Post on 06-Jan-2016
description
Geostrophic and thermal wind
Weather charts, 3 July 2006
300 mb
Consider the geopotential gradient across the solid red line.
Δz = 952 – 912 Dm = 400 m, ΔΦ = 4000 m2 s-2
Δx = 5.5 deg lat = 616 km (1 deg = 111 km)
pΦ = 4000 / 616000 = 0.0065 m s-2
Ug = f-1 k x pΦ = 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)
700 mb
Consider the geopotential gradient across the solid red line.
Δz = 316 – 300 Dm = 160 m, ΔΦ = 1600 m2 s-2
Δx = 5.5 deg lat = 616 km (1 deg = 111 km)
pΦ = 1600 / 616000 = 0.0026 m s-2
Ug = f-1 k x pΦ = 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)
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
ΔUg = - (r/f) T Δln 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.
Surface chart
Temperature gradient coincides with a front