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03jtrickey

Tight Pressure Gradient Around Greenland - So Why Aren't The Winds Very Strong?

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I was browsing through this morning's 06Z GFS run and noticed that at +168 (well into FI I know), there is a really strong pressure gradient showing around Greenland.

h850t850eu.png

I've spotted this phenomenon on many occasions before but it only just occurred to me that - if the isobars are so tight - why aren't the winds really strong?

The NA Wind map for the same time shows winds of "just" 40-44mph over Greenland, while simultaneously over Ireland, similarly-spaced (perhaps even less tight) isobars produce wind speeds into the high 60s mph.

windvector.png

Is there a reason for this? Perhaps because in Greenland the isobars are surrounding an area of HP whereas the winds over Ireland are from LP? Don't isobars of the same tightness always produce the same wind speed? Or does the Coriolis effect make the winds stronger to the RHS of an Atlantic low pressure system?

Perhaps it is just because near Ireland the winds are over open sea? But even so the wind does not look as strong as I would have expected given how packed the isobars are near Greenland.

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Well, the seemingly semi-permanent highs over the interior of Greenland are a product of dense cold air building up over the high ice plateaus of Greenland, this creates the higher surface pressure. This dense cold air sinks towards the lower coastal areas under gravity, known as a katabatic wind. Often there is steep pressure gradients that develop over the Greenland landmass, as low pressure systems often graze the southern most part.

Bit difficult to ascertain why the strong pressure gradient shown across the interior doesn’t manifest as strong winds on the wind charts though. One thought that crosses my mind maybe because the wind speeds are mapped at a set pressure level, bearing in mind much of interior Greenland is at high elevation, so maybe the winds are not representative of that level? Maybe John Holmes has an answer?

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Thanks for this Nick... that explains the high pressure.

I wonder if there is a definitive explanation for the relatively low winds considering the tight gradient etc. :o

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Is there a reason for this? Perhaps because in Greenland the isobars are surrounding an area of HP whereas the winds over Ireland are from LP? Don't isobars of the same tightness always produce the same wind speed? Or does the Coriolis effect make the winds stronger to the RHS of an Atlantic low pressure system?

Nick gave some reasons, but I think the a main issue is latitude.

Regarding the above posts, some of the assumptions are incorrect.

1) Air going around a high will move faster than air going around a low. This sounds counter-intuitive because we associate lows with strong winds. But, when isobars are curved anticyclonically the wind is super-geostrophic whilst curving cyclonically is sub-geostrophic. Essentially that means that compared with straight isobars of the same spacing, and assuming the geostrophic balance holds, super-geostrophic is stronger than straight isobar wind, and sub-geostrophic is weaker than straight isobar wind.

2) Isobars of the same tightness don't produce the same wind speed. Latitude is one factor, and I think it's the main one here. It's another counter-intuitive effect, I suppose. We tend to link high latitudes with high winds. But actually the high latitudes are somewhat crippled by the mathematics of the atmosphere. For a given isobar spacing (assuming the isobars are straight), the wind at 75N will be weaker than at 55N which will be weaker than at 35N...and so on. When you get into the deep tropics (say between 15-20 degrees of latitude) it gets more complicated because geostrophy is not such a good approximation.

However, it is not a linear relationship. The critical part of the relationship is the sine of latitude (in degrees). On older weather charts you will see something in the corner that gives a correction of wind speed for latitude.

There's a good explanation of all this stuff here:

http://vaac.metservice.com/default/index.php?alias=howtoreadmaps234396

Quote: "on a weather map with isobars 4 hectoPascals apart, a spacing of about two degrees latitude (with straight isobars) means fresh winds about Auckland but a gale over Fiji. "

He also gives a more easily understandable explanation of sub/super geostrophy:

Because of the 'spin-out' effect when turning corners, the wind speed can be:

  • Up to 20% higher than the isobar spacing would suggest as the air turns around (and out of) a High
  • As much as 20-40% lighter than the isobar spacing would suggest as the air turns around (and into) a Low.

