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Geostrophic and thermal wind. Reminder Geostrophic wind in pressure coordinates In the free atmosphere, wind is usually close to geostrophic. The departure.

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Presentation on theme: "Geostrophic and thermal wind. Reminder Geostrophic wind in pressure coordinates In the free atmosphere, wind is usually close to geostrophic. The departure."— Presentation transcript:

1 Geostrophic and thermal wind

2 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:

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

4 Weather charts, 10 July 2006 300 mb Consider the geopotential gradient across the solid red line. Δz = 952 – 912 Dm = 400 m,  ΔΦ = 4000 m 2 s -2 Δx = 5.5 deg lat = 616 km (1 deg = 111 km)  p Φ = 4000 / 616000 = 0.0065 m s -2  U g = f -1 k x  p Φ = 54 ms -1 (f = 1.12 x10 -4 )  U g = 105 kt compared with 100-105 kt measured (1 knot = I nautical mile hr -1 = 1852 m hr -1 = 0.514 ms -1 )

5 700 mb Consider the geopotential gradient across the solid red line. Δz = 316 – 300 Dm = 160 m,  ΔΦ = 1600 m 2 s -2 Δx = 5.5 deg lat = 616 km (1 deg = 111 km)  p Φ = 1600 / 616000 = 0.0026 m s -2  U g = f -1 k x  p Φ = 23 ms -1 (f = 1.12 x10 -4 )  U g = 45 kt compared with 45 kt measured (1 knot = I nautical mile hr -1 = 1852 m hr -1 = 0.514 ms -1 )

6 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° ΔT = 6° at 700 mb, 3° at 300 mb, mean around 4.5° Δx = 3.7 degrees latitude = 411 km   T = 1.13x10 -5 K m -1 ΔU g = - (r/f)  T Δln p = (286x10 4 ) 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.

7 Surface chart Temperature gradient coincides with a front


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