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atmo 170A Class Notes Week 6

by: Savannah Way

atmo 170A Class Notes Week 6 ATMO 170 A1

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Savannah Way

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Class notes cover scales of atmospheric motion, local wind systems, global winds, and atmosphere-ocean interactions.
Intro to Weather and Climate
Dr. Mullen
Class Notes
weather, climate
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This 10 page Class Notes was uploaded by Savannah Way on Thursday October 6, 2016. The Class Notes belongs to ATMO 170 A1 at University of Arizona taught by Dr. Mullen in Fall 2016. Since its upload, it has received 3 views.


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Date Created: 10/06/16
WEEK 6 (184-221) SCALES OF ATMOSPHERIC MOTION Scales of motion: hierarchy of motion from tiny gusts to giant storms  Microscale: the smallest scale of motion o eddies with diameters of a few meters of less not only disperse smoke, they also sway branches and swirl dust and papers into the air  Mesoscale: circulation of city air constitutes the next layer scale o Typical mesoscale winds range from a few kilometers to about a hundred kilometers in diameter  Mesoscale – land breeze is the most accurate association  Macroscale: weather map scale, synoptic scale o Circulations of this magnitude dominate regions of hundreds to even thousands of square kilometers and although the life spans of these features vary, they typically last for days and sometimes weeks  Air aloft that changes directions or speed Eddies: when the wind encounters a solid object, a whirl of air, an eddy – forms on the object’s downwind side  the size and shape of the eddy often depends on the size and shape of the obstacle and on the speed of the wind  light winds produce small stationary eddies  wind moving past trees, shrubs, and even bodies can produce small eddies  rotors: violent vertical motions that produce extreme turbulence and hazardous flying conditions o strong winds blowing over a mountain in stable air may produce a mountain wave eddy on the downwind side, with a reverse flow near the ground  Turbulent eddies form aloft as well as near the surface o Turbulence aloft can occur suddenly and unexpectedly, especially where the wind changes its speed or direction abruptly o Wind shear: the shearing creates forces that produce eddies along a mixing zone  if the eddies form in the clear air, this form of turbulence is called clear air turbulence LOCAL WIND SYSTEMS Thermal circulations: circulations brought on by changes in air temperature, in which warmer air rises and colder air sinks  Thermal (cold core) highs and thermal (warm core) lows: region of surface high and low atmospheric pressure created as the atmosphere either cools or warms o shallow systems, usually extending no more than a few kilometers above the ground  Sea breeze: coastal showers are caused by a local wind system, as the cooler ocean air pours inland, it forces the warmer, unstable humid air to side and condense, producing rain  During the summertime along the coast, a sea breeze is usually strongest and best developed in the afternoon  A sea breeze circulation will reverse and become a land breeze circulation once per day o Sea breeze front: leading edge of the sea breeze, a rapid drop in temperature occurs just behind o Sea breeze convergence zone: formation of opposing breezes meeting on the opposite side of the obstruction  In Florida, when a sea breeze moving west meets a sea breeze moving east, they form a sea breeze convergence zone.  Land breeze: a breeze at the surface that flows from the land toward the water; typically, weaker than a sea breeze o Night counterpart of a sea breeze  Valley breeze: heated air, being less dense than the air of the same altitude above the valley, rises as a gentle upslope wind  A valley breeze would develop its maximum strength in the early afternoon  While fly fishing in a mountain stream, you notice that the wind is blowing upstream. From this you could deduce that the wind is a valley breeze  Cumulus clouds that appear above isolated mountain peaks are often the result of valley breezes.  Mountain breeze: cooler, more-dense air glides downslope into the valley; gravity is the force that directs these winds downhill, also referred to as gravity winds  Katabatic wind: any downslope wind that is much stronger than mountain breezes o Rush down elevated slopes at hurricane speeds, but most are not that intense and many are on the border of 10 knots or less o Bora: cold, gusty, northeasterly wind with speeds sometimes in excess of 100 knots o Mistral: less violent, cold wind  A katabatic winds is a cold and downslope wind  The Columbia Gorge wind: ocean side of the Cascade Mountains much colder than the marine air it replaces, often the harbinger of a prolonged cold spell  Chinook wind: warm, dry, downslope wind that descends the eastern slope of the Rocky Mountains o rather narrow and extends from northeastern New Mexico northward into Canada o chinook wall cloud: along the front range of the Rockies, a bank of clouds forming on the windward side of the mountains is a telltale sign of impending chinook, usually remains stationary as air rises, condenses, and then rapidly descends the leeward slopes, often causing strong winds in foothill communities  the heat from a chinook wind is generated mainly by compressional heating.  