Note that the difference in wind speed between Auckland and Fiji (about 20 degrees of latitude) would actually be greater than the roughly 20 degrees of latitude between Greenland and Ireland. This is because the relationship is non-linear. However, I still think it may be sufficient to explain much of the difference you are seeing.

This page has a written example, though he's dealing with winds at 500hPa the idea holds:

http://bangladeshtornadoes.org/geostropapx.html

For Winnipeg, Manitoba (50N): Ug = -13.4/(-.766)= 17.5m/s = 34kts

For Minneapolis, MN(45N): Ug= -13.4/(-.707)= 19m/s = 36.8kts

For Amarillo, TX (35N): Ug = -13.4/(-.573)=23.4m/s = 45.4kts

For Dhaka, Bangladesh(23.8N): Ug = -13.4/(-.403)=33.25m/s = 64.6kts

You can see from this the major corrections for latitude between a borderline tropical location and a mid/high latitude location, using the same geostrophic wind for each location. Also note the non-linear relationship. The jump between 35N and 24N is massive compared with the jump from 45N to 35N.

There's so many factors that affect wind speed. We have to consider all sorts of issues, and also the weaknesses of the geostrophic approximation, as well as whether the isobars curve cyclonically or anticyclonically, and thermal effects (notably the katabatic).

Another issue is the isallobaric effect. When a low deepens rapidly, there is an additional component of the wind to want to blow in the direction of the increasing pressure gradient due to deepening of the low.

Finally, even though the latitude is pretty high, Antarctica manages to overcome this. It's the windiest continent on earth.

Just always so much going on, when the wind blows.

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Nick gave some reasons, but I think the a main issue is latitude.

...

Just always so much going on, when the wind blows.

Thanks for your really helpful reply! That makes a lot of sense. So, in general, you need tigher isobars to produce the same wind speeds at higher latitudes.

I found another site here with some details: http://www.tpub.com/weather2/7-9.htm

And now I know what the Geostrophic wind scale is for on the synoptic charts! http://gliding.eusu.ed.ac.uk/met/syn.jpg

What I'm still not quite sure about now is why latitude affects the wind speed. I've been trying to find out online and in "Atmosphere, Weather and Climate" but it seems extremely complex and I can't find a definitive answer! What is it that makes winds less strong at higher latitudes for a given pressure gradient?

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Thanks for your really helpful reply! That makes a lot of sense. So, in general, you need tigher isobars to produce the same wind speeds at higher latitudes.

I found another site here with some details: http://www.tpub.com/weather2/7-9.htm

And now I know what the Geostrophic wind scale is for on the synoptic charts! http://gliding.eusu.....uk/met/syn.jpg

What I'm still not quite sure about now is why latitude affects the wind speed. I've been trying to find out online and in "Atmosphere, Weather and Climate" but it seems extremely complex and I can't find a definitive answer! What is it that makes winds less strong at higher latitudes for a given pressure gradient?

I'll try and come back on that in a bit but the Fax chart you show nicely illustrates how for a given width apart of isobars, the further south you the stronger the 'gradient' is likely to be.

Remember that the Geostrophic scale is not the surface wind, its the anticipated wind just above the friction layer normally taken as 2,000ft above sea level. The surface wind is backed from (anti-clockwise from) and less than the 2000ft Geostrophic wind.

I'd best tell you that you have picked on a very complex item, not that many in meteorology are not complex.

this first class weather site may give you some insight into a variety of items; most of its input was originally from Martin Rowley ex senior forecaster in UK Met-hope it helps anyway

http://weatherfaqs.org.uk/

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I’m not sure I can explain it simply but I’ll try.

Assume a particle of air moving, at say 20 degrees north and another one at 60 degrees north. The angle that both subtend to the centre of the earth are obviously different, that at 60 is greater than that at 20 north.

Due to the earths’ rotation any air is given an acceleration that depends on its distance from the equator. Wind strength is dependent on a number of factors, the initial speed of the air, the effect of Coriolis Force, the force imparted due to the earths spin and where on its surface that air is.

Air tries to flow from high to low pressure and as soon as it does the above begins to act on it.

