Hadley cells

FIGURE 21.1 Zonally averaged components of the absorbed solar flux and emitted thermal infrared flux at the top of the atmosphere. The + and — signs denote energy gain and loss, respectively. (From Radiation and Cloud Processes in the Atmosphere Theory Observation and Modeling by Kuo-Nan Liou. Copyright 1992 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc.) 

FIGURE 21.2 Three-cell representation of global circulation of the atmosphere. 

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Because of the rotation of the Earth and because of the warm equatorial and cool polar regions, the atmosphere divides itself into six regions (roughly corresponding to the climatic zones). These atmospheric regions are called the northern and southern Polar, Ferrel, and Hadley cells (so called because they are roll cells ) see Figure 3.2. This division of the atmosphere tends to slow the mixing... 

These three cells of convective circulation occur within the troposphere, contributing to relatively rapid and complete mixing of this layer of the atmosphere. The tropical cells, located north and south of the equator, often are called Hadley cells. The circulation patterns immediately north and south... 

Mean motions in the lower stratosphere also are driven by the Hadley circulation, because its ascending branch causes tropospheric air to enter the stratosphere. The direction of motion in the lower stratosphere is northward in December-February, southward in July-August. The maximum value of the integrated mass flux associated with the Hadley cell is 200Tg/s in the middle troposphere and 6Tg/s in the stratosphere. 

The descending branch of the Hadley cell gives rise to a high-pressure belt at about 30° latitude. Further poleward, the meridional circulation is opposite in direction to that required for thermally driven motion. These features, which are called Ferrel cells, must be indirect circulation systems. Yet another pair of cells is evident in the polar regions. They are very weak systems. 

In the tropics the situation is somewhat different. Here, the mean motion is directed upward as it follows the ascending branch of the Hadley cell, thus counteracting the downward flux of ozone by eddy diffusion. The ozone concentration rises more gradually, and the gradient at the tropopause is essentially negligible. Since in addition the tropopause level is fairly stable, there is practically no influx of ozone into the troposphere. Instead, ozone undergoes lateral, poleward transport and can then enter the troposphere at midlatitudes, in part via the subtropical tropopause breaks... 

At 30°N, the air descending to the surface that does not move south toward the equator in the Hadley cell begins to move north. As it does so, it is deflected toward the right by the Coriolis force. In these regions north and south of the trade wind belt (in the Northern and Southern Hemispheres, respectively), winds tend to blow from west to east (westerlies) these are referred to as westerly wind belts. 

As air flows northward in the upper level of the Hadley cell, the Coriolis force turns it to the right (eastward). The magnitude of the Coriolis force increases as the air flows toward the poles, and by the time the air reaches about 30°N latitude, the Coriolis force has turned the flow entirely from west to east (see Figure 21.2). To conserve angular momentum, the speed of the wind also increases as the air moves toward the pole. 

As we discussed earlier, in the equatorial region, warm air expands upward and creates a poleward pressure gradient force at the upper altitudes, where air flows poleward from the equator. This air, as it moves poleward, cools and sinks in the subtropical high-pressure belts ( — 30°) and returns to the equator at the surface. This thermally driven circulation between the equator and the subtropics is referred to as the Hadley cell. In the polar regions, a similar thermally driven circulation occurs. An airflow exists at upper levels toward the equator and at lower levels toward the poles, producing a Hadley circulation between the poles and the subpolar low-pressure regions. 

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