Geostrophic winds cannot be exactly parallel to isobars
 

I keep seeing descriptions (e.g.
http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/fw/fric.rxml) of winds
blowing parallel to isobars. Yet surely this is not physically possible?
The only force driving the wind is the pressure gradient force. The
Coriolis force always acts perpendicular to the direction of movement,
so it cannot do work--that is, change the kinetic energy of the wind in
in any way.

Look at it this way: the pressure gradient force is always perpendicular
to the isobars. The Coriolis force is always perpendicular to the
direction of the wind. So when the wind is moving parallel to the
isobars, you have both forces acting perpendicular to its motion,
leaving _no_ force acting in the direction of its motion. So what's
keeping the wind moving against the drag of the ground?

Geostrophic winds cannot be exactly parallel to isobars
 

On Fri, 20 Aug 2004 22:57:41 +1200,
Lawrence D¹Oliveiro , in
wrote:
+ I keep seeing descriptions (e.g.
+ http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/fw/fric.rxml) of winds
+ blowing parallel to isobars. Yet surely this is not physically possible?

Geostrophic winds happen rarely in the real world. And they are, by
definition, the component parallel to isobars. The ageostrophic
component is perpendicular to the isobars. So, in reality, the winds
look like this:

wind = geostrophic + ageostrophic

Now that's a bit of a simplification since those are vectors
(direction and speed).

James
--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

Geostrophic winds cannot be exactly parallel to isobars
 

In article ,
(I R A Darth Aggie) wrote:

On Fri, 20 Aug 2004 22:57:41 +1200,
Lawrence D¹Oliveiro , in
wrote:
+ I keep seeing descriptions (e.g.
+ http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/fw/fric.rxml) of winds
+ blowing parallel to isobars. Yet surely this is not physically possible?

Geostrophic winds happen rarely in the real world. And they are, by
definition, the component parallel to isobars.

I thought the definition
http://ww2010.atmos.uiuc.edu/(Gl)/ww...hret=/guides/m
tr/fw/fric.rxml was that the Coriolis force was in balance with the
pressure gradient force. Which I took to mean, that component of the
pressure gradient force perpendicular to the direction of the wind.

As long as the drag is nonzero, there must be a component of the
pressure gradient force in the direction of the motion of the wind, to
offset the drag. So the wind can never be exactly parallel to the
isobars.

Geostrophic winds cannot be exactly parallel to isobars
 

"Lawrence D¹Oliveiro" wrote in message
...
... Which I took to mean, that component of the
pressure gradient force perpendicular to the direction of the wind.

As long as the drag is nonzero, there must be a component of the
pressure gradient force in the direction of the motion of the wind, to
offset the drag. So the wind can never be exactly parallel to the
isobars.

Why?

The fact that the air is moving at all, is determined by the pressure
gradient force (PGF). The fact that it's direction is 90 degrees offset to
the PGF is a function of the coriolis effect... you cannot ask for *more*
PGF in that direction.

Geostrophic winds cannot be exactly parallel to isobars
 

"Lawrence D¹Oliveiro" wrote in message
...
| I keep seeing descriptions (e.g.
| http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/fw/fric.rxml) of winds
| blowing parallel to isobars. Yet surely this is not physically possible?
| The only force driving the wind is the pressure gradient force. The
| Coriolis force always acts perpendicular to the direction of movement,
| so it cannot do work--that is, change the kinetic energy of the wind in
| in any way.
|
| Look at it this way: the pressure gradient force is always perpendicular
| to the isobars. The Coriolis force is always perpendicular to the
| direction of the wind. So when the wind is moving parallel to the
| isobars, you have both forces acting perpendicular to its motion,
| leaving _no_ force acting in the direction of its motion. So what's
| keeping the wind moving against the drag of the ground?

The geostrophic wind is not a "real" wind. It is an "ideal" wind based on
certain assumptions. One of these is that there is no friction against the
ground, which is why it does not appear in the formulae used to calculate
this wind.

In the absence of ground friction, (which applies very nearly in the free
atmosphere a kilometre or two up or more), the geostrophic wind is a very
close approximation, providing the systems are not moving too quickly or are
developing/decaying rapidly. These circumstances also produce effects not
included in the geostrophic wind calculation.

