Angular Momentum in Planetary Atmospheres
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Transcript of Angular Momentum in Planetary Atmospheres
Angular Momentum in Planetary Atmospheres
Buffalo Astronomical AssociationMay 8, 2009
Jude S. SabatoAssistant Professor of Earth Science
Buffalo State College
Outline
1. Overview of planetary atmospheres2. Angular momentum in rotating atmospheres3. Earth’s Hadley Circulation and Jet Streams4. Mars’ Hadley Circulation5. Super-rotation on Venus and Titan6. Jet formation on Jupiter
Outline
1. Overview of planetary atmospheres2. Angular momentum in rotating atmospheres3. Earth’s Hadley Circulation and Jet Streams4. Mars’ Hadley Circulation5. Super-rotation on Venus and Titan6. Jet formation on Jupiter
Planetary Atmospheres
Atmospheres we’ll talk about today…
• Earth• Mars• Venus• Titan (Saturn’s largest moon)• Jupiter
Planetary AtmospheresObject Composition
(“dry”)Condensibles Surface
PressureSurface Temperature
Atmospheric Dynamics
Venus 97% CO23% N2
SO2H2SO4
90,000 mbar 750 K Super-rotating
Earth 78% N221% O21% Ar
H2O 1000 mbar 288 K Hadley CellsJet StreamsMonsoons
Mars 96% CO22.5% N21.5% Ar
CO2H2O (trace)
10 mbar 220 K Hadley CellsJet StreamsDry Monsoons?
Jupiter 90% H210% He
NH3H5NSH2O?
No solid surface
165 K (at 1000 mbar)
Multiple jetsMacroturbulence
Titan 100% N2 CH4 1500 mbar 95 K Global Hadley CellSuper-rotationMethane cycle
Outline
1. Overview of planetary atmospheres2. Angular momentum in rotating atmospheres3. Earth’s Hadley Circulation and Jet Streams4. Mars’ Hadley Circulation5. Super-rotation on Venus and Titan6. Jet formation on Jupiter
Momentum
Momentum measures motion and mass:
momentum = mass x velocity
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
Momentum
Angular Momentum
Angular Momentum measures spinning motion:
Angular Momentum = radius x mass x velocity
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
Angular Momentum
Let’s break down the atmosphere into symmetric and wavy components…
Atmospheric Angular MomentumJet Streams and Storms
Flow variable (Wind, Temperature, Pressure, etc.)
Symmetric part Wavy part
= +Symmetric part
conserves its angular
momentum…
…if there are no waves
Take home points:• Atmospheric angular momentum is conserved
if 1. There are no torques on the atmosphere2. 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
Atmospheric Angular Momentum
Outline
1. Overview of planetary atmospheres2. Angular momentum in rotating atmospheres3. Earth’s Hadley Circulation and Jet Streams4. Mars’ Hadley Circulation5. Super-rotation on Venus and Titan6. Jet formation on Jupiter
EarthThere 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!
Earth
Earth
Earth
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
Earth
Monsoons by angular momentum too!
After Bordoni and Schneider 2008,
Nature Geoscience
Outline
1. Overview of planetary atmospheres2. Angular momentum in rotating atmospheres3. Earth’s Hadley Circulation and Jet Streams4. Mars’ Hadley Circulation5. Super-rotation on Venus and Titan6. Jet formation on Jupiter
MarsMars 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
Mars
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)
Outline
1. Overview of planetary atmospheres2. Angular momentum in rotating atmospheres3. Earth’s Hadley Circulation and Jet Streams4. Mars’ Hadley Circulation5. Super-rotation on Venus and Titan6. Jet formation on Jupiter
Venus
Venus in the ultraviolet
VenusVenus’ 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!
Titan
Titan in the infrared
Titan
• Titan has a global Hadley Cell
• Titan’s upper atmosphere is in a state of super-rotation, like Venus
Titan
George Hadley’s original idea to explain the trade winds (1735)
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
Titan
False color RADAR image
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.
The fly-wheel crust of Titan?
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
Outline
1. Overview of planetary atmospheres2. Angular momentum in rotating atmospheres3. Earth’s Hadley Circulation and Jet Streams4. Mars’ Hadley Circulation5. Super-rotation on Venus and Titan6. Jet formation on Jupiter
Jupiter
Multiple Jets and macroturbulence
Jupiter
Jet Formation
Stirring
wavebreaking
wavebreaking
E-W Wind
Angular momentum divergence
Angular momentum divergence
Angular momentum convergence
Jet Formation
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:
SummaryAngular 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
SummaryAngular 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
SummaryAngular 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
SummaryAngular 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
THANK YOU!