Re: Geostrophic winds cannot be exactly parallel to isobars

From: Scott (ScottLWI_at_mouse-potato.com)
Date: 08/25/04 

Date: Wed, 25 Aug 2004 06:12:28 -0500

Lawrence DčOliveiro wrote:
> In article <ldo-84681C.12150123082004@news.wave.co.nz>,
> I wrote:
>
>
>>Another thing to keep in mind is that the Coriolis force increases with
>>latitude. This means that, even in the complete absence of drag, the
>>wind cannot follow a closed path (which is what an isobar is), as that
>>would cause a pressure build-up at some point, which would stop the wind
>>flowing.
>>
>>Thus, even neglecting drag, the idea of winds flowing parallel to
>>isobars is still unrealistic.
>
>
> Let me explain this a bit further. Assume you have a wind flowing around
> a closed path with zero drag and air viscosity. Due to the Coriolis
> force, this flow will be anticlockwise in the northern hemisphere, and
> clockwise in the southern hemisphere. Or to put it another way, the half
> flowing east-to-west is closer to the respective pole than the half
> flowing west-to-east.
>
> But closer to the pole, the Coriolis force is stronger. That means that
> the half of the wind flowing closer to the pole must be moving on
> average faster than that half further from the pole, otherwise the
> deflection in direction caused by the Coriolis force would mean their
> paths would not join up.
>
> But if the east-to-west part is flowing faster, then there must be a
> buildup of pressure at the western side of the path, and a corresponding
> reduction in pressure at the eastern side. These changes in pressure
> represent transfers between the kinetic energy of the wind and the
> potential energy of the atmospheric pressure. But the Coriolis force
> cannot perform such transfers of energy--it can do no work, since it
> always acts perpendicular to the direction of motion. Therefore any such
> pressure buildup would stop the wind from flowing.
>
> Therefore the closed-path wind motion is not physically possible.

I know of no one who suggests that strongly accelerating
flow, such as your flow around a closed path, would be
geostrophic. Also, I'm not sure why you have assumed
that the pressure gradient would be the same initially
all around this low. If your initial conditions are not
physically realistic, I'm not surprised that your concluding
the flow is not physically possible.

Scott

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