experimental Results
Basic Motion
There was sinking of water around the cold canister, outward motion at the base of the tank, rising motion at the rim, and a return flow in shallow water. Figure 12 shows the movement of permanganate, which is a substance that sinks in water. As can be seen, the permanganate moves away from the tank, thus revealing the outward movement of water at the base of the tank.
The overturning circulation described was not the only type of movement observed. In fact overturning circulation such as this would happen in a completely stationary tank. The more interesting observation, from the experiment, shown in Figure 13, was that a dome of cold air formed around the central canister.
Two simultaneous processes happened to cause this cone to form. There was clockwise sinking of cold water, that moved outward as it descended. Additionally, there was anticlockwise inward rising of warm water. The anticlockwise motion of warm water was in the same direction as the tank’s rotation. In Hadley circulation, westerlies are in the same direction as Earth’s rotation; hence, the anticlockwise motion represents the westerlies. The clockwise motion associated with sinking cold water is similar to the atmospheric easteries.
Heat Transport
In Figure 13, the temperatures of the different sensors are plotted.
