Hello,

The answer to your question is: Only in the idealized model of the geostrophic wind resulting from the exact balance between the Coriolis force and the pressure gradient force. The comprehensive discussion of this important wind system is presented in

https://en.m.wikipedia.org/wiki/Geostrophic_wind

It is important to note that the geostrophic wind approximation is most appropriate for the mid latitudes.

Concerning the second part of your question, in order to describe the flow field in the general situation we need to consider the effect of friction, vertical stratification, thermal gradients at the surface and terrain slopes as well as the flow patterns associated with convective clouds.

Dear Dan and Janusz

To complete the answer of Janusz, we have also to take into account the isallobaric wind which is also due to synoptic pressure forces as well.. It is the  component of the wind arising from the spatial non-uniformity of local rates of change of pressure.

While geostrophic wind is due to spatial pressure gradient,. isallobaric wind is due to the time variation of pressure  (i.e. δP/δt, pressure time tendency). At the synoptic scale, isallobaric winds can be very important if synoptic systems are moving rapidly.

Isallobaric gradients are important components of the wind as they increase or decrease the geostrophic wind.

https://www.eas.ualberta.ca/jdwilson/EAS372_15/eas372_isallobaricwind.pdf

Regards

Dear Alain and Dan,

The idea of Alain to include the transient effects is indeed interesting.

I have examined the Brunt and Douglas (1928) theory of the isallobaric wind. Upon my review I would like to suggest to consult Haurwitz (1946): "On the relation between the wind field and pressure changes" available at

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281946%29003%3C0095%3AOTRBTW%3E2.0.CO%3B2

The main result of this paper is suggestion to abandon the concept of the issalobaric wind altogether because the simplifications needed to derive the governing equations are not justified.

The arguments from Haurwitz (1946) seems to be valid; I wonder if the model of issalobaric wind is still useful?

Regards,

Janusz

Dear Janusz and Alain, 

thank you for your answers. How would you comment Conradsen, Nielsen, Prahm

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0450(1984)023%3C1173:ROWSFE%3E2.0.CO;2

in Introduction "it is believed that for a climate like the Danish, with flat land and wind speeds mainly generated by synoptic pressure forces, the true wind speed distribution is unimodal" ... and at the end of section 2 again "In Denmark the wind is generally driven by the synoptic pressure pattern".

The "middle" of  Denmark is approximately at 450 km far from Oslo and from Hannover and the dominant wind direction is west. The wind over a spatial scale of 800-900 km of flat land could be considered as driven only by synoptic pressure pattern ?

Thank you,

Dan

hello

Experienced weather forecasters who I talk to mentioned to me that the isallobaric wind is especially important in the Canadian Prairies (flat land like Denmark) particularly in situation behind cold fronts with a high pressure system poking in behind. Pressure tendencies are then high at times and isallobaric having an important contribution in that case.

In the Canadian Meteorological Center (CMC), they use the maximum isallobaric wind as a measure of a potential maximum gust winds. Here attached the product they use (see figure).

So isallobaric wind is not a myth it is real but in many occasions masked by the advective geostrophic wind which could be stronger and opposite direction.

Commenting your article:  I think it is fair but including isallobaric wind is necessary in the situation mentioned above.

I would not abandoned isallobaric wind even if most of the time it could be neglected (weaker component than other member of ageostrophic wind).

On occasion it cannot be neglected (strong pressure tendency horizontal gradient).

Denis Bachand, a meteorologist for CMC has obtained this formula:

isallobaric wind (kts) = 2.5 * ni    where ni is the number of isallobars with interval of 1 mb/3 hrs (within 5 degrees of latitude).

It is still used at CMC on specific cases to forecast  max potential wind from isallobaric wind.

Regards

Alain

Hello Dan,

The statements:

1. "it is believed that for a climate like the Danish, with flat land and wind speeds mainly generated by synoptic pressure forces, the true wind speed distribution is unimodal"

2. "In Denmark the wind is generally driven by the synoptic pressure pattern"

are certainly approximations but they are justified considering the most typical weather patterns in Denmark.

Please note the use of words: "mainly" in (1) and "generally" in (2). I would be even more pleased to see "predominantly" instead of "generally" in (2)

Consequently I would suggest to add the word "predominantly" in your last statement:

"The wind over a spatial scale of 800-900 km of flat land is driven predominantly by the synoptic pressure pattern"

The situation is more complicated close to the shoreline.

You could be also interested in the recent study of the wind field in Denmark reported in

http://iopscience.iop.org/article/10.1088/1742-6596/524/1/012118/pdf

addressing some aspects of the three-dimensional structure of the wind field.

Sincerely,

Janusz

Hello Alain,

Thank you for preparing such a comprehensive answer. The problem is complicated because the isallobaric wind is one of the components of the ageostrophic wind. Considering the serious problems with the formula for the former I would suggest just to speak about the ageostrophic wind component. I still do not see anywhere a meaningful rebuttal of the critical statements from Haurwitz (1946).

Sincerely,

Janusz

Dear Janusz, 

thank you very much for your comments and for the paper.

Regards,

Dan

Dear Alain,

Thank you very much for your comments and for the attached maps.

Regards,

Dan

Thanks Janusz.

There are controversy about isallobaric wind.

See the attached article which is interesting but I still believe isallobaric wind should not be dropped.

http://ramrodmines.com/the-isallobaric-myth/

Regards

Alain

Hello Alain,

Thank you for the link; the opinion of Haurwitz (1946) is more restrained:

"While it has been shown that the simple relation between the isallobaric gradient and the geostrophic deviation derived by Hesselberg and Brunt and Douglas does not hold, it is, of course, not suggested that the two quantities are entirely unrelated"

-end of quote-

We definitely need to develop a more general expression for the non-geostrophic wind component. Perhaps this was already accomplished in the synoptic practice.

I think also that the work of McVittie (1951) "Theory of development and of thickness pattern" is the relevant reference to derive such an equation (please read the text on page 15) (the text is available when you enter the title of the paper in the Google search).

Kind regards,

Janusz

Thanks Janusz

I Appreciate

Wind is generated because of pressure gradient between regions of different pressure. The wind blows from high pressure region toward low pressure region. However, the pressure gradient force is not the only force that affects wind. The Coriolis force that turns wind toward right (in Northern hemisphere) also affects the value and direction of wind. Hence, pressure gradient force,Coriolis force, in addition to friction force all play their role in the synoptic situation of wind.

The pressure gradient is the change in barometric pressure over a distance. Big changes within shorter distances equals high wind speeds, while environments that exhibit less change in pressure with distance generate lower or non-existent winds. This is because higher-pressure air always moves toward air of lower pressure in an attempt to gain balance within the atmosphere. Steeper gradients result in a stronger push.