What happens with ground friction is that the wind speed is reduced from the
geostrophic and so the coriolis force term drops. The pressure gradient
force is then left unbalanced and the wind is deflected to blow at an angle
towards low pressure, effectively filling it up. Just as well, really, for
this limits the practical depth to which depressions can get and causes them
to fill when the processes generating them are finished. Otherwise there is
no telling how strong a hurricane could get (the same argument applies with
cyclostrophic balance, which is when the pressure gradient force provides
the acceleration required to maintain the moving air in a circular path - it
balances the "centrigugal force" which also drops with windspeed). That
ground friction drag prevents things being much worse than they actually are
in the real world.
--
- Yokel -
oo oo
OOO OOO
OO 0 OO
) ( I ) (
) ( /\ ) (

"Yokel" now posts via a spam-trap account.
Replace my alias with stevejudd to reply.

Geostrophic winds cannot be exactly parallel to isobars
 

In article ers.com,
"Icebound" wrote:

"Lawrence D¹Oliveiro" wrote in message
...
... Which I took to mean, that component of the
pressure gradient force perpendicular to the direction of the wind.

As long as the drag is nonzero, there must be a component of the
pressure gradient force in the direction of the motion of the wind, to
offset the drag. So the wind can never be exactly parallel to the
isobars.

Why?

The fact that the air is moving at all, is determined by the pressure
gradient force (PGF). The fact that it's direction is 90 degrees offset to
the PGF is a function of the coriolis effect... you cannot ask for *more*
PGF in that direction.

The work done by a force is the dot product of the force and the
displacement of the object it's pushing on. In particular, if the two
vectors (force and displacement) are at right angles, then the dot
product is zero. Which means if the wind is moving at right angles to
the pressure gradient force, then the pressure gradient force cannot
transfer any kinetic energy to the wind, so it cannot cause the wind to
blow.

Geostrophic winds cannot be exactly parallel to isobars
 

In article ,
"Yokel" wrote:

In the absence of ground friction, (which applies very nearly in the free
atmosphere a kilometre or two up or more), the geostrophic wind is a very
close approximation, providing the systems are not moving too quickly or are
developing/decaying rapidly. These circumstances also produce effects not
included in the geostrophic wind calculation.

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.

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote:
In article ,
"Yokel" wrote:


In the absence of ground friction, (which applies very nearly in the free
atmosphere a kilometre or two up or more), the geostrophic wind is a very
close approximation, providing the systems are not moving too quickly or are
developing/decaying rapidly. These circumstances also produce effects not
included in the geostrophic wind calculation.


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.

I guess someone oughta tell the 500-mb flow that, then ;)

Scott

Geostrophic winds cannot be exactly parallel to isobars
 

In article ,
Scott wrote:

Lawrence D¹Oliveiro wrote:
In article ,
"Yokel" wrote:


In the absence of ground friction, (which applies very nearly in the free
atmosphere a kilometre or two up or more), the geostrophic wind is a very
close approximation, providing the systems are not moving too quickly or are
developing/decaying rapidly. These circumstances also produce effects not
included in the geostrophic wind calculation.


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.

I guess someone oughta tell the 500-mb flow that, then ;)

You have some empirical evidence contradicting my claim?

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote:

In article ,
Scott wrote:

Lawrence D¹Oliveiro wrote:
In article ,
"Yokel" wrote:


In the absence of ground friction, (which applies very nearly in the free
atmosphere a kilometre or two up or more), the geostrophic wind is a very
close approximation, providing the systems are not moving too quickly or are
developing/decaying rapidly. These circumstances also produce effects not
included in the geostrophic wind calculation.


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.

I guess someone oughta tell the 500-mb flow that, then ;)

You have some empirical evidence contradicting my claim?

If you're talking *exactly* parallel to the isobars at all times, no.
But the 500 mb winds do flow generally approximately parallel to the
isobars, to within a good enough approximation for many purposes,
especially in areas of respectable gradients. A look at a couple of
500 mb synoptic charts will show that. For instance see
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html .
Note that the contours are isoheights of the 500 mb surface, not
isobars, but it makes no difference to Scott's point. If you want
to dig out a meteorology text from the 1940s or 1950s, when they
still used isobars on a constant height surface, be my guest.

