Atmospheric Forces and Balances

1. Atmospheric Forces and Balances AOS101 Lecture 10

2. Review from last week • A severe thunderstorm is defined as a thunderstorm that produces - Hail of 1 inch diameter (in central US) or larger and/or wind gusts 58 mph or greater and/or a tornado • Occurs most frequently during the spring and summer when there are the following atmospheric conditions: • Conditionally unstable atmosphere • Moisture • Upward vertical motion (“Lifting”) • Wind shear

4. Review from last week • A tornado is defined as “a violently rotating column of air descending from a thunderstorm and IN CONTACTwith the ground.” –NWS • Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical. • An area of rotation, 2-6 miles wide, now extends through much of the storm. • Most strong and violent tornadoes form within this area of strong rotation

6. Review from last week • Several favorable environmental conditions must be in place before a tropical cyclone can form: • Some initial disturbance such as a thunderstorm complex, which may slowly develop • Warm ocean waters (at least 80°F) • Potentially unstable atmosphere favorable to convection • Moist air near the middle of the troposphere • Low values of vertical wind shear between the surface and upper troposphere • If these conditions persist for several days, a tropical cyclone may form

7. Stages of Hurricane Development • When these disturbances first appear, they are called tropical depressions • Not named yet • Once the disturbance has developed with surface wind speeds stronger than 39 mph, the storm is classified as a tropical storm • After further strengthening and surface wind speeds greater than 74 mph, the system is upgraded to a hurricane • Atlantic Hurricane season is June 1 – November 30

8. RAINBAND EYEWALL EYE HURRICANE KATRINA

9. Why does the wind blow? • What makes the wind blow? • We need to think about Newton's Laws • 1st Law • An object at rest will remain at rest; an object in motion will remain in motion as long as no force is exerted on the object. • 2nd Law • The total force exerted on an object is equal to the acceleration of the object times its mass.

10. Forces that Influence the Wind • Pressure Gradient Force (PGF) • Coriolis force (CF) • Centripetal force • Frictional Force

11. Why we care • Our atmosphere is full of forces that become balanced • As a result, we can say something about it’s motion • Balanced forces tell us many things. • For example, the wind direction is a balance between the Coriolis force, PGF, and frictional force

12. Pressure Gradient Force (PGF) • The pressure gradient is a change in pressure over a given distance. Pressure gradient force compels fluids to move from high pressure to lower pressure. The PGF acts to increase lower pressure and decrease higher pressure

13. Pressure Gradient Force (PGF) • Direction of PGF – always points from HIGHpressure toward LOWpressure, directly perpendicular to an isobar • Magnitude of PGF- strength is directly related to the strength of the pressure gradient • The PGF is the force that causes the wind to blow! Pressure gradient = 4 mb per 100 km

14. When isobars are very close together, the numerator in the PGF equation is large (a very large change in pressure) • So the pressure gradient is large, and thus, the PGF is very strong.

15. Pressure Gradient Force

17. The Coriolis Force (CF) • The Coriolis force is an apparent force that results from the constant rotation of the Earth. • In N. Hemisphere, acts at a 90° angle to the right of the object in motion (such as the wind) • This means that a wind from the south would have a CF acting toward the east

18. Imagine Dallas, TX fires a missile at Winnipeg, Manitoba…

19. Imagine Dallas, TX fires a missile at Winnipeg, Manitoba… Missile starts at Dallas, which is at a latitude of 37.28 N, rotates with the Earth at a speed of 465.11 m/s.

