Hadley, Ferrel, and Polar Cells by Brianna Rapoza on Prezi
day meeting was held at the International Pacific Research Center, Hono- much more intense than the Ferrel and polar cells in the extratropics. The Seager, R., N. Harnik, Y. Kushnir, W. Robinson, and J. Miller. The cells tend to shrink (expand) during the warm (cold) phase of ENSO and during . a feature that is strongly related to the annular mode (Robinson ). . to the weaker Ferrel cells in the midlatitudes and the even weaker polar cells at .. Surface Temperature dataset (HadISST) v SST from the Met Office Hadley. In each hemisphere there are three cells (Hadley cell, Ferrel cell and Polar cell) in which air circulates through the entire depth of the.
The Hadley cell is a closed circulation loop which begins at the equator.
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There, moist air is warmed by the Earth's surface, decreases in density and rises. A similar air mass rising on the other side of the equator forces those rising air masses to move poleward. The rising air creates a low pressure zone near the equator.
As the air moves poleward, it cools, becomes denser, and descends at about the 30th parallelcreating a high-pressure area. The descended air then travels toward the equator along the surface, replacing the air that rose from the equatorial zone, closing the loop of the Hadley cell. The poleward movement of the air in the upper part of the troposphere deviates toward the east, caused by the coriolis acceleration a manifestation of conservation of angular momentum.
At the ground level, however, the movement of the air toward the equator in the lower troposphere deviates toward the west, producing a wind from the east. The winds that flow to the west from the east, easterly wind at the ground level in the Hadley cell are called the Trade Winds. Though the Hadley cell is described as located at the equator, in the northern hemisphere it shifts to higher latitudes in June and July and toward lower latitudes in December and January, which is the result of the Sun's heating of the surface.
The zone where the greatest heating takes place is called the " thermal equator ". As the southern hemisphere summer is December to March, the movement of the thermal equator to higher southern latitudes takes place then. The Hadley system provides an example of a thermally direct circulation. The power of the Hadley system, considered as a heat engine, is estimated at tera watts. There it subsides and strengthens the high pressure ridges beneath.
A large part of the energy that drives the Ferrel cell is provided by the polar and Hadley cells circulating on either side and that drag the Ferrel cell with it. It might be thought of as an eddy created by the Hadley and polar cells. In the upper atmosphere of the Ferrel cell, the air moving toward the equator deviates toward the west. Both of those deviations, as in the case of the Hadley and polar cells, are driven by conservation of angular momentum.
As a result, just as the easterly Trade Winds are found below the Hadley cell, the Westerlies are found beneath the Ferrel cell. The Ferrel cell is weak, because It has neither a strong source of heat nor a strong sink, so the airflow and temperatures within it are variable. For this reason, the mid-latitudes are sometimes known as the "zone of mixing. The weaker Westerlies of the Ferrel cell, however, can be disrupted. The local passage of a cold front may change that in a matter of minutes, and frequently does.
As a result, at the surface, winds can vary abruptly in direction. But the winds above the surface, where they are less disrupted by terrain, are essentially westerly. A strong high, moving polewards may bring westerly winds for days.
The Ferrel system acts as a heat pump with a coefficient of performance of Polar vortex and Polar easterlies The Polar cell is a simple system with strong convection drivers. Though cool and dry relative to equatorial air, the air masses at the 60th parallel are still sufficiently warm and moist to undergo convection and drive a thermal loop.
As it does so, the upper level air mass deviates toward the east. When the air reaches the polar areas, it has cooled and is considerably denser than the underlying air. It descends, creating a cold, dry high-pressure area. At the polar surface level, the mass of air is driven toward the 60th parallel, replacing the air that rose there, and the polar circulation cell is complete. As the air at the surface moves toward the equator, it deviates toward the west. Again, the deviations of the air masses are the result of the Coriolis effect.