On the eastern side of the Rockies, chinook winds are driest when clouds form and precipitation falls on the upwind side of the mountains  Santa Ana Winds: warm, dry wind that blows downhill from the east or northeast into southern California o As the air descends from the elevated desert plateau, it funnels through mountain canyons o Compressional heating: primary source or warming over the region of high pressures  Strong Santa Ana winds develop in Los Angeles during the fall when a high pressure center forms to the northeast of Los Angeles over the Great Basin  Desert Winds: winds all sizes developing over the desert and huge dust storms form in dry regions o Haboob: storm composed of dust or sand and forms as cold downdrafts along the leading edge of a thunderstorm lift dust or sand into a huge, tumbling dark cloud  Most common in African Sudan o Dust devil: whirlwinds, dry areas, the wind may also produce rising, spinning columns of air that pick up dust or sand Monsoon: changes direction seasonally, blowing from one direction in summer and from the opposite direction in winter BRIEF REVIEW  The size of atmospheric circulations ranges from the smallest microscale to the larger mesoscale, to the largest macroscale.  Thermal pressure systems are shallow pressure systems that are driven by the unequal heating and cooling of the earth’s surface.  The sea breeze and the land breeze are types of thermal circulations that are due to uneven heating and cooling rates of land and water.  At the surface, a sea breeze blows from water to land; whereas a land breeze blows from land to water.  A valley breeze blows uphill during the day and a mountain breeze blows downhill at night.  Chinook (foehn) winds are warm, dry winds that blow downhill along the eastern side of the Rocky Mountains.  The main source of warmth for the chinook is compressional heating.  Santa Ana winds are warm, dry downslope winds that warm by compressional heating and blow from the east or northeast into Southern California.  Dust devils tend to form over dry terrain on clear, hot days. They are not tornadoes, although the winds of a large dust devil may cause minor damage to structures.  Monsoon winds are winds that change direction seasonally. In southern Asia, the winter monsoon, which blows from land to water, is dry; the summer monsoon, which blows from water to land, is wet. GLOBAL WINDS General Circulation of the Atmosphere: winds throughout the world are averaged over a long period of time, the local wind patterns vanish, and what we see is a picture of the winds on a global scale Single-cell model 1. The earth’s surface is uniformly covered with water (so that differential heating between land and water does not come into play). 2. The sun is always directly over the equator (so that the winds will not shift seasonally). 3. The earth does not rotate (so that the only force we need deal with is the pressure gradient force). Hadley cell: named after meteorologist, George Hadley, it is driven by the energy from the sun  Excessive heating of the equatorial area produces a broad region of surface low pressure, while at the poles excessive cooling creates a region of surface high pressure  The horizontal pressure gradient, cold surface polar air flows equatorward, while the higher levels air flows toward the poles Three-cell model: if we allow the earth to spin, the simple convection system breaks into a series of cells, the tropical regions still receive an excess of heat and the poles a deficit. In each hemisphere, three cells instead of one have the task of energy redistribution  middle cell is completed when surface air from the horse latitudes flows poleward toward the polar front Doldrums: the air is warm, horizontal pressure gradients are weak, and winds are light over equatorial water Subtropical highs: the convergence of air produces a belt of high pressure  horse latitudes are when weak pressure gradients in the center of the high produce only weak winds and horses were either thrown overboard or eaten according to legends trade winds: steady winds from the northeast in the Northern hemisphere and from the southeast in the Southern hemisphere intertropical convergence zone (ITCZ): near the equator, the northeast trades converge with the southeast trades westerlies: air moving towards the poles and deflecting towards the east polar front: separation of two air masses of contrasting temperature subpolar low: a zone of low pressure where surface air converges and rises, and storms and clouds develop polar easterlies: behind the polar front the cold air from the poles is deflected by the Coriolis force, so that the general flow of air is from the northeast  polar cell is when air aloft eventually reaches the poles, slowly sinks to the surface, and flows back toward the polar front Four semi-permanent pressure systems:  subtropical anticyclones that develop in response to the convergence of air aloft, since surface winds blow clockwise around these systems, we find the trade winds to the south and the prevailing westerlies to the