Cheers,
Russell
--
All too often the study of data requires care.

Geostrophic winds cannot be exactly parallel to isobars
 

In article ,
"R. Martin" wrote:

Lawrence D¹Oliveiro wrote:

In article ,
Scott wrote:

Lawrence D¹Oliveiro wrote:

Thus, even neglecting drag, the idea of winds flowing parallel to
isobars is still unrealistic.

I guess someone oughta tell the 500-mb flow that, then ;)

You have some empirical evidence contradicting my claim?

If you're talking *exactly* parallel to the isobars at all times, no.
But the 500 mb winds do flow generally approximately parallel to the
isobars, to within a good enough approximation for many purposes,
especially in areas of respectable gradients. A look at a couple of
500 mb synoptic charts will show that. For instance see
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html .
Note that the contours are isoheights of the 500 mb surface, not
isobars, but it makes no difference to Scott's point.

And at what heights were the wind directions determined--ground level?
So the isoheights don't correspond to the wind directions on that chart
at all, and any parallelism between the two is really accidental.

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote:
In article ,
"R. Martin" wrote:


Lawrence D¹Oliveiro wrote:

In article ,
Scott wrote:


Lawrence D¹Oliveiro wrote:


Thus, even neglecting drag, the idea of winds flowing parallel to
isobars is still unrealistic.

I guess someone oughta tell the 500-mb flow that, then ;)

You have some empirical evidence contradicting my claim?

If you're talking *exactly* parallel to the isobars at all times, no.
But the 500 mb winds do flow generally approximately parallel to the
isobars, to within a good enough approximation for many purposes,
especially in areas of respectable gradients. A look at a couple of
500 mb synoptic charts will show that. For instance see
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html .
Note that the contours are isoheights of the 500 mb surface, not
isobars, but it makes no difference to Scott's point.


And at what heights were the wind directions determined--ground level?
So the isoheights don't correspond to the wind directions on that chart
at all, and any parallelism between the two is really accidental.

What ARE you talking about?

The heights of the wind at 500mb are the height of the 500mb
surface, which varies between 5000 and 6000 geopotential meters,
approximately.

Wind barbs will be parallel to the height contours on an
isobaric surface or, equivalently, isobaric contours on
a height surface if the flow is geostrophic. Geostrophic
flow is unaccelerated. For straight, slowly-varying,
frictionless flow at 500 mb, that's not a bad approximation.

Scott

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote:

In article ,
"R. Martin" wrote:

Lawrence D¹Oliveiro wrote:

In article ,
Scott wrote:

Lawrence D¹Oliveiro wrote:

Thus, even neglecting drag, the idea of winds flowing parallel to
isobars is still unrealistic.

I guess someone oughta tell the 500-mb flow that, then ;)

You have some empirical evidence contradicting my claim?

If you're talking *exactly* parallel to the isobars at all times, no.
But the 500 mb winds do flow generally approximately parallel to the
isobars, to within a good enough approximation for many purposes,
especially in areas of respectable gradients. A look at a couple of
500 mb synoptic charts will show that. For instance see
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html .
Note that the contours are isoheights of the 500 mb surface, not
isobars, but it makes no difference to Scott's point.

And at what heights were the wind directions determined--ground level?
So the isoheights don't correspond to the wind directions on that chart
at all, and any parallelism between the two is really accidental.

You clearly don't know what you're talking about. I suggest you
take a course or two in meteorology. If you have, I suggest you
demand your money back, since you received very poor instruction.

Cheers,
Russell
--
All too often the study of data requires care.

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote:

In article ,
"R. Martin" wrote:

Lawrence D¹Oliveiro wrote:

In article ,
Scott wrote:

Lawrence D¹Oliveiro wrote:

Thus, even neglecting drag, the idea of winds flowing parallel to
isobars is still unrealistic.

I guess someone oughta tell the 500-mb flow that, then ;)

You have some empirical evidence contradicting my claim?