The air flows at the surface are called the polar easterlies. The outflow of air mass from the cell creates harmonic waves in the atmosphere known as Rossby waves. These ultra-long waves determine the path of the polar jet streamwhich travels within the transitional zone between the tropopause and the Ferrel cell.
By acting as a heat sink, the polar cell moves the abundant heat from the equator toward the polar regions. The Hadley cell and the polar cell are similar in that they are thermally direct; in other words, they exist as a direct consequence of surface temperatures.
Their thermal characteristics drive the weather in their domain. The sheer volume of energy that the Hadley cell transports, and the depth of the heat sink that is the polar cell, ensures that the effects of transient weather phenomena do not only have negligible effect on the system as a whole, but — except under unusual circumstances — do not form.
There are some notable exceptions to this rule. In Europe, unstable weather extends to at least the 70th parallel north. A portion of the air moves back toward the equator completing the circulation system known as the Hadley cell.
This moving air is also deflected by the Coriolis effect to create the Northeast Trades right deflection and Southeast Trades left deflection. The surface air moving towards the poles from the subtropical high zone from 30 latitude to 60 is also deflected by Coriolis acceleration producing the Westerlies. This collision results in frontal uplift and the creation of the sub polar lows or mid- latitude cyclones.
Most of this lifted air is directed to the polar vortex where it moves downward to create the polar high. Figure 2 - Simplified global three-cell surface and upper air circulation patterns. The circulation patterns produced by these pressures seem different somewhat from the three cell model in Figure 2.
These differences are caused primarily by two factors. First, the earth's surface is not composed of uniform materials.
The two surface materials that dominate are water and land. These two materials behave differently in terms of heating and cooling, causing latitudinal pressure zones to be less uniform. The second factor influencing actual circulation patterns is elevation. Elevation tends to cause pressure centers to become intensified when altitude is increased.
This is especially true of high pressure systems. In figures 3 and 4 monthly average sea-level pressure and prevailing winds for the Earth's surface are for the years Atmosphere pressure values are adjusted for elevation and are described relative to sea-level.
Pressure values are indicated by color shading see the legend in the graphic.
Global circulation patterns
Blue shades indicate pressure lower than the global average, while yellow to orange shades are higher than average measurements. Wind direction is shown with arrows. Wind speed is indicated by the length of these arrows see the legend on the graphic. The intertropical convergence zone is identified on the figures by a red line.
The formation of this band of low pressure is the result of solar heating and the convergence of the trade winds. In January, the intertropical convergence zone is found south of the equator Figure 3. During this time period, the southern hemisphere is tilted towards the sun and receives higher inputs of shortwave radiation. Note that the line representing the intertropical convergence zone is not straight and parallel to the lines of latitude.
Bends in the line occur because of the different heating characteristics of land and water. This phenomenon occurs because land heats up faster then ocean. Figure 3 - Mean January prevailing surface winds and centers of atmospheric pressure, The red line on this image represents the intertropical convergence zone ITCZ. Centers of high and low pressure have also been labeled. This shift in position occurs because the altitude of the sun is now higher in the Northern Hemisphere.
In the winter months, the intertropical convergence zone is pushed south by the development of an intense high pressure system over central Asia compare Figures 3 and 4. The extreme movement of the ITCZ in this part of the world also helps to intensify the development of a regional winds system called the Asian monsoon. Figure 4 - Mean July prevailing surface winds and centers of atmospheric pressure, Instead, the system consists of several localized anticyclonic cells of high pressure.
These systems are located roughly at about 20 to 30 degrees of latitude and are labeled with the letter H on Figures 3 and 4. The subtropical high pressure systems develop because of the presence of descending air currents from the Hadley cell. These systems intensify over the ocean during the summer or high sun season. During this season, the air over the ocean bodies remains relatively cool because of the slower heating of water relative to land surfaces. Over land, intensification takes place in the winter months.
At this time, land cools off quickly, relative to ocean, forming large cold continental air masses. The intensity of the sub polar lows varies with season. This zone is most intense during southern hemisphere summer Figure 4. At this time, greater differences in temperature exist between air masses found either side of this zone.
