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12.003Atmosphere, Ocean and Climate Dynamics
Lecture XIIHadley Circulation
Thursday, October 6, 2011
Lecture XII Outline
1. Horizontal motions in the atmosphere2. Angular momentum conservation and the Hadley Circulation3. Midlatidute eddies4. The atmospheric energy transport
Thursday, October 6, 2011
Radiative-convective equilibrium
High p
Low p
High p
Low p
Thursday, October 6, 2011
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
Thursday, October 6, 2011
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
Thursday, October 6, 2011
Observations: Meridional Overturning Circulation
Thursday, October 6, 2011
Observations: Zona-Average Zonal Winds
Thursday, October 6, 2011
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)
1
Thursday, October 6, 2011
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)
1
Thursday, October 6, 2011
Breaking up of Hadley’s cell
Thursday, October 6, 2011
Breaking up of Hadley’s cellSpinup of an atmospheric circulation model (simulation starts from radiative-convective equilibrium solution)
Thursday, October 6, 2011
Descending branch of Hadley’s cell
Thursday, October 6, 2011
Hadley cell and moisture transportrt
Schematic of circulation Annual-mean zonal-mean relative humidity (%)
Thursday, October 6, 2011
Atmospheric energy transport
Thursday, October 6, 2011
Atmospheric circulation
Non-rotating planet Rotating planet
Thursday, October 6, 2011
Energy balance modelsEnergy budget for an atmospheric latitude band, i.e.• zonally averaged band• vertically averaged band
Thursday, October 6, 2011
Surface temperature and heat transport
Thursday, October 6, 2011
Energy transport by the Hadley cellHeat transport
Thursday, October 6, 2011
Energy transport by the Hadley cellHeat plus potential energy (dry static energy) transport
Thursday, October 6, 2011
Energy transport by the Hadley cellHeat plus potential energy plus latent heat
(moist static energy) transport
Thursday, October 6, 2011
Energy transport at midlatitudesEnergy transport is by geostrophic eddies
Thursday, October 6, 2011
Total atmospheric energy transport
Thursday, October 6, 2011
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
Thursday, October 6, 2011
Energy balance of the Earth
Thursday, October 6, 2011
Surface temperature and heat transport
Thursday, October 6, 2011