GGR214 EXAM Part 3.docx

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University of Toronto Mississauga
David Passmore

Lecture 14 – Chapter 12 General Circulation of the atmosphere • Incoming and outgoing radiation are not balanced at individual latitudes - This drives the general circulation of the atmosphere. • The circulation system operates differently in the tropics than outside the tropics – o Tropics weather = relatively stable o extratropical weather = more variables • This is because there is a surplus of radiation between 40 *N and 40* S, where there is a deficit of radiation poleward of these latitudes. (Left side of diagram) As a result, there must be a poleward transfer of energy to prevent Polar Regions from getting colder and tropical regions from getting warmer. (Right side of diagram) Easterly winds in the tropical latitudes gain momentum from Earth and westerly winds of the mid latitudes lose momentum to Earth. Momentum is the product of mass and velocity - Angular momentum: the product of mass, is conserved in a rotating system. (right side)  m × v × r - if we want to conserve momentum, it must be transferred o ex. Transferred from the easterly winds to the westerly winds to maintain current momentum The general circulation: must transport both energy and momentum toward the poles. - Tropics: accomplished by simple convection - Mid-latitudes: accomplished by weather systems in the form or eddies and waves Descriptions of the General Circulation Simple Case - (2 assumptions) • The sun is always overhead at the equator • Earth’s surface is composed of one surface type Surface wind form in response to the following Pressure patters: (middle up) - Equatorial low - Subtropical highs - Subpolar lows - Polar highs *Remember the winds are named for the direction they come from. Trade Winds: steady wind from the tropical region - Subtropical highs to equator low are deflected by Coriolis force Equatorial Low: or Intertropical Convergence zone (ITCZ) is where trade winds from two hemisphere converges Mid-latitude westerlies: are created between subtropical highs and subpolar lows - they experience much greater deflection because poleward increases the strength of coriolis force. Polar esterlies: are created between polar highs and subpolar lows *The equatorial low and polar highs are THERMAL in origin *The subtropical highs and subpolar lows are DYNAMIC in origin Annually averaged pressure and wind patterns at the Earth’s surface. Alternating pressure and wind patterns. • Annually averaged pressure and wind patterns aloft • High pressure over the equator • Low pressure over poles. • Coriolis produces prevailing westerlies. The real case: • The assumptions no longer apply. • The sun is directly 23*N in the June Solstice and 23*S on the December solstice. The surface is not homogenous and so it takes into account of the influence of the differences in heating and cooling of land and water on surface pressure patterns. • Location of maximum solar angle shifts with the seasons – ITCZ moves south in January and north in July. Other pressure systems shift seasonally as well. • Continents produce uneven heating and cooling. o Aleutian low & Icelandic low: Thermal lows over warmers continents in summer. o Canadian High and Siberian high: Thermal highs over cooler continents in winter Monsoon: A circulation pattern that leads to very wet summers and very dry winters - Thermal lows over the continents in July are important for this process The tropical Circulation Haley cell – a thermally direct cell with warm air rising at the equator and colder air sinking in the subtropics. - It produces equatorial (thermal) low along the ITCZ. It also produces subtropical (dynamic) highs at around 30*. - The clouds created are cumulonimbus clouds and has copious amounts of rainfall. The ITCZ is also known for being a location of very weak and variable winds due to lack of strong pressure gradients (doldrums). - In the center of subtropical highs is another region of light and variable surface winds (horse latitudes) The extratropical Circulation – • Mid-latitude westerly – not steady as the trade winds. Westerly on average. This is because flow is disturbed by the travelling cyclones and anticyclones responsible for the variable weather on mid latitudes. IN DETAIL: Just as warm air flows toward the pole from subtropical highs, cold air flows toward the equator from the polar highs. Remember the flow is deflected by Coriolis force, thus producing the polar easterlies. Polar front: ideally represents the meeting of a cold polar air and warm tropical air The circulation Aloft • Equatorial thermal low becomes high pressure aloft • Polar thermal low becomes low pressure aloft • Coriolis force produces upper air westerlies. • The equatorial to pole temperature gradient leads to a surface pressure gradient directed from the poles to the equator. The pressure gradient reverses and strengthens with height. The subtropical Jet streams align sinking air and subtropical highs at the surface (30*) • • Conservatio n of momentum (poleward movement causes velocity to greatly increase. Polar front jetstreams: have more variable paths. Net flow is zonal but meandering produces meridional components. Rossby waves or long waves are associated with polar front jet streams. • Strongly influence mid latitude weather (influences: temperature advection and storm development) Earth’s general atmospheric circulation : -transfers energy and momentum - tropical