GEOG 1112L 7

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Geography

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Feb 20, 2024

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Name: Lab day: GEOG 1112L Lab 7: General Circulation The different forces at work in the atmosphere lead to the wind flow that creates the weather that we experience every day. The average wind flow that is the result of the interaction of these forces is known as the General Circulation. This lab will focus on aspects of general circulation and how surface and upper-level wind flows interact with each other. Topics covered will include: o Semi-permanent surface features Impact of semi-permanent surface features Circulation models The impact of ocean-atmosphere interaction The connection between surface and upper-level features General Circulation: The Three-Cell Model Studying general circulation can help us determine the predominant climate conditions at various locations across the globe (see the figure below). | North Pole | G e gl SRy High latitudes Arctic Circle : REG;QN ikl - b Middle latitudes Tropic of Cancer | ”?{’;'_’Efifi‘?~”°“‘- e Equator '@,r‘ TROPICS . ' Low latitudes el e afflhb Tropic of Capricorn e L : : ERATEZONE L s ‘"“"w’z ) Middle latitudes omarclc " POLARREGION st i " High latitudes South Pole The reason we have a general circulation at all is because the Earth’s surface is unequally heated. Think back to Lab 1 and the sun angle calculations that you did. After performing these calculations, you should have found that the city of New Orleans had consistently higher sun angles than those of Helsinki. Different Jatitudes receive different amounts of incoming solar Scanned with CamScanner

radiation, and as a result there is an imbalance of energy across latitudes. To balance this out, the atmosphere will move warmer air towards the poles and colder air towards the tropics. The Three-Cell Model helps to explain this movement. Because the Earth rotates, the best model we can use to describe its general circulation is the Three-Cell Model. This model breaks the vertical circulation of air across latitudes into three cells, all stemming from this assumption: the tropics receive the most radiation and the poles receive the least radiation. The Hadley Cell Because the Earth is tilted, the tropics will receive the most radiation. This means that the tropics will be the warmest region. Because the hottest air is located in the tropics, this air will rise into the atmosphere. This rising motion leads to the movement of mass away from the surface, which means there is less weighing down on the surface. This creates an area of low pressure encircling the tropics. The air from the tropics will continue to rise until it reaches the tropopause, the border between the troposphere and the stratosphere. The tropopause will prevent the air from rising any further, and since the air has to go somewhere, it begins moving poleward. As it does so and approaches the subtropics, it will lose some of its heat and converge with other air masses moving equatorward towards the subtropics. This convergence around 30° leads to sinking motion there, and the air will warm as it sinks. This creates a band of high pressure encircling the area around 30°. This type of cell, with rising warm air and sinking cold air, is called a thermally direct cell. The Ferrel Cell This cell functions as a kind of “transition cell” between the Hadley and the Polar cells. This type of cell is called a thermally indirect cell. Sinking motion occurs around 30°, where air from poleward moving warm air of the Hadley cell meets the equatorward moving air from the midlatitudes. Once the sinking air reaches the surface, it has to go somewhere, so it begins to flow back towards the mid-latitudes. Air begins rising again around 60°. The Polar Cell This cell, extending from 60°-90°, is a key driver of much of the weather in the mid-latitudes. This cell stems from the Polar High, a permanent surface high over the poles. Due to the imbalance in received radiation, the poles are the coldest latitudes of the Earth. To try and correct this imbalance, the cold air at the poles begins to move equatorward along the surface. As it moves, it becomes warmer, but not as warm as the air over the tropics. This warmer air will then converge with poleward moving air from the Ferrel Cell around 60°. This convergence and rising motion leads to another band of surface low pressure, called the polar front. As in the Hadley Cell, this rising air will eventually be stopped by the tropopause and will then begin to move poleward. Air will cool as it moves poleward and then sink once it reaches the poles. Scanned with CamScanner