If you're talking *exactly* parallel to the isobars at all times, no.
But the 500 mb winds do flow generally approximately parallel to the
isobars, to within a good enough approximation for many purposes,
especially in areas of respectable gradients. A look at a couple of
500 mb synoptic charts will show that. For instance see
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html .
Note that the contours are isoheights of the 500 mb surface, not
isobars, but it makes no difference to Scott's point.

And at what heights were the wind directions determined--ground level?
So the isoheights don't correspond to the wind directions on that chart
at all, and any parallelism between the two is really accidental.

P.S. I'm going offline for a while, so consider my previous comment
my last on the topic since I won't be able to argue with you even if
I wanted to waste my time.

Cheers,
Russell
--
All too often the study of data requires care.

Geostrophic winds cannot be exactly parallel to isobars
 

On Sat, 21 Aug 2004 22:44:54 +1200,
Lawrence D¹Oliveiro , in
wrote:

+ I thought the definition
+ http://ww2010.atmos.uiuc.edu/(Gl)/ww...hret=/guides/m
+ tr/fw/fric.rxml was that the Coriolis force was in balance with the
+ pressure gradient force. Which I took to mean, that component of the
+ pressure gradient force perpendicular to the direction of the wind.
+
+ As long as the drag is nonzero, there must be a component of the
+ pressure gradient force in the direction of the motion of the wind, to
+ offset the drag. So the wind can never be exactly parallel to the
+ isobars.

You're gonna make me drag out Holton? that's gonna cost you.

The geostrophic wind is a *mathematical* solution to the horizontal
equation of motion given a certain set of assumptions. That does not
mean that it provides all possible solutions (it doesn't), but "As
discussed in Section 2.4.1, the geostrophic wind is generally a good
approximation to the actual wind in extratropical synoptic-scle
disturbances."

Particularly once you get above the boundary layer, where drag is for
the most part negligible.

Reference:

Holton, James R., An Introduction to Dynamic Meteorology, Third
Edition, 1992, ISBN 0-12-354355-X. See Chapter 3.

James
--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

Geostrophic winds cannot be exactly parallel to isobars
 

On Mon, 23 Aug 2004 23:56:43 GMT,
R. Martin , in
wrote:

+ If you want to dig out a meteorology text from the 1940s or 1950s,
+ when they still used isobars on a constant height surface, be my
+ guest.

After rereading Holton's chapter 3, I remember why I like an isobaric
coordinate system...if you think our friend would quibble over
the geostrophic wind, I'm sure he'd have a cow if we had to play with
value of density...

James
--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote in message ...
In article ,
"R. Martin" wrote:

Lawrence D¹Oliveiro wrote:

In article ,
Scott wrote:

Lawrence D¹Oliveiro wrote:

Thus, even neglecting drag, the idea of winds flowing parallel to
isobars is still unrealistic.

I guess someone oughta tell the 500-mb flow that, then ;)

You have some empirical evidence contradicting my claim?

If you're talking *exactly* parallel to the isobars at all times, no.
But the 500 mb winds do flow generally approximately parallel to the
isobars, to within a good enough approximation for many purposes,
especially in areas of respectable gradients. A look at a couple of
500 mb synoptic charts will show that. For instance see
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html .
Note that the contours are isoheights of the 500 mb surface, not
isobars, but it makes no difference to Scott's point.

And at what heights were the wind directions determined--ground level?
So the isoheights don't correspond to the wind directions on that chart
at all, and any parallelism between the two is really accidental.

900 or 850 mb maybe, lacking professional meterological education I
can't tell you what approach that is most common or useful. What I can
tell you is after reading this thread, I get the impression that you
are caught in a not pariculary useful vorticy generated by the small
differences between idealized models(tools used in understanding,
theaching and prediction) and real observations. Or perhaps 'trolling'
to create an entertaining thread on the subject.

Either way, you might benefit from first using the idealized modells
to grasp the big picture and then go back and try to
understand/improve the models you used. It is for instance less
tempting to go back and improve something you have used a lot of time
and effort on.

Imagine that one way to get the big picture is to find ground level
winds, 850 mb heights/winds, 500 mb heights/winds and 300 mb
heights/winds analysis maps for a tropical cyclone.

