Geostrophic and thermal wind

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Geostrophic and thermal wind
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Reminder

• Geostrophic wind in pressure coordinates
• In the free atmosphere, wind is usually close to
  geostrophic.
• The departure from geostrophy is the ageostrophic
  wind
• Thermal Wind

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Worked Example
10 July 2006
Jet Stream, wind up to 60 m/s
300 mb
Same direction, but 20 m/s
700 mb
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Weather charts, 10 July 2006
300 mb

• Consider the geopotential gradient across the
  solid red line.
• ?z 952 912 Dm 400 m, ? ?F 4000 m2 s-2
• ?x 5.5 deg lat 616 km (1 deg 111 km)
• ?pF 4000 / 616000 0.0065 m s-2
• Ug f-1 k x ?pF 54 ms-1 (f
  1.12 x10-4)
• Ug 105 kt compared with 100-105 kt measured
• (1 knot I nautical mile hr-1 1852 m hr-1
  0.514 ms-1)

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• Consider the geopotential gradient across the
  solid red line.
• ?z 316 300 Dm 160 m, ? ?F 1600 m2 s-2
• ?x 5.5 deg lat 616 km (1 deg 111 km)
• ?pF 1600 / 616000 0.0026 m s-2
• Ug f-1 k x ?pF 23 ms-1 (f
  1.12 x10-4)
• Ug 45 kt compared with 45 kt measured
• (1 knot I nautical mile hr-1 1852 m hr-1
  0.514 ms-1)

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Thermal wind
700 mb
300 mb
Camborne temperature at 700 mb 5, at 300 mb
-37 Valentia temperature at 700 mb -1, at 300
mb -40 ?T 6 at 700 mb, 3 at 300 mb, mean
around 4.5 ?x 3.7 degrees latitude 411 km ?
?T 1.13x10-5 K m-1 ?Ug - (r/f) ?T ?ln p
(286x104) 1.13x10-5 ln(7/3) 27
ms-1 Actual value is 30 ms-1 but the calculation
is considerably cruder than the 3-figure
precision implies.
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Surface chart
Temperature gradient coincides with a front