Changes in the hydrological cycle associated with anthropogenic climate change

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Changes in the hydrological cycle associated with
anthropogenic climate change

• John L McBride
• Bureau of Meteorology Research Centre Melbourne

Acknowledgements K. McInnes (CSIRO Atmospheric
Research) J. Arblaster (National Center for
Atmospheric Research)
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What are the mechanisms causing rainfall in
Australia during winter?

• Baroclinic instability associated with polar
  front? ----- NO
• Storm tracks in the circumpolar vortex? NO
• Subtropical ridge over continent / westerlies
  south of continent / lows in the westerlies with
  cold-front rainband crossing the continent? ---
  NO

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What are the mechanisms causing rainfall in
Australia during winter?

• Split jet
• Cut-off low
• Equatorward baroclinic zone associated with
  sub-tropical jet
• Moisture inflow from tropics

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How will this mechanism change in enhanced
greenhouse climate

• Scenarios say decreased winter precip --why?
• Subtropical ridge strengthens and moves southward
  --- How will it affect the mechanism we have just
  looked at?
• Baroclinic instability associated with Eady
  growth rate --- Why is there a split jet and an
  equatorward baroclinic zone in the first place?
• Where in the text-books and literature are
  studies of mechanisms and reasons for
  cut-off/subtropical-jet developments?

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CSIRO Mark 2 coupled AOGCM 2 X CO2 vs 1 X
CO2
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So. Whats the point of all this?

• Merely, that there is still a lot of work to do
  in
• A) understanding the mechanisms causing
  Australian rainfall in terms of large scale
  structure of the atmosphere
• B) understanding phenomenologically how it will
  change under a changed climate

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2nd part of talk
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Idealised two level model for global energy
budget
So (1-a)
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More complicated models

• Add stratospheric level (high value of ?, low
  value of e )
• Add 1, then 2, then 3, then n tropospheric
  levelsobtain analytical solution as function of
  n
• Add cloud at level i, with cloud albedo ai, and
  with e0 below cloud

• Use feedback analysis techniques (e.g Dutton 95)
  to separate CO2 and H2O contributions to e
• Parameterise variables (e.g Q as function of
  lapse rate)
• Etc., etc. (Two adjoining boxes upward and
  downward motion sectors)
• Land and sea sectors
• Hadley cell and higher latitude sectors

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Two major points

• This approach helps me, at least, to understand
  the system Can address questions such as why
  Hadley cell extent may change, why relative
  humidity stays effectively constant as
  temperature increases, etc.

• The entire system is dominated by water
• Why do we have the current value of emmisivity e
  ? Mainly because of water vapour content.
• Why current value of albedo a? Ice at poles,
  clouds (liquid water)
• The reason the planet has its current
  climate/current value of global-mean surface
  temperature is because of presence of large
  quantities of water in three phases solid,
  liquid, gas

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Changes in the hydrological cycle associated with
anthropogenic climate change

• The point?
• We looked at rain mechanisms for Australia
  poorly understood but a rich field for study
• We looked at simple models (zero-d, 1-d etc.) A
  rich field of study for understanding climate
  change
• Someone in our community should be doing this
  kind of work

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annual temperature difference CSIRO-2 2
times CO2 minus 1 times CO2
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At low levels T increases Relative humidity
stays approximately same
Relative humidity, q, increases.
Air rises in Hadley cell ?E cpT gz Lq is
conserved.
At greater heights/colder temperatures, air can
hold much smaller amount of water vapour so Lq ?
CpT