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Angular Momentum in Planetary Atmospheres

Angular Momentum in Planetary Atmospheres. Buffalo Astronomical Association May 8, 2009 Jude S. Sabato Assistant Professor of Earth Science Buffalo State College. Outline. Overview of planetary atmospheres Angular momentum in rotating atmospheres Earth’s Hadley Circulation and Jet Streams

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Angular Momentum in Planetary Atmospheres

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  1. Angular Momentum in Planetary Atmospheres Buffalo Astronomical Association May 8, 2009 Jude S. Sabato Assistant Professor of Earth Science Buffalo State College

  2. Outline • Overview of planetary atmospheres • Angular momentum in rotating atmospheres • Earth’s Hadley Circulation and Jet Streams • Mars’ Hadley Circulation • Super-rotation on Venus and Titan • Jet formation on Jupiter

  3. Outline • Overview of planetary atmospheres • Angular momentum in rotating atmospheres • Earth’s Hadley Circulation and Jet Streams • Mars’ Hadley Circulation • Super-rotation on Venus and Titan • Jet formation on Jupiter

  4. Planetary Atmospheres Atmospheres we’ll talk about today… • Earth • Mars • Venus • Titan (Saturn’s largest moon) • Jupiter

  5. Planetary Atmospheres

  6. Outline • Overview of planetary atmospheres • Angular momentum in rotating atmospheres • Earth’s Hadley Circulation and Jet Streams • Mars’ Hadley Circulation • Super-rotation on Venus and Titan • Jet formation on Jupiter

  7. Momentum Momentum measures motion and mass: momentum = mass x velocity

  8. Momentum Newton’s First Law: “An object at rest will remain at rest and an object in motion will move in a straight line at constant speed, unless acted on by a force.” force = change in momentum

  9. Momentum

  10. Angular Momentum Angular Momentum measures spinning motion: Angular Momentum = radius x mass x velocity

  11. Angular Momentum Newton’s First Law (revisited): “An object that is not spinning will remain so and a spinning object will continue spinning at constant speed and in the same orientation, unless acted on by a twisting force (torque).” torque = change in angular momentum

  12. Angular Momentum

  13. Atmospheric Angular MomentumJet Streams and Storms Let’s break down the atmosphere into symmetric and wavy components… Symmetric part conserves its angular momentum… = + …if there are no waves Flow variable (Wind, Temperature, Pressure, etc.) Symmetric part Wavy part

  14. Atmospheric Angular Momentum Take home points: • Atmospheric angular momentum is conserved if • There are no torques on the atmosphere • There are no atmospheric waves • Atmospheric waves open the door to super-rotation • angular momentum transfer associated with atmospheric waves can generate E-W jets

  15. Outline • Overview of planetary atmospheres • Angular momentum in rotating atmospheres • Earth’s Hadley Circulation and Jet Streams • Mars’ Hadley Circulation • Super-rotation on Venus and Titan • Jet formation on Jupiter

  16. Earth There are so many interesting dynamical phenomena in Earth’s atmosphere! We’ll focus on the Hadley Circulation and Jet Streams… • Hadley Cells Driven by low latitude convection • Hadley Cells approximately conserve angular momentum • Angular momentum conservation means fluid moves in rings around the planet – not at all true!

  17. Earth

  18. Earth

  19. Earth

  20. Earth • Angular momentum conservation in the Hadley Cell generates a subtropical Jet Stream • Subtropical jet is unstable and becomes wavy • These atmospheric waves (midlatitude storms) can sometimes generate a second jet stream

  21. Earth

  22. Monsoons by angular momentum too! After Bordoni and Schneider 2008, Nature Geoscience

  23. Outline • Overview of planetary atmospheres • Angular momentum in rotating atmospheres • Earth’s Hadley Circulation and Jet Streams • Mars’ Hadley Circulation • Super-rotation on Venus and Titan • Jet formation on Jupiter

  24. Mars Mars has a Hadley Circulation too… • Driven by convection • Much greater degree of angular momentum conservation, however… • Angular momentum conservation means fluid moves in rings around the planet – probably not true for Mars either • Jet stream is unstable and becomes wavy (still true for Mars) • Atmospheric waves (midlatitude storms) do not generate a second jet because the planet is too small • Topography/surface heating can force waves that move the atmospheric angular momentum from place to place

