The Hadley Cell

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The Hadley Cell
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• What does it look like?
• How do we know this?
• Why is it the way it is?
• Is it changing or expected to change?

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• First - the time- and zonally-averaged
  circulation.
• What does it look like?
• How do we know this?
• How is this circulation maintained?
• What do we mean by maintained?

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• Features of the averaged circulation
• Zonal wind
• Meridional wind
• Vertical wind
• Streamfunction
• See Holton for figures.

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• How is this circulation maintained?
• Maintained means that if we let go,
  frictional dissipation will dissipate the winds,
  and thermal dissipation (radiative cooling) will
  remove the accompanying temperature field (cools
  to near zero).

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• For example, we looked at the radiative-convective
  Em temperature.
• If we turn off solar heating, the T field relaxes
  back to near abs zero, and thus the field is
  maintained by continued radiative heating.

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• Incidentally, recall that the radiative-convectiv
  e Em temperature differs sharply from observed
  averaged temperatures, especially at high
  latitudes.
• This means that radiation alone cannot explain
  observed temperatures. No other diabatic effects
  can help explain this (e.g., latent heat
  release).
• The resolution is that dynamics supports the
  observed temperature distribution. Dynamics
  provides the warming, through direct descent
  adiabatic warming.
• Eddies are involved in this process - we may
  look at this further in 205B.

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• So how is the mean circulation maintained?
• Holton

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• Now lets focus on the Hadley cell.
• Questions
• Why does it have the observed latitudinal
  extent?
• What factors control this?
• Could it extend pole-equator?
• What controls its strength?
• Answers were provided in a paper by Held Hou,
  which we will examine.