circulation is relatively simple - extra tropical circulation is relatively complex. Lecture 15 – Chapter 13 Describe the characteristics of an air-mass source region and distinguish between five major air mass types (Chapter 13); Describe and account for air mass paths in North America and explain how these masses change as they move (Chapter 13); Describe weather changes that can occur when air masses move into a region (Chapter 13); Distinguish between the four types of fronts and describe their structures and weather changes that occur during their passage (Chapter 13) Air masses are large bodies of air with similar (horizontal) temperature and humidity characteristics. Reflect energy and water transfer between Earth’s surface and air layers in contact with the surface. The best source regions are areas that have uniform surface properties stretching over thousands of square kilometers. They must be relatively flat (land or water but not both). Winds must be light because air must remain stagnant for up to two weeks. Good source regions are therefore associated with large, semi-permanent anticyclones of the general circulation, polar high and subtropical highs. Mid latitudes are not good source regions • Air over warm surfaces becomes warm • Air over moist surfaces become humid • Air over cold surfaces loses heat Recap** Air masses form as: - Heat is transferred between the surface and the air - Water vapour is transferred between the surface and the air Air-mass source regions are - Thousands of square kilometers in area - Of uniform surface type - Usually associated with semi-permanent high-pressure cells The path an air mass takes when it moves is determined by the upper airflow. As air masses move, they influence the weather in places they pass through and, in turn, they are modified by their new surroundings. For example, a cold air mass moving southeastward from northern Canada in winter will warm as it travels. As a result, it may cause large temperature drop in southern Ontario but it will not have a strong effect on temperature by the time it reaches Florida. An air mass that is colder than the surface over which it is travelling will be warmed from and made unstable. On the other hand, an air mass that is warmer than the surface over which it is travelling will be cooled from below and become stable. Lake- effect snows: snowfall that occurs downwind of large, frozen lakes As cP air moves south it can pick up moisture. When it picks up a lot of moisture from large, unfrozen bodies of water (Great Lakes) it produces lake-effect snows. As the cP air flows over these lakes from north or the west it is warmed below and picks up moisture. Warming makes the air unstable and the instability leads to the formation of clouds and release moisture as snow. These snows are unique in that they cause areas just downwind of the Great Lakes to have the greatest snowfall accumulations of the region. Pineapple express – mT air from the Pacific (Hawaii) reaches North America at least once a year during fall or winter. Very heavy rainfalls and snowmelt Dry line: a boundary separating warm moist air from warm dry air • Transition zone between cT (dry) and mT (moist, less dense) air. • mT air rises over cT air – thunderstorms. • Not a true “front” because temperatures are similar on both sides of these lines. Fronts Following changes are associated with the passage of fronts - Increase or decrease in temperature - Increase or decrease in dew-point temperature - An occurrence of clouds and precipitation - Decrease in pressure - A shift in wind direction • Boundaries between airs of different properties – most commonly different temperatures. • Uplift of less dense air mass (clouds, precipitation, drop in pressure) • Wind direction shifts clockwise • Temperature and dew point temperature are likely to change as front moves past. Stationary front: no significant movement Frontogenesis: the process of increasing a temperature gradient e.g. surface convergence, differential heating / cooling Frontolysis: weakening or dissipation of a front e.g. surface divergence, differential heating / cooling Weather associated with Cold Fronts Cold Fronts have cold air advancing and replacing warm air - can produce strong winds, cumulonimbus clouds, large raindrops, heavy showers, lighting and thunder, hair and the possibility of tornadoes. Weather associated with Warm Fronts Warm Fronts have cold air retreating and warm air moving in Gentle lifting of warm, moist air – nimbostratus and stratus clouds, drizzly rain showers; sleet or snow possible in winter. Occluded Fronts: front separates a cold air mass from a cool one, and low pressure center from warm air in a mid-latitude cyclone • Occur if a cold front catches up and overtakes a warm front • Occlusion – closed off • Coming together of cold, cool and warm air masses. Looking at the picture above. A) The occluded front separates the two coldest air masses at the surface, and it lies between the low-pressure center and the warm air mass. B) A cross-section along the line AB showing where the warm front and cold front would be located on the surface C) A cross section along the line CD showing where the occluded front and the TROWAL would be located on the surface 8Warm air is lifted up forming a trough above the surface. Air behind the cold front meets the air ahead of the warm front. Remember it as… TROWAL: TROugh of Warm air Aloft Occlusion: Process that gr
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