12.003 Atmosphere, Ocean and Climate Dynamics Lecture ...raffaele/12.003/Lectures_files/...Thursday,...
Transcript of 12.003 Atmosphere, Ocean and Climate Dynamics Lecture ...raffaele/12.003/Lectures_files/...Thursday,...
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12.003Atmosphere, Ocean and Climate Dynamics
Lecture XIIHadley Circulation
Thursday, October 6, 2011
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Lecture XII Outline
1. Horizontal motions in the atmosphere2. Angular momentum conservation and the Hadley Circulation3. Midlatidute eddies4. The atmospheric energy transport
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Radiative-convective equilibrium
High p
Low p
High p
Low p
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Hadley Model George Hadley (1685-1768), British meteorologist, speculated that pressure gradients drive a pole-to-eq1taor overturning cell (the Hadley cell)
➡ heating in tropics drives ascending motions➡ pressure gradients at tropopause drive northward flow➡ cooling at poles drive descending motions
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Hadley Model Hadley realized that air must conserve angular momentum
➡ as air move poleward it gets closer to rotation axis➡ air must spin faster along➡ westerly winds
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Observations: Meridional Overturning Circulation
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Observations: Zona-Average Zonal Winds
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Conservation of angular momentumM0 = mΩa2 = mΩa2 cos2 θ +uacosθ (1)
dTdz
=− 1+L/RT q∗
1+Lβ/cp q∗Γd ≈−
1+31 q∗
1+167 q∗Γd (2)
dTdz
�
Tropopause≈−1+31×1×10
−3
1+167 1×10−3 Γd ≈ 0.9Γd ≈ 9K/Km (3)
dTdz
�
Tropics≈−1+31×17×10
−3
1+167 17×10−3 Γd ≈ 0.9Γd ≈ 4K/Km (4)
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Thursday, October 6, 2011
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Conservation of angular momentumM0 = mΩa2 = mΩa2 cos2 θ +uacosθ (1)
dTdz
=− 1+L/RT q∗
1+Lβ/cp q∗Γd ≈−
1+31 q∗
1+167 q∗Γd (2)
dTdz
�
Tropopause≈−1+31×1×10
−3
1+167 1×10−3 Γd ≈ 0.9Γd ≈ 9K/Km (3)
dTdz
�
Tropics≈−1+31×17×10
−3
1+167 17×10−3 Γd ≈ 0.9Γd ≈ 4K/Km (4)
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Thursday, October 6, 2011
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Breaking up of Hadley’s cell
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Breaking up of Hadley’s cellSpinup of an atmospheric circulation model (simulation starts from radiative-convective equilibrium solution)
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Descending branch of Hadley’s cell
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Hadley cell and moisture transportrt
Schematic of circulation Annual-mean zonal-mean relative humidity (%)
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Atmospheric energy transport
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Atmospheric circulation
Non-rotating planet Rotating planet
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Energy balance modelsEnergy budget for an atmospheric latitude band, i.e.• zonally averaged band• vertically averaged band
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Surface temperature and heat transport
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Energy transport by the Hadley cellHeat transport
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Energy transport by the Hadley cellHeat plus potential energy (dry static energy) transport
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Energy transport by the Hadley cellHeat plus potential energy plus latent heat
(moist static energy) transport
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Energy transport at midlatitudesEnergy transport is by geostrophic eddies
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Total atmospheric energy transport
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Total atmospheric energy transport• Hadley cell transport energy in the tropics➡ weak meridional temperature gradients below 30o
• Eddies transport energy at midlatitudes➡ strong temperature gradients beyond 30o
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Energy balance of the Earth
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Surface temperature and heat transport
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