Horizontal wind in the Three-Cell Model The previous three paragraphs described the vertical motion (rising and sinking) of air in the Three-Cell Model. However, there is also horizontal motion associated with this model as well. Between 30° and the equator (the tropics), air begins to move back towards the equator. Due to the Coriolis Force, it will not move in a straight line directly from one latitude to the other. The deflection from Coriolis Force will cause the wind to become northeasterly (from the northeast) in the Northern Hemisphere, and southeasterly (from the southeast) in the Southern Hemisphere. These winds are called the tradewinds. Near the equator, the north and south trades will converge along a boundary called the Intertropical Convergence Zone (ITCZ). This is a boundary associated with surface convergence of warm air (brought by the trades) and rising motion. Due to the warm, moist nature of the air masses brought by the trades, rain and thunderstorms are very common along the ITCZ. This boundary also follows the warm air, meaning that it will move to warmer areas as seasons change throughout the year. There is also horizontal motion within the Ferrel Cell. As the sinking warm air begins to move back towards the poles, some of this air will also be deflected by the Coriolis Force. Because of this deflection, the wind will now have a westerly component (coming from the west). These winds, prevalent in both hemispheres, are called the westerlies. In the Polar Cell, wind moving equatorward is also deflected by the Coriolis Force. This deflection, similar to what occurs within the Hadley Cell, causes the wind to have an easterly component as it approaches 60°N. These winds are referred to as the Polar Easterlies. 1. Please use the figure below to depict the general circulation (the Three-Cell Model). a) Label latitude lines b) Nllustrate the Hadley, Ferrel, and Polar cells (both hemispheres) ¢) Label with and “L” or an “H” the location of any low- or high-pressure bands d) Label the approximate location of the ITCZ (average location) ¢) Draw and label the predominant wind direction for each latitude band. Scanned with CamScanner

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e e P Ce\\ gh laktuaes L VWi d ale \atitunédes oW L Fikundes padale latitu b ey \hqia Latiudes Please answer following questions. 2. At which latitudes does rising motion occur? ® YO 3. At which latitudes does sinking motion occur? =0° 4. At which latitude would you expect the highest amount of precipitation to fall? Why? e NAhesr amopnk of Precip iarion occuvs gx §° Iddivude Vecausr ov W TTCL. Scanned with CamScanner

5. At which latitude would you expect the lowest amount of precipitation? Why? Avortd 20° laxinde Wi Ylee Novkhern and Soutbevn nemisphevey \ave e lowesk amouny o Precv@iration. Wumi g v 1w and e fewmps ave Yoo cad for Surface Highs and Lows QVARPovat\oN - atev Yo OCCAY . Because of the circulation of the atmosphere (described with the Three-Cell Model), there are regions where surface pressure features persist throughout the year. These are referred to as semi-permanent highs and lows, because these are surface highs and lows that tend to occupy the same general locations from year to year. Sinking motion in the poleward branch of the Hadley cells leads to a band of high-pressure centers we call the subtropical highs. Conversely rising motion along the poleward branch of the Ferrel cells is associated with a band of low- pressure centers we call the subpolar lows. The strength and location of these high- and low-pressure systems does vary seasonally in a similar manner each year, though. The subtropical highs strengthen during the summer in each hemisphere. During winter, the subpolar lows emerge as the subtropical highs weaken. As a low or high strengthens or weakens throughout the year, the resulting weather can also change. Oftentimes we compare the seasons of winter and summer, represented by January and July in the Northern Hemisphere, respectively. Look at the images below for January (the top image) and July (the bottom image) showing the distribution of sea level pressure patterns. Scanned with CamScanner

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Please fill in the blank, circle the correct answer, and/or answer following questions: 6. The subtropical highs are located at approximately 3(5 degrees latitude and are driven by the@ siilking branch of the .H()‘Me% cells. 7. The subpolar lows are located at approximately !Q degrees latitude and are driven by the rising / .branch of the P() \O\C cells. 8. TheITCZisa high pressure band associated wit sinking air. 9. What is responsible for this rising or sinking motion that characterizes the ITCZ? WO wakex 0RE Sunw Gvona e eguator Wears Y oy W e LTCL . W Cases Mre Numidiby and makes de one \ouo ank. g Peocess W fe assience of e Trade wmies Yo £ive o rona. . 10. Loék at éree Now"ern\l‘:lemlspl‘;ere \Wuhzlt t)lgv%‘-lpress&re systems nnpact?\I rtfl( America that become prominent during winter? What two high-pressure systems dominate during summer? Puvwg e upnrer Yae Aleution ond Teelandic [wos ave Cromiment, Vuvwg e Spmimenr N Bevmuda and Lot \W\ans ove Provimienit, 11. During which season are the semi-permanent lows the strongest? Wy ey 12. During which season are the semi-permanent highs the strongest? Sumwier 13. How does the ITCZ shift throughout the year? What is responsible for this shift? T ’ITCZ Moves Nocd 1y e Nocern \—\cw\‘\gphevt Spmamtex” OV toves Souta ™ W Nacthern ltevaispheve gyimiter. T HCZ Srers. yhve Yl Season followimg n locaﬂm of greatest \visslasy a. As the ITCZ passes over different locations throughout the year, what lémguf face warmivg, weather will it bring? When Yre ETCZ 1S du Yle South vf Ve eduator e Nor¥a- edsk LIWNAS peevail \Or‘\vw){\"‘”} &Y\,) Condions. Whien 1 Movey ks Mae Nov¥evn |YCWisphere e Soutt Wested) Lowmds prevails \Dv'w\mwo; Wil (ondihiovs. Scanned with CamScanner

