Julian Scarfe wrote:
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
In addition to the other reasons already posted:
The difference of 2000 feet near sea level, corresponds to a
pressure-level difference of approximately 60 mb. Because of horizontal
temperature differences, or other dynamic effects, the pattern of
pressure distribution at 2000 ft is different than that at the surface,
and thus the pressure forces producing the airflow at 2000 feet, are
different than the pressure forces producing the airflow at the surface.