Climatology Lecture 7

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ClimatologyLecture 7

• Horizontal Motion in the Atmosphere
• Continued
• Michael Palmer

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Wind

• Pressure gradient force
• Coriolis Force
• Geostrophic wind
• Gradient wind
• Friction and the Ekman spiral

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Wind

• Pressure gradient force
• Coriolis Force
• Geostrophic wind
• Gradient wind
• Friction and the Ekman spiral

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• Geostrophic wind only applies to straight
  parallel isobars, but isobars are usually curved,
  and often arent parallel
• For air moving in a curve, we must consider an
  additional force

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Centripetal force

• Tie a Spice Girls CD to some string and spin it
  around your head
• The tension you maintain on the rope pulls the CD
  around in a circle
• Inward force - centripetal

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Centrifugal force

• Fairground ride
• Body thrown out against side
• Centrifugal force - outward

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Which force do we work with?

• Both the same!
• Force apparent to a rotating body
• outward centrifugal
• Force apparent to external observer
• inward - centripetal

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Pressure gradient force

• Identical pressure gradient in each case

LOW
LOW
LOW
HIGH
HIGH
HIGH
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Centrifugal force

• Force due to circular motion, as viewed by air

LOW
LOW
LOW
HIGH
HIGH
HIGH
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Sub-geostrophic winds

• Wind speed less than geostrophic
• Cyclonic curvature - sub-geostrophic

Geostrophic Balance PGF CF
PGFCF Ce
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Super-geostrophic winds

• Wind speed more than geostrophic
• Anticyclonic curvature - super-geostrophic

Geostrophic Balance PGF CF
PGFCeCF
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??? Question ???

• When air blows around curved isobars will the
  wind speed be the same as of straight isobars?
• assuming the pressure gradient is the same

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??? Question ???

• When air blows around curved isobars will the
  wind speed be the same as of straight isobars?
• assuming the pressure gradient is the same
• Answer No, and the size of the difference
  depends on amount of curvature

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Gradient wind

• Pressure gradient force
• Coriolis force 2wVsin Ø
• Centrifugal force V2/r
• V is the wind speed
• r is the radius of curvature
• Balance

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Wind

• Pressure gradient force
• Coriolis Force
• Geostrophic wind
• Gradient wind
• Friction and the Ekman spiral

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Friction

• Friction reduces wind speed
• Coriolis force is reduced
• Flow crosses isobars from high towards low
  pressure
• Applies near surface but not at upper levels
  where friction is insignificant

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Cyclonic

• Cyclonic circulation produces convergence near
  surface

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Anticyclonic

• Anticyclonic circulation produces divergence near
  surface

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• Friction reduces with altitude

Free atmosphere
LOW
Near surface
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Surface pressure change

• Net inflow Net
  outflow
• P Increase P
  Decrease
• System Declines System Declines

LOW
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Vertical motion

• Upward Downward

Free atmosphere
LOW
Near surface
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Conclusions

• Curved flow is described by gradient wind
• Cyclonic curvature gives sub-geostrophic winds
• Anticyclonic curvature gives super-geostrophic
  winds
• PGF, CF, Friction and centrifugal forces describe
  wind systems adequately

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Readings for todays lecture

• Barry and Chorley 1997 p76-86
• Briggs et al. 1997 Fundamentals of the Physical
  Environment p78-88
• Henderson-Sellers and Robinson 1999 p56-74
• Linacre and Geerts 1997 Climates and Weather
  Explained p127-145
• McIlveen 1992 Chp 7 p181?