Climatology Lecture 6

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ClimatologyLecture 6

• Horizontal Motion in the Atmosphere

Michael Palmer Room 119, Atmospheric
Physics mpalmer_at_atm.ox.ac.uk
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Course Outline

• Heat and the Earths Atmosphere
• Vertical Motion Stability
• Horizontal Motion Winds
• The General Circulation Midlatitudes
• The General Circulation Tropics
• Variability of the General Circulation

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WIND
Why do we have wind??!Which direction does it
go in?What governs how strong it is??!
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Wind

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

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Pressure Gradient Force
LOW pressure
Height (meters)
Pressure (hPA)
HIGH pressure
surface
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Pressure Gradient Force
LOW
HIGH
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Pressure Gradient Force

• Wind is driven by differences in pressure
• Differences in pressure arise both vertically and
  horizontally
• Vertical differences are balanced by gravity
  (hydrostatic equilibrium)
• Differences in pressure arise on a large scale
• PGF is directed from areas of high pressure to
  low pressure
• It is perpendicular to isobars (lines of equal
  pressure) and the closer the isobar spacing, the
  greater the PGF.

PGF
? density change of pressure over
distance
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Wind

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

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Coriolis Force

• deflection force
• depends on latitude, wind speed, earths
  rotation rate
• 2.w.V.sinØ
• sine of latitude Ø equator zero poles 1
• acts to the left in SH and to the right in NH
• acts at right angles to the wind
• deflection is proportion to wind speed

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A
r
O
Consider a toy car, initially at rest on a large
disc, then pushed so that it travels from O to A
at constant velocity V. It will move through
distance r in time t r V.t (distance
velocity x time) No problem..But what if the
disc is rotating??!
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w
w rotation rate (degrees per sec or radians
per sec.. 1 radian 180/p deg)
A
r
B
?
O
r V.t (distance velocity x time) In the
time (t) it takes for the car to go from O to A,
A has moved round to B. The disc has rotated
through angle OAB or ? ? w.t (angle turned
through angular velocity x time)
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w
w rotation rate (degrees per sec or radians
per sec.. 1 radian 180/p deg)
A
r
B
?
O
r V.t (distance velocity x time) In the
time (t) it takes for the car to go from O to A,
A has moved round to B. The disc has rotated
through angle OAB or ? ? w.t (angle turned
through angular velocity x time)
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w
A
w rotation rate (degrees per sec or radians per
sec.. 1 radian 180/pi deg)
r
B
?
O
r V.t (distance velocity x time) ? w.t
(angle turned through angular velocity x time)
The distance AB is given by AB r ?
V.t.w.t The distance from A to B may also be
written AB a.t2/2 So a.t2/2 V.t.w.t
a 2.w.V.
Coriolis acceleration,
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But, unfortunately, we know that the world is not
a flat disc! So a little adjustment is needed.
pole
sin Ø 1
w
wsinØ
Ø
sin Ø 0
equator
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But, unfortunately, we know that the world is not
a flat disc! So a little adjustment is needed.
pole
sin Ø 1
w
Rotating disc a 2.w.V. Rotating earth a
2.w.V.sin Ø
wsinØ
Ø
sin Ø 0
equator
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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

• Balance between PGF and Coriolis Force
• above the surface (above 1km) in extratropics
• wind blows parallel to isobars

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Geostrophic Wind
PGF Coriolis Force
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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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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. Esp p186-190