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Old February 5th 04, 10:23 AM posted to sci.geo.meteorology,uk.sci.weather
John Whitby John Whitby is offline
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First recorded activity by Weather-Banter: Oct 2003
Posts: 53
Default Surface wind and geostrophic wind

A rather complex subject Julian. Stability is also a factor. In very stable
conditions it is not *that* unusual for the the surface wind to be 40 or 50
deg backed from Geostrophic. Your 'rule of thumb' of a 50% increase in speed
between the surface and 2000FT is rather large and probably applies mainly
to daytime and unstable airmasses. At low wind speeds local topography
becomes very important.
Cheers
John
York,
North Yorkshire.
(Norman Virus Protected)

"Julian Scarfe" wrote in message
...
I'm puzzled. Why is the rotation of wind direction between surface and

say
2000 ft as low as it is?

The classic explanation of the difference between surface wind and
geostrophic wind, e.g.

http://ww2010.atmos.uiuc.edu/(Gl)/gu...r/fw/fric.rxml

leads to a fairly easy quantitative conclusion. Looking at the diagram on
that page, you can do some trivial trigonometry and conclude that

(Coriolis force at surface) = (Pressure gradient force) *
cos(angle_of_veer)

[where angle_of_veer is the angle between the surface wind and the
geostrophic wind]

so

(Coriolis force at surface) = (Coriolis force of geostrophic wind) *
cos(angle_of_veer)

But since the coriolis force is proportional to the wind speed, then

(Wind speed at surface) = (Geostrophic wind speed) * cos(angle_of_veer)

So we should be able to relate the change in wind speed to the
angle_of_veer.

Angle Ratio of Surface wind to geostrophic wind

10 98.5%
20 94%
30 87%
60 50%

So far so good, but I don't think it tallies with reality. The pilot's
rule-of-thumb is that the wind at altitude veers 30 degrees and doubles in
strength. It varies but that's not unusual. It's not uncommon to see
doubling or tripling of wind speed as you cross the boundary layer, but

veer
angles don't often exceed 30 degrees. A 60 degree veer seems very

unusual.

But according to the formula above, a ratio of 50% should be associated

with
a 60 degree veer, or putting it the other way round a 30 degree veer

should
be associated with a much smaller increase in wind speed.

So where does the model above break down?

Julian Scarfe