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Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

    Hi all. In relation to the discussion on troughs, I have re-read the following link WHAT GOES ON ALOFT?

    In the section on diffluence and confluence, it states that both troughs and ridges in long wave systems can be either diffluent (air entering at speed and slowing as it leaves) or confluent (slow entry, quick exit). This I understand. However, it then talks about jet streaks, which sound rather like diffluence, but it describes a northwards angling of air on entry to the streak due to pressure difference and a southward angling on exit due to the Coriolis force, which, according to their diagram, means that every jet streak has both right entrance and right exit as I understand it. Yet this suggests confluence, not diffluence, and therefore -ve vorticity. To top it all, it then starts referring to left entry which (to my understanding) it has just described as not happening.

    As you can see, I'm confused. To me, the page seems to contradict itself. Does the air flow into a jetstreak always enter left or right, and does it always exit left or right? Or can it vary between jetstreaks? I can understand that diffluence would generate +ve vorticity and therefore cyclogenesis and low pressure formation, and that confluence would generate -ve vorticity. What I don't get is how both diffluence and confluence are defined by what the air does both in entry and exit, yet entry can be both diffluent or confluent and so can exit. Aaargh!wallbash.gifhelp.gif

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    Not at all straightforward but when I get time I'll try and explain as best I can, unless someone wishes to do it before me-please?

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    Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

    Thanks for replying John. I think I may have missed a vital point in there somewhere to do with convergence being at the base of +ve vorticty and divergence being at the top, creating low pressure. Having said that, it still doesn't make much sense to me.doh.gif

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    Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

    OK, to help anybody brave enough to try to get the theory of this into my thick skull, let me summarise what I think I know:

    1. Long-wave features are caused by the polar jet swinging north to form ridge of warm air and south to form a trough of cold air.

    2. Ridges are associated with higher heights; troughs with lower heights. This is due to fact that warm air rises.

    3. At a trough, the acceleration of the jet steam around the base of the cold polar air lowers the pressure slightly along with the fact that the cold air mass is thinner and therefore exerts lower pressure than warm air. In addition, the curve of the jetstream establishes positive (anticlockwise/cyclonic) vorticity. This vorticity, together with the lowering of pressure, establishes a spinning column of air, generating a vacuum, thus causing air to flood in at lower levels to fill that vacuum. If the rate at which the air at upper levels diverges is greater than the rate at which the air flooding in at the base converges, then the pressure will drop, so the low deepens; if the rate of convergence at the base exceeds the rate of divergence at the top, the pressure rises and the low is filled.

    4. As the polar front dips southwards at the trough, just ahead of that trough, a warm front is established as the vorticity of the low builds and warm air is dragged upwards from the south. This generates a warm sector, behind which is a cold front which delineates the point at which the warm air is followed by colder air from the north.

    5. Due to the fact that the warm air at the warm front tends to ride up over the colder air, convection is limited, and precipitation tends to be light. Conversely, at the cold front, cold air undercuts warm air and the rising warm air generates convection - thunder and heavy precipitation associated with cumulo- and stratonimbus clouds are commonly observed.

    6. Presumably, a warm front must then pass over after the depression at the point where the cold air associated with the trough gives way to the warm air of the next ridge.

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    hi Chris

    This is not an answer to the various questions you raise but an observation. Please don't think I'm having a dig I'm not. But I do think you are trying to run before you can walk - in terms of meteorology.

    I would, if I were you, forget for a while vorticity and concentrate on more basic elements. Yes, vorticity is a key but there are other things which perhaps are easier to understand and come before trying to understand the bigger picture of the major troughs and ridges.

    I'm not blowing my own trumpet but have a look in our Guides at, first of all air masses, then read the one on the Polar Front. I'm known for trying to talk as basic as possible in the belief if anyone starts at total basic understanding then they can gradually build up to understanding more complex issues. If you don't fancy reading NW Guides then try the Met O site for their basic weather understanding and there are other sites that will help. Just 'Google' north Atlantic weather systems

    polar front weather systems

    or such similar headings.

