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whitestuff

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Posted
  • Location: Rowley Regis
  • Location: Rowley Regis

    When looking at the 18Z run yesterday I tracked a high pressure system that I think is called the azures high cross the atlantic and end up over the British Isles at the end of the run.

    Looking at the 00z run this morning, the high pressure is at least 500 miles further south at the end of the run.

    Why is there so much difference in the two runs and is the data used from a different source?

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    Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL
    When looking at the 18Z run yesterday I tracked a high pressure system that I think is called the azures high cross the atlantic and end up over the British Isles at the end of the run.

    Looking at the 00z run this morning, the high pressure is at least 500 miles further south at the end of the run.

    Why is there so much difference in the two runs and is the data used from a different source?

    I don't know the specifics from a meteorological point of view, but the atmosphere is a highly non-linear system. In fact, pretty much everything in the world is non-linear, but sometimes we can approximate them as being linear. For example, a simple pendulum swinging is non-linear, but due to a fortunate "trick" it can be approximated as a linear system and that model works pretty well.

    When it comes to the atmosphere, everything is interdependent, basically meaning it's a highly non-linear dynamical system. And it gets even worse. It can't be approximated linearly (please someone correct me if I'm wrong). And also it exhibits a phenomenon known as "chaos". Along with a few other tedious mathematical requirements, this means that a small change in initial conditions can lead to large, and unpredictable results.

    I wish I could draw a picture to demonstrate this, but essentially, if you imagine a graph which is a straight, diagonal line, that is linear. Now, on the far left side of the graph, imagine that it starts just a little bit higher up. In a truly linear system, you will end up with an identical, parallel line which is just a small bit above the original one.

    However, for a non-linear case, this very small initial perturbation will not produce a predictable result. It can set the resulting curve plunging low, oscillating down there is seemingly random fashion, then bursting up above the original line, oscillating for a bit more maybe, then going down some more, getting stuck in another pattern, then bursting out of that one. And so on.

    The other problem is that non-linear systems can very rarely be solved analytically; this means that numerical methods are used. Essentially, this means that the "solution" to the initial problem is still just an approximation in itself, based upon some numerical iteration worked out by a computer.

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    Posted
  • Location: Ashford, Kent
  • Weather Preferences: Anything
  • Location: Ashford, Kent
    When looking at the 18Z run yesterday I tracked a high pressure system that I think is called the azures high cross the atlantic and end up over the British Isles at the end of the run.

    Looking at the 00z run this morning, the high pressure is at least 500 miles further south at the end of the run.

    Why is there so much difference in the two runs and is the data used from a different source?

    Hi there Whitestuff!

    Good to have you posting. I think you will find that the name is the Azores High (hi howyadoin? :) ). I'm not really an expert but I think that the data input for each run is taken from many different sources, weather balloons, ground stations, bouys, etc. The Idea is to get as many readings as possible at the same time, the computer does lots of smart things with some bells and whistles and predicts the situation 6 hours ahead, it then feeds the results into itself again and continues to expand on it's predictions from there onwards. The flaw with this method is that any errors with the original source data are extrapolated and become larger errors over time.

    To this end there are several runs done with slight built in errors to account for differences in source data. These are called ensembles.

    Also different runs have different sources of data, which is why some runs differ slightly from the run before.

    Someone else will be able to explain it a whole lot better I'm sure but in the meantime check out the learners area. I'm sure there will be some guidance there.

    Couple of links to get you started.....

    Data sources

    Weather computers

    Hope this helps

    AH

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    Posted
  • Location: 115meters ASL, Andover,Hampshire
  • Location: 115meters ASL, Andover,Hampshire

    Also i think you will find that the input infomation can be from different sources on each run I.E 6z may run with Bouys and Ballons 12 may run with aircraft infomation and bouys and ballons

    i am sure a more knowledgable poster will fill in the detale

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    • 3 weeks later...
    Posted
  • Location: Upton, Wirral (44m ASL)
  • Location: Upton, Wirral (44m ASL)

    On a similar topic so I posted in this thread. Does anyone know how much historical data is input and/or retained in each new forecasting run? Also any idea on how often the "crunching" program is updated/changed to reflect errors between what was predicted and what actually happened? Finally, is it valid to compare like for like when runs have been generated using different versions of the algorithms?

    Just curious really - have been fascinated by the fact that computers models are used to predict atmospheric conditions for years but never realised that there was so much info available on the subject until I found this site! If you read the BBC website about how they do the monthly outlook it really only gives the impression that they use historical data and hence a LR forecast based on the probability or likelyhood of what has gone before happening again based on the current prevailing conditions, time of year etc etc.

    Great site - keep up the good work

    Wysi :)

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