  25. Mars

  26. Mars • Mars topography/surface thermal inertia may have an “elevated heat island” effect • Elevated heat island drives Indian Monsoon (maybe, or better partially) • Is there a “dry monsoon” on Mars? • One way or another the atmosphere is not “moving in rings” (transport properties are not axisymmetric)

  27. Outline • Overview of planetary atmospheres • Angular momentum in rotating atmospheres • Earth’s Hadley Circulation and Jet Streams • Mars’ Hadley Circulation • Super-rotation on Venus and Titan • Jet formation on Jupiter

  28. Venus Venus in the ultraviolet

  29. Venus Venus’ atmosphere appears to be super-rotating… Super-rotation: winds aloft at the equator are faster than the planet’s rotation This is akin to stirring a cup of coffee and observing that the coffee is circulating faster than your spoon!

  30. Titan Titan in the infrared

  31. Titan • Titan has a global Hadley Cell • Titan’s upper atmosphere is in a state of super-rotation, like Venus

  32. Titan George Hadley’s original idea to explain the trade winds (1735)

  33. Titan • CH4 on Titan behaves very much like water on Earth (“methanological” cycle) • Links between seasonal and methanological cycle could drive angular momentum changes in the atmosphere and the solid surface

  34. Titan Recent observations show a slight change in Titan’s spin rate… This could be evidence of a liquid water ocean between the solid interior and icy surface. What’s the culprit? It could be angular momentum transfer between the surface and the atmosphere. False color RADAR image

  35. The fly-wheel crust of Titan?

  36. So what about super-rotation… Any East-West asymmetries could be responsible • On Titan: ??? • On Venus: • “moving candle” = Venus is rotating very slowly; the Sun heats one side for quite a while; radiative cooling on the other side • Atmospheric waves, from wind over mountains, propagate upward and deposit momentum in the upper atmosphere • They’re both slow-rotators ---- easy to get super-rotation in a model with slow rotation • Bottom line: we know what kinds of mechanisms can generate super-rotation but we don’t know which of these, if any, are operating in which atmosphere

  37. Outline • Overview of planetary atmospheres • Angular momentum in rotating atmospheres • Earth’s Hadley Circulation and Jet Streams • Mars’ Hadley Circulation • Super-rotation on Venus and Titan • Jet formation on Jupiter

  38. Jupiter Multiple Jets and macroturbulence

  39. Jupiter

  40. Jet Formation wave breaking Angular momentum divergence Stirring Angular momentum convergence Angular momentum divergence wave breaking E-W Wind

  41. Jet Formation

  42. Jupiter Jets form by stirring at small scales, exciting waves and transporting angular momentum across latitude circles. • Stirring is thought to be by “thunderstorms” • Equatorial super-rotation requires atmospheric waves to travel across the equator • Why so many jets? That is, what determines the jet width? • size of the planet • speed of the wind • rotation rate of the planet • Rhines Length:

  43. Summary Angular momentum is a unifying concept in atmospheric dynamics. Earth • Earth’s Hadley Cell is approximately angular momentum conserving (sometimes, sort of) • Angular momentum conserving theories accurately predict width of the cells and the existence of a jet stream • Monsoons may be a result of dynamical regime shifts between nearly (symmetric) angular momentum conserving flow to wave driven flow

  44. Summary Angular momentum is a unifying concept in atmospheric dynamics. Mars • Mars’ Hadley Cell is much more angular momentum conserving than Earth’s but is still not “rings of fluid” • Angular momentum conserving theories accurately predict width of the cells on Mars as well • A type of dry Monsoons may be driving non-axisymmetric transport of H2O, CO2 and dust

  45. Summary Angular momentum is a unifying concept in atmospheric dynamics. Venus and Titan • Super-rotation in both atmospheres • Several mechanisms are possible causes but none are certain (and may be different for each atmosphere) • Titan’s atmosphere may be exchanging significant angular momentum with the surface, causing spin rate changes

  46. Summary Angular momentum is a unifying concept in atmospheric dynamics. Jupiter • Multiple jets and macroturbulence • Equatorial super-rotation as well • Angular momentum transport can form jets, while a planet’s size, rotation rate and atmospheric wind speeds determine their width/number

  47. THANK YOU!

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