On Wednesday, 17 June 2020 17:41:20 UTC+1, Metman2012 wrote:
On 17/06/2020 17:15, JGD wrote:
On 17/06/2020 16:47, Norman Lynagh wrote:
JGD wrote:

A question that someone here might be able to help with please:

Is it possible to get any idea of the likely pressure gradient
associated with wind over relatively short distances, eg 500-1000m -
say one end of a level field to the other?

Presumably there has to be some difference in pressure for the wind
to blow at all, but I'm guessing that even for a significant say 20kt
wind blowing directly along the length of the field then the
difference over 1km might be relatively tiny, say 0.1mb?

The wind is dependent on the broadscale pressure field. Over very small
distances the atmospheric pressure may be subject to small changes due
to the wind blowing over the relatively rough terrain (humps and
hollows, walls, trees etc) i.e. over very small distances the pressure
will fluctuate due to the wind rather than the other way round.


OK, thanks for the answer - much appreciated. But I'm less interested in
the mechanism than trying to get even a rough estimate of the likely
pressure gradient at small linear scale. Say there was a steady eg 20kt
10m wind blowing at sea then, is it possible to express the pressure
gradient as mb/km?

And BTW there is a point behind this seemingly idle question in a debate
about spacing of sensors for dynamic atmospheric pressure measurements.
But it would be really useful to have even a rough estimate of that
mb/km figure.

The best guess I can come up with is around 0.01mb/km for a sustained
20kt wind but it would be good for someone more expert than me to
confirm or to say rubbish!

I think you need to look up geostrophic wind and the scales which help
give wind speeds from isobar spacing. I think what you want will require
a lot of complicated maths. There's also the gradient wind. And remember
that the sea is rarely flat, so the friction effects Norman speaks of
will also happen there. It's a long time since I dabbled in such esoterica!

As Metman2012 says, it would be good to read up on geostrophic wind scales - that will give you some insight into the complexities. For a given isobaric spacing (and ignoring the effects of rough terrain) there would be a range of possible values owing to isobaric curvature and the presence (or lack of) vertical motion.

--
Freddie
Dorrington
Shropshire
115m AMSL
http://www.hosiene.co.uk/weather/
Stats for the month so far: https://www.hosiene.co.uk/weather/st...cs/latest.xlsx