Assume - from the coriolis effect and bernoulli equation(If your not
familiar with the bernoulli equation, google it) - that the 850 mb
heights/winds below the convective cloud cover will have a relative
large ageostrophic component towards the center and thus get stronger
untill the centrifugal forces annihilates the agestrophic
component(the eye).

- And that the 300 or 200 mb heights/winds will have a agestrophic
component away from the center and slow down as they blow outwards.


*Don't think this type of trolling is especially bad or deceitful,
more like talking about the weather to a pretty female TV
meterologist(or weather man if being a woman).

Geostrophic winds cannot be exactly parallel to isobars
 

In article ,
"R. Martin" wrote:

You clearly don't know what you're talking about.

And yet if you look carefully at the chart you mentioned
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html , you
can see a few wind vectors pointing _away_ from the low.

Do you understand why that's not physically possible?

Geostrophic winds cannot be exactly parallel to isobars
 

In article ,
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.

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote:
In article ,
"R. Martin" wrote:


You clearly don't know what you're talking about.


And yet if you look carefully at the chart you mentioned
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html , you
can see a few wind vectors pointing _away_ from the low.

Do you understand why that's not physically possible?

Well, I carefully looked, and saw nothing unusual.

I think it's important, when looking at an analysis like
that to ask yourself: what is the scale of the analysis,
and what is the scale of the observations? The 500-mb
chart you've linked to is obviously a synoptic-scale
analysis, and I'll suggest it's first guess field is
derived from a model, which model results may or may
not jibe with reality near the radiosonde observations.
Depending on how the analysis is constructed, those
observations at variance with the model forecast may
or may not be considered by the analysis, and for that
reason you may have wind vectors that appear not to
follow the flow -- of course, the analysis is just
wrong, or the wind vector is significantly ageostrophic
(which means the wind vectors are accelerating). Or both!

Scott

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote:
In article ,
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

Geostrophic winds cannot be exactly parallel to isobars
 

Now you have got me twisting in agony, the coriolis force is an
imagined force with the purpose of saving artillerist from complicated
math and learn school children basic meterological skills. Its real
name is _the coriolis effect_, an artifact caused by the conservation
of momentum(F=ma) and the earths rotation.

If you have to go ballistic on an imagined subject, try this one:
http://www.eoascientific.com/campus/...ew_interactive

Scott wrote in message . ..
Lawrence D¹Oliveiro wrote:
In article ,
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.

Would a circular tube of fast flowing air located around the North
Pole, lets say 30 to 60 degrees north, also be impossible ?

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.

It cannot do work since it is imagined, the coriolis effect does
actually seem to speed up air moving northward, but it is really the
land surface slowing down and the air flowing relative frictionless
above groundlevel keeping its momentum .



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

Geostrophic winds cannot be exactly parallel to isobars
 

"Scott" wrote in message
...
| Lawrence D¹Oliveiro wrote:
| In article ,
| 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.
|
|

There was a discussion about geostrophic flow on curved isobars a month or
two back on this very newsgroup (in June, thread "Wind Velocities"). The
curvature adds an extra term to the formula (the "cyclostrophic term")
representing the force required to accelerate the air and maintain it on the
curved path of the isobars.

It is possible for a balance to be set up between this cyclostrophic force
and the pressure gradient force alone, without regard to the coriolis force.
To all intents and purposes, this is what happens in a tornado, which it is
why it is possible to get an "anticyclonic" or "wrong way" tornado - the
coriolis force is insignificant on this scale (even though it may have an
effect on the scale of the cloud generating the tornado).

I would refer you back to that thread by the usual methods of reading old ng
postings.
--
- Yokel -
oo oo
OOO OOO
OO 0 OO
) ( I ) (
) ( /\ ) (

"Yokel" now posts via a spam-trap account.
Replace my alias with stevejudd to reply.

Geostrophic winds cannot be exactly parallel to isobars
 

Scott wrote in message . ..
Lawrence D¹Oliveiro wrote:
In article ,
"R. Martin" wrote:


You clearly don't know what you're talking about.


And yet if you look carefully at the chart you mentioned
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html , you
can see a few wind vectors pointing _away_ from the low.