    I'm sure it will help you get a better basic understanding-don't get me wrong, you obviously have a pretty good idea of the bigger picture already.

    As I said please don't feel I'm having a go I am trying hard to help. The questions you are asking will I am sure help others who want to learn about meteorology as well. Also others, as Brick has already done, will drop in and give their ideas.

    Anyway please keep asking and I promise to come back with some ideas on confluence and divergence in both ridges and troughs.

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    try this Chris or anyone regarding a basic idea about the large scale troughs-ridges-vorticity

    its on its way as a pdf file

    Jet streams or Jet streaks-5 Mar 10.pdf

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    Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

    Thanks John - I understand the polar front and the formation of warm and cold fronts as I've read, re-read and inwardly digested your guides. What I am struggling with is reconciling the pattern of cool air, followed by a warm sector, then a cold front bringing in cold air behind, with the longwave pattern of a trough being a lobe of cold polar air being brought south by a southward lobe of the jet stream.

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    Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

    Also, John, just a quick question about your explanatory PDF file - are you saying that most streaks lead to the formation of both high and low pressures or that the pattern you describe is only if the flow is perfectly symmetrical? If that is the case, does any asymmetry in the flow lead to the formation of either a high or low in the jet streak?

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    I'll have a go at answeing your supplemtaries tomorrow Chris- well later today as its 0005.

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    I'm in the process of trying to make a reply to the two questions posed by Chris.

    I hope what has been explained so far is a help to some reading it and equally that further explanations will also help.

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    Below as a pdf is my attempt to answer the two questions posed by Chris.

    Please do ask questions and I'll try and answer them or suggest a link which does.

    Jet Streaks and replies part 2 sun7 mar10.pdf

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    good-its very complex so I try to simplify it as much as I can then we can grade it up once anyone feels comfortable with that particular aspect.

    There is a plan, no more than that, eventually, for me to do a suite of technical tutorials dealing with what is already available in NW Guides with new professional diagrams along with actual charts and me doing a voice over.

    gawd knows when/if it will come to fruition but its a plan both with Paul and myself.

    There will then be another video link within the NW video area, not just for my stuff but also inputs from other forecast members of the team-storms-long range etc?

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    Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

    Sounds good John. Just one final quick question - you explained that a convergent trough can give rise to a high pressure area - so can divergence at a ridge form a low-pressure area?

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    Sounds good John. Just one final quick question - you explained that a convergent trough can give rise to a high pressure area - so can divergence at a ridge form a low-pressure area?

    yet another simple question Chris and its a much more complex answer.

    I'm just about to start and do a full explanation of trough-ridges-diffluent/convergent and what each will produce.

    I may have it done later today but its about to be started?

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    Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

    Thanks again John. I see now why prediction low or high pressure formation is so tricky!

    Just to make certain that I understand. A surface low will start forming in a confluent ridge behind (to the west of) a trough, then it will deepen in the left exit of the dffluent trough as the streak moves along the lobes of the stream. Therefore, the low will be found just ahead of the trough, explaining how the pattern of warm front, warm sector and cold front forms, with the cold front being at the leading edge of the polar air brought south in the trough lobe. Conversely, high pressures develop in diffluent ridges and mature in confluent troughs, but (for a reason I don't think you mentioned - probably just as well for my and your sanity!) they are found behind a trough, not ahead of it.

    Is that a correct understanding?

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    yep, about right Chris

    I'll get round to talking about highs, be they formed in a similar fashion as outlined in my pdf above or the more 'robust' type which we have seen quite a lot of this season 2009-10, but not sure when that will be.

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    Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

    yep, about right Chris

    I'll get round to talking about highs, be they formed in a similar fashion as outlined in my pdf above or the more 'robust' type which we have seen quite a lot of this season 2009-10, but not sure when that will be.

    I think you deserve a rest after your sterling work teaching me this! :unsure: :good:

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    thanks Chris-it takes some time to put it together but I'm a weather nut like most on here so I enjoy doing this sort of thing when I have time.

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