Slide1: Understanding the Weather EAS-107 Chapter 10 Wind: Global Systems


Slide2: Understanding the Weather EAS-107 Chapter 10 Overview General Circulation of the Atmosphere Jet Streams Atmosphere-Ocean Interactions


Slide3: Scales of Motion How large of an area does a weather system affect? Synoptic scale motions cover the US, 100’s to 1000’s of km. Mesoscale motions are the size of thunderstorms, 10’s of km. Microscale motions are 10’s to 100’s of meters. Over the next few Chapters lectures will focus on the large scale and move downscale to make the discussion as easy as possible to understand. Understanding the Weather EAS-107


Slide4: General Circulation of the Atmosphere General circulation of the atmosphere only represents the average (daily or monthly) air flow around the world and actual winds may vary significantly at any one place and time from this average. It is difficult to “see” the large scale motions when all the “noise” of the smaller scale motions are super-imposed on the large scale motions. Averages over several days or months are a way to remove the “noise” so the large scale motions are visible. Understanding the Weather EAS-107


Slide5: General Circulation of the Atmosphere We learned in Chapter 2 that, averaged over the entire earth, incoming solar radiation is roughly equal to the outgoing earth’s radiation. In that discussion it was shown that there was an uneven heating of the earth. The energy balance is not maintained for each latitude as there is a net gain of energy at the tropics and a net loss at the poles. As a result the poles are cool and the equator is warm. The question is how does the earth restore the balance since we know that the poles are not at absolute zero and the equator does not melt lead. To restore balance, the atmosphere transports warm air poleward and cool air equatorward. Understanding the Weather EAS-107


Slide6: General Circulation of the Atmosphere (Single-Cell Model) The british meteorologist George Hadley devised a gedanken to try to understand how the earth comes into balance. Single-Cell model assumes that: Earth’s surface is uniformly covered with water (so that differential heating between the land and water does not come into play). The sun is always directly over the equator (so that the winds will not shift seasonally). The earth does not rotate (so that the only force we need to deal with is the pressure gradient force). Understanding the Weather EAS-107


Slide7: General Circulation of the Atmosphere (Single-Cell Model) For the single-cell model described above, the air near the equator heats and as it does it becomes less dense. The air near the equator then begins to rise and the surface pressure drops. Conversely, the air near the poles cools and becomes more dense. The air near the poles then begins to sink and the surface pressure rises. Understanding the Weather EAS-107


Slide8: General Circulation of the Atmosphere (Single-Cell Model) At this point, a pressure gradient force develops between the poles and the equator and the cold air flows from the high pressure at the poles to the low pressure at the equator. The opposite occurs in the upper portion of the troposphere. The rising air at the equator strikes the tropopause and must flow both north and south towards the poles which creates high pressure aloft. At the poles, the sinking air creates low pressure aloft and another pressure gradient develops in the upper part of the troposphere between the poles and equator. Understanding the Weather EAS-107


Slide9: General Circulation of the Atmosphere (Single-Cell Model) Understanding the Weather EAS-107


Slide10: General Circulation of the Atmosphere (Single-Cell Model) Obviously, this circulation does not really occur on the earth. Earth is not uniformly covered with water. The sun does not always stay above the equator. Coriolis forces will turn the winds to the right. However, Hadley’s gedanken did provide grounds for further study. Understanding the Weather EAS-107


Slide11: General Circulation of the Atmosphere (Three-Cell Model) In the three-cell model, the first step is to remove the restriction of a non-rotating earth. Although this model is much more complex than the single-cell model, there are some similarities. As with the single cell model, the air at the equator is heated by the sun, becomes less dense, and rises to the tropopause creating a low pressure at the surface. In addition, above the equator, the rising air creates high pressure aloft. The poleward flow of air at upper-levels is then deflected by the Coriolis force toward the right in the NH and to the left in the SH. Understanding the Weather EAS-107


Slide12: General Circulation of the Atmosphere (Three-Cell Model) As the air aloft moves northward, it’s volume is slowly reduced. The volume is reduced because of the spherical surface. As you move closer to the poles the latitude lines get closer together to reflect the reduced radius. The slowly reducing volume creates a region of convergence near 30°. The air becomes so compacted at 30° that it must sink. The sinking air strikes the surface and spreads out in all directions. This creates a belt of high pressure called subtropical highs. Understanding the Weather EAS-107


Slide13: General Circulation of the Atmosphere (Three-Cell Model) The air from the high pressure belt that is directed south curves to the right and eventually flows into the equatorial surface low (trade winds). The air from the high pressure belt that is directed northwards is again deflected to the right (westerlies). The subsiding air produces generally clear skies and warm surface temperatures. Here are where the major deserts of the world are found. This region is know as the horse latitudes. Understanding the Weather EAS-107


Slide14: General Circulation of the Atmosphere (Three-Cell Model) The mild air from the high pressure belt that is called the westerlies moves poleward and encounters cold air moving down from the poles. The cold air from the poles meets the air from the subtropical high in the horse latitudes. This convergent zone causes the air to rise once again, and is called the polar front. Understanding the Weather EAS-107


Slide15: General Circulation of the Atmosphere (Three-Cell Model) Understanding the Weather EAS-107


Slide16: General Circulation of the Atmosphere (Three-Cell Model) Understanding the Weather EAS-107 Click image to play in web browser.


Slide17: Sea-level Pressure and Surface Winds (January) Understanding the Weather EAS-107


Slide18: Sea-level Pressure and Surface Winds (July) Understanding the Weather EAS-107