Do you understand why that's not physically possible?

Well, I carefully looked, and saw nothing unusual.

I think it's important, when looking at an analysis like
that to ask yourself: what is the scale of the analysis,
and what is the scale of the observations? The 500-mb
chart you've linked to is obviously a synoptic-scale
analysis, and I'll suggest it's first guess field is
derived from a model, which model results may or may
not jibe with reality near the radiosonde observations.
Depending on how the analysis is constructed, those
observations at variance with the model forecast may
or may not be considered by the analysis, and for that
reason you may have wind vectors that appear not to
follow the flow -- of course, the analysis is just
wrong, or the wind vector is significantly ageostrophic
(which means the wind vectors are accelerating). Or both!

Scott

Suppose 100% accurate analysis. Doesn't it look like the 552 low on
the westcoast get its horseshoe form because coriolis forces -acting
on northerly winds from the high- is causing a vacuum effect all along
the rockies, except straight west from the low where southwesterly
winds is piling up. The ageostrophic anomalies should therefore not be
any mystic at all with both piling up, windshear from the Rocky
Mountains and not unlikely some shear from a sharp turn in the
jetstream nearby.

No need to attack this particulary model/modeller you know, it contain
no traces of G.warming.;-).

Geostrophic winds cannot be exactly parallel to isobars
 

O18-C-O16 wrote:
Now you have got me twisting in agony, the coriolis force is an
imagined force with the purpose of saving artillerist from complicated
math and learn school children basic meterological skills. Its real

That's okay, the geostrophic wind is imaginary too :)

Scott

Geostrophic winds cannot be exactly parallel to isobars
 

O18-C-O16 wrote:
Scott wrote in message . ..

Lawrence D¹Oliveiro wrote:

In article ,
"R. Martin" wrote:



You clearly don't know what you're talking about.


And yet if you look carefully at the chart you mentioned
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html , you
can see a few wind vectors pointing _away_ from the low.

Do you understand why that's not physically possible?

Well, I carefully looked, and saw nothing unusual.

I think it's important, when looking at an analysis like
that to ask yourself: what is the scale of the analysis,
and what is the scale of the observations? The 500-mb
chart you've linked to is obviously a synoptic-scale
analysis, and I'll suggest it's first guess field is
derived from a model, which model results may or may
not jibe with reality near the radiosonde observations.
Depending on how the analysis is constructed, those
observations at variance with the model forecast may
or may not be considered by the analysis, and for that
reason you may have wind vectors that appear not to
follow the flow -- of course, the analysis is just
wrong, or the wind vector is significantly ageostrophic
(which means the wind vectors are accelerating). Or both!

Scott


Suppose 100% accurate analysis. Doesn't it look like the 552 low on
the westcoast get its horseshoe form because coriolis forces -acting
on northerly winds from the high- is causing a vacuum effect all along
the rockies, except straight west from the low where southwesterly
winds is piling up. The ageostrophic anomalies should therefore not be
any mystic at all with both piling up, windshear from the Rocky
Mountains and not unlikely some shear from a sharp turn in the
jetstream nearby.

No need to attack this particulary model/modeller you know, it contain
no traces of G.warming.;-).

Well, I look at the isoheights around that 522 low off the
west coast and see different jets moving around it.
Propagating jets = very strong accelerations = very
non-geostrophic flow. So the 522 low has a horseshoe
shape because there are jets, which are related to
the horizontal temperature gradients at levels below
500, among other things.

I find your discussion a little confusing, though, so
maybe I'm missing your meaning. How can an ageostropic
anomaly be mystic, for example?

Scott

Geostrophic winds cannot be exactly parallel to isobars
 

On Fri, 27 Aug 2004 10:49:27 -0500,
Scott , in
wrote:
+ O18-C-O16 wrote:
+ Now you have got me twisting in agony, the coriolis force is an
+ imagined force with the purpose of saving artillerist from complicated
+ math and learn school children basic meterological skills. Its real
+
+ That's okay, the geostrophic wind is imaginary too :)

Heh. I've had baby meteorologists try to tell me the thermal wind is
real, too...

