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CPL MET Book Question 9, Page 31

• Sasut2
• Topic Author

Sasut2 created the topic: CPL MET Book Question 9, Page 31

Hi Bob and Team.

I have a quick question - I am having some difficulty understanding question 9 on page 31 which states:
"At which latitude would there be the greatest difference between the gradient wind and the actual wind at 5,000ft?", the answer being 5 degrees South.

I understand that the purpose of including the 5,000ft figure is to discount the effect of friction from finding the answer. I am confused by the following:

1) What is the meaning of "actual wind"?
In working backward from the answer I had contemplated that the meaning of actual wind could be interpreted as the wind that is the result of the pressure-gradient force where there is no effect of Coriolis. I am unsure if this is correct.

2) Assuming my thoughts in point 1 are correct, the gradient wind and actual wind at/near the equator will essentially be the same accounting for the lack of Coriolis at the equator. As such I thought that as you moved further toward a pole, the Coriolis would increase and there would be a greater difference between the wind that is the result of the gradient wind (interaction between PGF, Coriolis and centrifugal force) and the actual wind (wind due to PGF alone).

I am confused as to why 5 degrees South is the correct answer? If my rationale is correct I would have assumed that the further you move toward a pole (away from the Equator), the greater the difference. Could you please assist in clarifying?

Regards.

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MissSoph replied the topic: CPL MET Book Question 9, Page 31

Did you work this question out at all Sasut2?
I’m having a bit of a hard time working out how they say that answer too...

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• Sasut2
• Topic Author

Sasut2 replied the topic: CPL MET Book Question 9, Page 31

MissSoph wrote: Did you work this question out at all Sasut2?
I’m having a bit of a hard time working out how they say that answer too...

Hi MissSoph. Unfortunately I never received a response from this query. I am yet to figure out why. Maybe you will have more luck. I am sorry I cannot provide any further clarity.

Regards,
Shaun

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Carello replied the topic: CPL MET Book Question 9, Page 31

Hi Guys
I don't have the book and therefore do not have the full context of this question, however, the following may be helpful.

First, some definitions.
1) The gradient wind is typically taken to be the wind between 2000 and 3000 ft AGL. A these levels the flow (wind) is considered free of frictional forces within the boundary layer.

2) The actual wind is as suggested in the name

Now, the gradient wind is a non-geostrophic wind that blows parallel to curved isobars above the boundary layer. The gradient wind represent a balance of three forces viz, Pressure Gradient Force (PGF), Coriolis (C) and Centrifugal Force (CF).

In summary (refer to your notes):
For cyclonic flow the balance is: PGF = C + CF which results in a sub-geostrophic wind speed.
For anticyclonic flow the balance is: C = PGF + CF which results in a super-geostrophic wind speed.

The above is all fine and well in the mid to high latitudes (where Coriolis is a significant force), but at low latitudes (tropical regions), the idea of Geostrophic and Gradient flow breaks down due to the breakdown of the Coriolis Force that reduces to zero at the equator. In other words, the gradient wind balance cannot be achieved at low latitudes and as a result the "Actual Wind" at low latitudes will not follow the curved isobars.

So, in answer to the question, the "Actual Wind" will depart from the Gradient Wind as latitude decreases. Theoretically this departure will be greatest at the equator.

Please not that the Gradient Flow is quite different to Cyclostrophic Flow around curved isobars at low latitudes.

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