Upper Level Winds Additionally, the general circulation impacts upper level wind patterns. This also has foundations in the varying amount of received radiation throughout the year. Look at the two 500mb maps below for January (the top) and July (the bottom) and use them to answer the following questions. =1 IS bt \ 3 e s vamer =T e e 3 R phetY ""w e wadfaveent NPy % o (i il ; . Tobo® ~-...l. .-‘»qu-'--:. A 5. L4y 3 .'1 AL T (ANl g L b ] G ¢ AN e z "..w,.-"" ‘}& 1 Tk P e —— v'__,,,‘..-—- i 07 - fn‘- ....... { ..... ,.‘_' T PP, v . § T e % ..... e e e b ol . 00 RS S E P . B ¥ e ..... "oy LR ER R Gl e . . Scanned with CamScanner

14. Look at the isoheights (black solid lines) and the isotherms (red dotted lines). How would you describe the relationship between temperature and the height of a pressure suface? e et o A eressuve Sucface Was a gftect on ey TL fe Pressave decyeades HMan 30 doey e Fempevane and ¥ Hee Pessuve ncteases so doey A demp, 15. During which season are the temperature and height gradients between the equator and the north pole strongest in the Northern Hemisphere? Why? (think back to the lab on insolation) puv‘\\f\q ~\4»( Luowier Sealon VY\,\(_ +eme ona Vw'(qh-!—- avadients petween e CAUAKOY and flovidih pole O e Shvongest. flhis 1S pecause Yhe flortn pole yeceves: Mwvwal sunlhdnt duving this time 16. What kind of wind speeds would you expect because of these strongest gradients? e Sxvang gvadeny W cawse Phere o he Vevy Stvowg) Wwdy, 17. Based on your previous answers, when are upper level winds strongest in the Northern Hemisphere? UPReY e\ LoWMaT pve steodest W Mt Wwiker The Interaction between the Surface and Upper-Levels It is important to note what happens in the upper-levels impacts what happens at the surface, and vice-versa. Remember that the presence of friction at the surface causes the air (wind) to flow across the isobars at an angle from high to low pressure. This results in flow inward toward the center of a low-pressure center, causing convergence to occur. Since the air needs to go somewhere when it converges at the surface, it will go up. Once it reaches the top of the troposphere, the air will diverge (spread out). The opposite is true of a surface high. Since air flows out of a surface high, there is convergence occurring at the upper-levels and sinking of air. Once it gets to the surface, it will diverge. These cycles are illustrated in the image below. Scanned with CamScanner

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-, Convergencs Divergence Please circle the correct answer. 18. A surface low is associated with rising otion. 19. A surface high is associated with sinking motion. Ocean-Atmosphere Interaction One final factor impacting general circulation is the interaction between the atmosphere and the ocean. Because both are dynamic fluid systems, they behave in a similar manner, in that there are general circulations that dominate their overall behavior. However, ocean currents don’t follow wind patterns exactly. They move in semi-closed circular paths called gyres. See the image below for a map of important currents. 10 Scanned with CamScanner

MM fo A'le (= 3 . ,,- ¢ ,“\x thlnthl! East Wind Drift Currents with red arrows repfesent those that will bring warm water from the tropics poleward, and bluc arrows represent currents that bring cold water equatorward from the poles. 20. Which currents will have the greatest impact on the United States? T Guls v and Calforiia cavweaens i Hove Yz geeatest wngact ot e Dnked Stakes | 21. Describe the temperature of the water that each of these currents will bring to the U.S. coast? "(\/\0 Q(}.\’Uéovr\'\m Coxvenid o\t \orwo C@\’é \UJQ\'CV O\VIA m C"\U\l‘(’ Sheeons \D\('W\q; VIOV VI woder . 22. Use your previous answers to make a statement about the difference between weather (temperature, specific humidity, and precipitation) along either side of the continent of North America. TNy CGu\E Shveomm on e @ast (oast Causey ¢ Woaemey Yemp evarues W Yhe Towber and Ccooler \’(W\v’% W A= [uwawmer . “(\N QO\\\%W\\& Been & Couses Cooler Yewmps on Ye Wegt Coagt. Scanned with CamScanner

GEOG 1112L 7

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