north o Bermuda high: eastern Atlantic, between latitudes 25 and 35N o Pacific high: in the Pacific Ocean  These zones of cyclonic activity actually represent regions where numerous storms, having traveled eastward, tend to converge, especially in winter o Icelandic low: in the North Atlantic, the Greenland-Icelandic low, which covers Iceland and southern Greenland o Aleutian low: sits over the Gulf of Alaska and the Bering Sea near the Aleutian Islands in the North Pacific  Other pressures systems, which are not semi-permanent in nature o Siberian high: huge but shallow thermal anticyclone that forms because of the intense cooling of the land Jet streams: where the upper level winds tend to concentrate into narrow bands, we find rivers of fast-flowing air  wind speeds in the central core of a jet stream often exceed 100 knots and occasionally 200 knots  band of strong winds, they form in the same manner as all winds, from horizontal differences in the air o subtropical jet stream: the jet stream situated near 30 latitudes at about 13 km above the subtropical high o polar front jet stream: to the north, the jet stream situated at a lower altitude at about 10 km near the polar front o tropical easterly jet: over the subtropics at the base of the tropopause in the summertime o stratospheric polar jet: during a dark polar winter, forms near the top of the stratosphere BREIF REVIEW  The two major semi-permanent subtropical highs that influence the weather of North America are the Pacific high situated off the west coast and the Bermuda high situated off the southeast coast.  The polar front is a zone of low pressure where cyclonic storms often form. It separates the mild westerlies of the middle latitudes from the cold, polar easterlies of the high latitudes.  In equatorial regions, the intertropical convergence zone (ITCZ) is a boundary where air rises in response to the convergence of the northeast trades and the southeast trades. Along the ITCZ huge thunderstorms produce heavy rain showers.  In the Northern Hemisphere, the major global pressure systems and wind belts shift northward in summer and southward in winter.  The northward movement of the Pacific high in summer tends to keep summer weather along the west coast of North America relatively dry.  Jet streams exist where strong winds become concentrated in narrow bands. The polar-front jet stream is associated with the polar front. The polar jet meanders in a wavy west-to-east pattern, becoming strongest in winter when the contrast in temperature along the front is greatest.  The subtropical jet stream is found on the poleward side of the Hadley cell, between 20° and 30° latitudes. It is normally observed at a higher altitude than the polar jet stream. ATMOSPHERE-OCEAN INTERACTIONS Gulf stream: warm water current that carries vast quantities of tropical water into higher latitudes  Great Britian typically has warmer winter temperatures than other areas of similar latitude because of the gulf stream. Upwelling: the rising of cold water from below  The cold water observed along the northern California coast in summer is due primarily to upwelling El Niño: large prolonged warming that develops at irregular intervals every three to seven years  surface temperatures across much of the tropical Pacific rises by 0.5 C or more for periods lasting few months or longer  large numbers of fish and marine plants may die  Southern Oscillation: reversing surface air pressure at opposite ends of the Pacific Ocean o ENSO or El Niño/Southern Oscillation is a phenomenon when the pressure reversals and the ocean warming are more or less simultaneous La Nina: cold water episode in the warm water and rainy weather confined to the western tropical Pacific  Exact opposite of el Niño Teleconnections: warmer or colder ocean surface can influence weather patterns in distant parts of the world Norther Atlantic Oscillation: reversal of pressure in the Atlantic and has an effect on the weather in Europe and along the east coast of North America  Positive (warm) phase occurs when strong westerlies in turn direct strong cyclonic storms on more northerly track into northern Europe  Negative (cool) phase occurs when the atmospheric pressure in the vicinity of Icelandic low rises, while the pressure drops in the region of the Bermuda high Artic Oscillation: where changes in atmospheric pressure between the Arctic and regions to the south cause changes in the upper-level westerly winds Pacific Decadal Oscillation: long-term Pacific Ocean temperature fluctuation because the ocean surface temperatures reverse every 20 to 30 years BRIEF REVIEW  Summertime weather in Northern California often consists of low clouds and fog due to cold coastal water  Surface winds and upper-level winds blow in the same direction in a thermal circulation F  Cumulus clouds formation above a mountain in the early afternoon could indicate a well-developed valley breeze T  Surface air is warmer and drier as a result of compressional heating in a downslope wind T  A warm northward flowing ocean current is found along the east coast and a cold southward moving current is found along the west coast T  Thunderstorms are likely to happen in the early evenings


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