20. Imagine Dallas, TX fires a missile at Winnipeg, Manitoba… Missile starts at Dallas, which is at a latitude of 37.28 N, rotates with the Earth at a speed of 465.11 m/s. Missile travels toward Winnepeg which, at a latitude of 52.00 N, rotates with the Earth at a speed of 286.35 m/s. The missile will conserve its angular momentum as it travels north, meaning it will travel around the Earth at the speed of the Earth’s rotation at Dallas, TX

21. Imagine Dallas, TX fires a nuclear missile at Winnipeg, Manitoba… Missile starts at Dallas, which is at a latitude of 37.28 N, rotates with the Earth at a speed of 465.11 m/s. Missile travels toward Winnepeg which, at a latitude of 52.00 N, rotates with the Earth at a speed of 286.35 m/s. The missile will conserve its angular momentum as it travels north, meaning it will travel around the Earth at the speed of the Earth’s rotation at Dallas, TX Since the Earth rotates slower the farther north you go, the missile appears to deflect to the right of its intended target

22. Imagine Dallas, TX fires a missile at Winnipeg, Manitoba… Missile starts at Dallas, which is at a latitude of 37.28 N, rotates with the Earth at a speed of 465.11 m/s. Missile travels toward Winnepeg which, at a latitude of 52.00 N, rotates with the Earth at a speed of 286.35 m/s. The missile will conserve its angular momentum as it travels north, meaning it will travel around the Earth at the speed of the Earth’s rotation at Dallas, TX Since the Earth rotates slower the farther north you go, the missile appears to deflect to the right of its intended target Missile lands north of Ottawa.

23. The Coriolis Force (CF) • We cannot see the planet rotating, so when something is moving, we perceive it as being deflected to the right of its intended path in the N. Hemisphere • Deflection is dependent on latitude • 0 at equator and maximum at the poles • Deflection intensity is directly related to wind speeds • Acts only as the wind starts to blow

25. Video Explanation • http://www.youtube.com/watch?v=mcPs_OdQOYU

26. Geostrophic Balance • A balance between the • Pressure gradient force • Coriolis force • Balance allows PGF to be equal and opposite the CF. This balance will tell use the magnitude of the geostrophic wind • Thegeostrophic windblowsparallelto lines of constant pressure, with low pressure on the left • Movie: What happens if its not in balance

27. Geostrophic Balance L 996 mb x 1000 mb 1004 mb H

28. Geostrophic Balance Pressure Gradient Force L 996 mb 1000 mb Coriolis Force 1004 mb H

29. Where the pressure gradient is small, the PGF is also small, resulting in a weak wind. Pressure Gradient Force L 996 mb Geostrophic Wind 1000 mb Coriolis Force 1004 mb H Where the pressure gradient is large, the PGF is also large, resulting in a strong wind.

30. Upper Level Flow • The wind can be approximated as nearly geostrophic in the upper levels of the troposphere. PGF CF How are there different heights at 500 mb?

32. PGF/ CF/ Centripetal

33. Frictional Force • Friction affects geostrophic balance by putting a drag-force on the air: friction always acts in the direction opposite the direction of the wind wind FR

34. Frictional Force • This throws the wind out of geostrophic balance– there is now a net force acting on the wind in the direction opposite its motion PGF FR wind CF

35. The Frictional Force How does friction affect geostrophic balance? • Since friction acts in the opposite direction of the wind, it slows the wind • Change in speed  change in magnitude of the Coriolis force • Friction + Coriolis force ~ PGF  no longer geostrophic balance and winds can cross the isobars

36. Friction L Upper Level Wind Balance: PGF/ CF Lower Level Wind Balance: PGF/ CF/ Friction Causes wind to cross isobars at ~30° angle at surface 996 mb 1000 mb 1004 mb H

37. More factors that affect the frictional force: • Height above the surface • The further away from the surface, the less friction • For instance, the winds at 300 mb experience less friction than the winds at the surface • Wind speed • The stronger the wind, the more friction will oppose the motion • Therefore, slower winds experience less friction than fast winds • Surface Type • The rougher the surface, the greater the friction • For example, the friction over an open body of water is weaker than that over a mountainous terrain

39. In regions (such as the surface) where friction is important, we expect the winds to point slightly toward lower pressure

40. In regions (upper levels) where friction is negligible, the winds are approximately geostrophic

41. Atmospheric Force Balances Around Low and High Pressure Centers