James
--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

Geostrophic winds cannot be exactly parallel to isobars
 

I R A Darth Aggie wrote:

On Mon, 23 Aug 2004 23:56:43 GMT,
R. Martin , in
wrote:

+ If you want to dig out a meteorology text from the 1940s or 1950s,
+ when they still used isobars on a constant height surface, be my
+ guest.

After rereading Holton's chapter 3, I remember why I like an isobaric
coordinate system...if you think our friend would quibble over
the geostrophic wind, I'm sure he'd have a cow if we had to play with
value of density...

James
--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

Agreed.

BTW, damned shame Jim Holton passed away.

Cheers,
Russell
--
All too often the study of data requires care.

Geostrophic winds cannot be exactly parallel to isobars
 

Scott wrote in message . ..
O18-C-O16 wrote:
Scott wrote in message . ..

Lawrence D¹Oliveiro wrote:

In article ,
"R. Martin" wrote:



You clearly don't know what you're talking about.


And yet if you look carefully at the chart you mentioned
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html , you
can see a few wind vectors pointing _away_ from the low.

Do you understand why that's not physically possible?

Well, I carefully looked, and saw nothing unusual.

I think it's important, when looking at an analysis like
that to ask yourself: what is the scale of the analysis,
and what is the scale of the observations? The 500-mb
chart you've linked to is obviously a synoptic-scale
analysis, and I'll suggest it's first guess field is
derived from a model, which model results may or may
not jibe with reality near the radiosonde observations.
Depending on how the analysis is constructed, those
observations at variance with the model forecast may
or may not be considered by the analysis, and for that
reason you may have wind vectors that appear not to
follow the flow -- of course, the analysis is just
wrong, or the wind vector is significantly ageostrophic
(which means the wind vectors are accelerating). Or both!

Scott


Suppose 100% accurate analysis. Doesn't it look like the 552 low on
the westcoast get its horseshoe form because coriolis forces -acting
on northerly winds from the high- is causing a vacuum effect all along
the rockies, except straight west from the low where southwesterly
winds is piling up. The ageostrophic anomalies should therefore not be
any mystic at all with both piling up, windshear from the Rocky
Mountains and not unlikely some shear from a sharp turn in the
jetstream nearby.

No need to attack this particulary model/modeller you know, it contain
no traces of G.warming.;-).

Well, I look at the isoheights around that 522 low off the
west coast and see different jets moving around it.
Propagating jets = very strong accelerations = very
non-geostrophic flow. So the 522 low has a horseshoe
shape because there are jets, which are related to
the horizontal temperature gradients at levels below
500, among other things.

I find your discussion a little confusing, though, so
maybe I'm missing your meaning. How can an ageostropic
anomaly be mystic, for example?

Scott

I don't know, but maybe Lawrence found to arrows indicating wind
blowing uphill(from low to high) and slowing down. He might have
conceived this inconsistent with the following statement because he
did not notice the initially at rest criteria and read, Wind blows
because air move from high to low pressure, instead: "An air parcel
initially at rest will move from high pressure to low pressure because
of the pressure gradient force (PGF)"
http://ww2010.atmos.uiuc.edu/(Gl)/gu...r/fw/geos.rxml

Shortening detailed and logical child/student misinterpretation proof
textbook definitions into short easy to remember lines with intuitive
appeal, often cause misperception of phenomena outside personal
experience.

....Individuals who remember such definitions AND that has the skill to
use them logical AND sufficient long span attention to avoid random
mistakes AND the motivation/will to actually do so, is a rare
species...

Geostrophic winds cannot be exactly parallel to isobars
 

On Fri, 27 Aug 2004 23:13:54 GMT,
R. Martin , in
wrote:

+ BTW, damned shame Jim Holton passed away.

I had not heard that. That is indeed a sad news.

James
--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

Geostrophic winds cannot be exactly parallel to isobars
 

I R A Darth Aggie wrote:

On Fri, 27 Aug 2004 23:13:54 GMT,
R. Martin , in
wrote:

+ BTW, damned shame Jim Holton passed away.

I had not heard that. That is indeed a sad news.

James

If you're interested:
seattletimes.nwsource.com/html/obituaries/2001878066_holtonobit13e.html

Russell
--
All too often the study of data requires care.