Another force called the Pressure Gradient Force also comes into play. As the Fax chart shows we can, without going into the complex mathematics and it shows clearly that a for similar isobar spacing that the wind strength is greater the further south one is.

Greenland is, or most of it is, well above seal level, much around 500ft with thick ice, so trying to work out what wind in any synoptic set up is even more complex along with katabatic winds also occurring as air stream off the ice.

Not a very good explanation but its very complex. Someone who has recently, well a lot more recent than 1971 when I did, a meteorology degree course may do it better.

If you can get your local library to send off for

Essential of Meteorology by DH McIntosh and AS Thom, it was in the Wykeham Science series, I got a copy, 2nd hand, off the web, it is an excellent book with diagrams and good explanations.

Again hope the above helps.

Anyone who spots errors please correct.

Thanks

jh

ps

a quick search showed this

http://www.amazon.co.uk/0851090400-Books/s?ie=UTF8&keywords=0851090400&rh=n%3A266239%2Ck%3A0851090400&page=1

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Thanks John and J07 for the more definitive feedback, 03jtrickey's initial query certainly had me stumped. But looks like your explanations have gone some way in explaning what seems a rather complex relationship between the synoptic patterns and what actually happens with regards to winds, temps etc, in these northerly areas up within the arctic circle. I had an idea of the katabatic processes over the icy interior of Greenland which creates the strong winds blowing out from the interior there. Certainly hadn't considered the effect of latitude when considering wind strength based on isobars.

Amazing how much there's still to learn in the world in weather, just when you think you've grasped alot already!

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I’m not sure I can explain it simply but I’ll try.

Assume a particle of air moving, at say 20 degrees north and another one at 60 degrees north. The angle that both subtend to the centre of the earth are obviously different, that at 60 is greater than that at 20 north.

Due to the earths’ rotation any air is given an acceleration that depends on its distance from the equator. Wind strength is dependent on a number of factors, the initial speed of the air, the effect of Coriolis Force, the force imparted due to the earths spin and where on its surface that air is.

Air tries to flow from high to low pressure and as soon as it does the above begins to act on it.

Another force called the Pressure Gradient Force also comes into play. As the Fax chart shows we can, without going into the complex mathematics and it shows clearly that a for similar isobar spacing that the wind strength is greater the further south one is.

Greenland is, or most of it is, well above seal level, much around 500ft with thick ice, so trying to work out what wind in any synoptic set up is even more complex along with katabatic winds also occurring as air stream off the ice.

Not a very good explanation but its very complex. Someone who has recently, well a lot more recent than 1971 when I did, a meteorology degree course may do it better.

If you can get your local library to send off for

Essential of Meteorology by DH McIntosh and AS Thom, it was in the Wykeham Science series, I got a copy, 2nd hand, off the web, it is an excellent book with diagrams and good explanations.

Again hope the above helps.

Anyone who spots errors please correct.

Thanks

jh

ps

a quick search showed this

http://www.amazon.co.uk/0851090400-Books/s?ie=UTF8&keywords=0851090400&rh=n%3A266239%2Ck%3A0851090400&page=1

Yes thanks for your detailed feedback, John. It certainly seems a very difficult subject, but now I have at least a reasonable explanation, without going into the detailed science and maths! With all those parameters consider it's incredible that computers can model and predict the weather. I'll see if can get a hold of that book sometime.

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Thanks John and J07 for the more definitive feedback, 03jtrickey's initial query certainly had me stumped. But looks like your explanations have gone some way in explaning what seems a rather complex relationship between the synoptic patterns and what actually happens with regards to winds, temps etc, in these northerly areas up within the arctic circle. I had an idea of the katabatic processes over the icy interior of Greenland which creates the strong winds blowing out from the interior there. Certainly hadn't considered the effect of latitude when considering wind strength based on isobars.

Amazing how much there's still to learn in the world in weather, just when you think you've grasped alot already!

we all learn something new each day Nick so don't worry about that.

as to the complexities of wind flow-I actually opened one of my theory books before answering it and shuddered at the memories it brought back. But that Mac and Thom book is well wort trying to get hold of a copy. I used it before my interview to get from observer to forecast grade-invaluable!

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