Atmospheric Motion

1. Reading: Chap 1.3 Atmospheric Motion • Wind • Pressure Gradient Force • Coriolis Effect • Cyclones • General Circulation of the Atmosphere

2. Why unequal? Atmospheric Motion • Air/atmosphere in constant motion due to unequal distribution of energy over earth’s surface Winds • Generally used to describe horizontal air movement • Result from pressure differences • Direction of flow affected by • Pressure gradient force (PGF) • Coriolis effect (CE) • Friction Why do we care about winds?

3. Is the wind stronger or weaker with steep gradients? Pressure Gradient Force • Force associated with pressure gradients • Gradients can be described in the context of spacing of isobars (areas of equal pressure) on weather maps • Steep gradients when isobars are close together • Small gradients when isobars are far apart Coriolis Effect • Apparent deflection of air as it flows northward or southward • Due to earth rotation • In N hemisphere, northward flow deflected to the NE • In N hemisphere, southward flow deflected to the SW • Depends on latitude and wind speed French mathematician Gaspard de Coriolis (1792–1843) www.theozonehole.com/coriolis.htm

4. Surface Weather Map jrscience.wcp.muohio.edu/.../computermodels.html

5. luna.tau.ac.il/.../Chapter3/Coriolis_Effect.htm Coriolis Effect www.newmediastudio.org/.../Geostrophic_Wind.html www.mhhe.com/.../fix/student/chapter8/08f06.html

6. Friction Fig 1.10 • Has a significant effect on air movement near the earth’s surface • Caused by • Surface roughness • Vertical air motion • Opposes PGF; Reduces wind velocity and turns wind in the direction of low pressure How does wind speed change wrt height? Any difference between day and night? Why? In what terrain Is friction larger?

7. Planetary Boundary Layer (PBL) • Lower 1-2 km of atmosphere • Air motion significantly affected by friction Free Troposphere • Above PBL • Not affected by friction • Wind primarily affected by CE and PGF which when in balance • Wind flows parallel to isobars at constant speed • Described as geostropic balance and therefore geostropic winds

8. Why? Cyclones and Anticyclones • Deflection of wind causes air to flow in a roughly circular pattern • This circular flow may be clockwise (anticyclonic) or counterclockwise (cyclonic) • On weather maps isobars describe circulation cells of high and low pressure • Air flows parallel to isobars in a curved path • Associated winds called gradient winds • Caused by the combined effects of PGF and CE • In N hemisphere air flow is counterclockwise in L pressure systems & clockwise in H pressure systems • This pattern is just the opposite in the S hemisphere http://www.tuckerman.org/avalanche/high%20and%20low%20pressure.jpg

9. Why? Low Pressure System • Air flows inward (convergence) and upward and outward (divergence) • Have cyclonic flows in the northern hemisphere • Represent very unstable air masses • Characterized by cloudiness, precipitation, storminess http://earthsci.org/processes/weather/weaimages/low_pressure.jpg

10. Why no precipitation? High Pressure System • Air flows downward (subsidence) and outward (divergence) • Have anticyclonic flows in the northern hemisphere • As air subsides it compresses air beneath it causing a warm air layer to form • Characterized by clear skies, no precipitation, low wind speeds and stability http://earthsci.org/processes/weather/weaimages/high_pressure.jpg http://earthsci.org/processes/weather/weaimages/evaporat.gif

11. High and Low Pressure Systems • Cross-sectional diameters of 100-1000 km • Commonly are migratory • In temperate latitudes affected by tropical & polar air • Move from West to East in mid-latitudes • Have a life span of 1-2 weeks

12. Deserts are common at 30o latitude? Why? Global Air Circulation • Best described by a three-zone model • Air at the equator flows poleward and descends at about 30o N & S latitude flowing along the surface back to the equator • Cold air flowing along the surface from the poles toward the equator is warmed and ascends toward poles • These flows are Hadley-type http://www.emc.maricopa.edu/faculty/farabee/biobk/deserts.gif • Air in middle latitudes forms only weak N-S circulation patterns because of the intrusion of tropical and polar air

13. Jet Streams • Formed in the discontinuities associated with circulation cells in the three zone model • Systems of fast moving air in upper troposphere (7.5 – 12 km) • Produced as a result of strong pressure differences at surface that produce strong pressure gradients aloft • Polar jet stream forms over middle latitudes associated with polar front http://science.enotes.com/images/earth/woes_01_img0095.jpg

14. Jet Streams • North polar jet stream meanders with movement of polar front • In winter may extend as far south as 30o N • In summer average position is 50o N • Because of this migration described as mid-latitude jet stream • Semi-permanent jet stream forms over tropics in winter • Slower than polar jet stream • Centered at about 25o N rst.gsfc.nasa.gov/Sect14/Sect14_1c.html

15. Why do air quality engineers care about jet streams and weather systems? Jet Streams • Important because they influence surface air flow patterns and weather • On acceleration they cause divergence aloft that promotes convergence near the surface and cyclonic motion • Supply energy to storm systems and direct their path • Also cause convergence aloft which intensifies high pressure systems

16. Quick Reflection