North of the sub polar low belt, summer heating warms subtropical air masses. South of the zone, the ice covered surface of Antarctica reflects much of the incoming solar radiation back to space. As a consequence, air masses above Antarctica remain cold because very little heating of the ground surface takes place.
The meeting of the warm subtropical and cold polar air masses at the sub polar low zone enhances frontal uplift and the formation of intense low pressure systems.
A new source of Southern Ocean and Antarctic aerosol from tropospheric polar cel
In the northern hemisphere, the sub polar lows do not form a continuous belt circling the globe Figures 3 and 4. Instead, they exist as localized cyclonic centers of low pressure. In the northern hemisphere winter, these pressure centers are intense and located over the oceans just to the south of Greenland and the Aleutian Islands. These areas of low pressure are responsible for spawning many mid-latitude cyclones.
The development of the sub polar lows in summer only occurs weakly Figure 4 - over Greenland and Baffin Island, Canadaunlike the southern hemisphere.
As a result, cold polar air masses generally do not form. These mid-latitude weather systems derive their energy from horizontal temperature contrasts between cold, polar air masses and warm, subtropical air masses.
Because the temperature contrasts between these air masses are greatest during winter, the frequency and intensity of European windstorms peak during this season as well. With respect to the origins of the cyclones, including the temperate difference between the different air masses referred to above, may be added the effect of the jet stream, and the presence of mountains or other surface boundary, e.
Tropical cyclones derive their energy from the vertical temperature contrast between the warm lower layer and cold upper reaches of the tropical atmosphere. Because the hurricane ultimately derives its energy from warm seawater, its winds quickly diminish when its eye moves over cooler water or land. The horizontal temperature gradient that powers an extratropical cyclone can persist as the storm center moves over land. Thus wind speeds in these storms can occasionally remain high, or even increase, after landfall.
Also, while a hurricane can sustain the same minimum central pressure for days, the energy that drives an extratropical cyclone rapidly decays as the air masses within it intermix—a single cyclone typically exists independently for three to five days.
Storms are steered by the polar jet stream, so the position of the jet stream defines the storm track. The polar jet stream and storm track move from month to month and from year to year, but storms moving along the storm track tend to reinforce the jet, and keep it from shifting. This is called eddy feedback. By reinforcing the jet, eddy feedback helps the jet and storm track "remember" their position over weeks to months, and thus enhances the persistence and predictability of mid-latitude variability.
It is also responsible for the tendency of European windstorms to occur in series. Eddy feedback is not, however, sufficiently robust to provide persistence from year to year. The storms that affect Europe are the eastern outliers of the approximately disturbances that form in the north Atlantic each year and track eastward along with the jet stream.
The track of the jet is affected by the position of the Azores high pressure cell and the Icelandic low. Shifts in the relative strength of these pressure cells control where a storm might make landfall in Europe. Flowing in a semi- continuous band around the globe from west to east, it is caused by the changes in air temperature which is greatest where the cold polar air moving towards the equator meets the warmer equatorial air moving polar ward.
The jet stream is found at heights between 10 to 15km above the ground. The pathway followed by jet streams is quite variable. They may break apart into two separate streams and then rejoin, or not. They also tend to meander north and south from a central west-east axis. The movement of the jet streams is an important factor in determining weather conditions in mid-latitude regions.
It is strongest in the winter due to higher thermal contrast. Its edges are very turbulent. The Polar Jet stream is stronger than the Subtropical Jet stream. The position of the polar jet stream varies with the seasons.
During the autumn and winter, the polar jet stream generally lies directly over the UK and Ireland, feeding in west to south-westerly winds off the Atlantic along with several low pressure systems, travelling from west to east. The polar jet stream is at its strongest during the autumn when the thermal contrast between the cold air to the north and warm air to the south is at its greatest. A stronger polar jet stream has a greater potential to produce deeper, more intense areas of low pressure and stronger winds.