Learning a bit of Meteorology

[franksingleton]

I appreciate the effort Frank, and if I understand you correctly, you're saying that upper level 'pertubations' can lead to (or at least aid in) the development of a low at the surface. I fully accept that, but that doesn't change the fact that the Jetstream is where it is, because it is above the boundary between two adjacent airmasses. If I've got it wrong, perhaps you could explain how the Jetstream moves the UK's typical Polar Maritime airmass down to Portugal?

This may or may not help.

Take a look at http://www.netweather.tv/index.cgi?action=jetstream;sess=. This is output direct from the GFS for the next two weeks. OK, I have grave doubts about its validity after 7 days but it will illustrate my point – I hope...

In the current situation, the jet stream is predominately strongly west to east. Surface weather systems will run quickly west to east. For much of the time the core of the jet is over the UK. Then, about 27 December the jet has worked itself south of Britain. A ridge of high pressure is starting to develop over the off the eastern sea board in association with a ridge in the jet stream.

By midday on the 28t the surface high pressure covers much of mid Atlantic. The high has strong southerly winds on its western side pushing warm air northwards. Northerly winds on the eastern side of the high push cold air southwards. The net result is an increase in the amplitude of the oscillation of the jet stream. That heads down towards the Iberian Peninsula giving a cold spell over Portugal.

On this forecast by the jet plunges as far south as North Africa. If the forecast is right the end of the year will be nasty and cold over the western med. By that time, over the Atlantic the jet is again main west to east with no great oscillation. Lows will again be running quickly west to east.

Which is cause and which is effect in the above is impossible to say. The development of the surface high occurs as the jets stream develops a marked oscillation.

Just looking at the jet stream forecast I can see roughly what the surface patterns will be. BUT I would not produce a good forecast chart. The computer model can do the sums that no human can do and produce mutually compatible upper air and surface level charts.

Whether this forecast is going to be good I cannot say. However, the point that I am trying to make is the interaction between all levels and all parts of the atmosphere. A phrase that I trot out from time to time is that “to know about the weather somewhere, you have to know about the weather everywhere.” That applies in 3D.

PS If anyone wonders where they can see forecast surface isobars out the 14 days, there are two sources that I know. One is the SailGrib app’ the other is Saildocs got by using email. There is another source but I cannot now remember where.

 

[Cruiser2B]

On this forecast by the jet plunges as far south as North Africa. If the forecast is right the end of the year will be nasty and cold over the western med. By that time, over the Atlantic the jet is again main west to east with no great oscillation. Lows will again be running quickly west to east.

Which is cause and which is effect in the above is impossible to say. The development of the surface high occurs as the jets stream develops a marked oscillation.

When you say it's impossible to say which is cause and which is effect, you're not convincing me the Jetstream does the driving. In your example above, it's not the jet moving the airmass south to Africa, but rather the airmass moving south and dragging the jet along with it. This airmass would likely be a very cold (and dense) Continental Polar from northern Canada - that's also responsible for building the insanely strong jet over North America. It moderates over the Atlantic, warming up, drawing up moisture and losing that density and leading to convective development.

 

[franksingleton]

…………………..
you're not convincing me the Jetstream does the driving. In your example above, it's not the jet moving the airmass south to Africa, but rather the airmass moving south and dragging the jet along with it.
……………

The mistake you are making is to try to separate out what happens at jet stream levels and what happens elsewhere. Those who talk about the importance of the jet stream do the same. You have to look at how the jet stream causes changes in pressure patterns low down as much as how what happens low down affects the jet stream. You have to look at the whole system.

Because the whole is so convoluted meteorologists too often try to simplify explanations or “scientific” journalists try to interpret meteorological speak into man on the Clapham omnibus speak. Such over simplifications just lead to categorical statements that fall apart when scrutinised – as you are trying to do here.

It was the impossibility of trying to separate out chicken from egg that really made the subjective approach to forecasting so difficult and, effectively, impossible. That is why I advise sailors not to try to use half understood ideas about jet streams or any other such ideas. They can do no better than use the output from numerical weather prediction computers.

NWP models use the best theory available, the best data analyses possible to produce the best forecasts possible. Note the order of the words. These are not the best possible forecasts. There will always be some errors. Occasionally the errors will be in the large scale development; more usually they will be in small scale detail. That is why some human input is usually required to get the best out of forecasts. Either your own or, if you want professional advice, someone like Simon – but NOT purely automated services despite their over egged claims for accuracy, precision etc.

That, really, is the message that I try to give in http://weather.mailasail.com/w/uploads/Franks-Weather/bookcover.pdf

 

[Babylon]

Whats the best way to do this?

I've done the RYA courses, I've looked at a few websites but I'm still a bit confused.

Recommendations would be welcome for good weather books to read etc.

(Or maybe being confused is something I just have to accept)

Have you made your choice yet?

 

[KellysEye]

>You're still confusing cause and effect. The dynamic boundary between polar and warmer temperate air masses causes the lows to go where they go. The same boundary determines the route of the jet stream. Hence your are wise and prudent to acquaint yourself with forecasts of jet stream location, but not precisely for the reason you suppose.

The great majority of people, including me, are not interested in details such as polar and warmer temperate masses or the cause of highs, storms or hurricanes. All we want to know is what weather we are going to get and how good or bad it is and which way it is moving. The Jetstream forecast and a synoptic chart forecast tell you that as I said in the beginning. I can understand why forecasters and weather geeks use things such as polar and warmer temperate air masses to help build the overall detail forecast. In other words the majority are interested effect not cause and we don't confuse the two because cause and effect is a well known phrase.

 

[Cruiser2B]

The mistake you are making is to try to separate out what happens at jet stream levels and what happens elsewhere. Those who talk about the importance of the jet stream do the same. You have to look at how the jet stream causes changes in pressure patterns low down as much as how what happens low down affects the jet stream. You have to look at the whole system.

Au contraire Frank. I understand there is a complex interplay between the various parts of the atmosphere and the external stimuli, that create what we mere mortals call "weather." To me though, the Jetstream is separate from the troposphere; sure there can be a heat exchange between the two, and the jet can be affected by variations in the trop, or convective effects that extend beyond the trop, and as you've pointed out a high-level ripple can grow to be a surface low; but that is as far as I see the connection. I've always felt I've had a good handle on how the Jetstream is formed and what it does. When Simon said that the Jetstream can steer surface lows (a concept that contradicts my understanding) I wanted to know what he meant. I want to know the mechanics of the interaction or at the very least, the theory behind it.

 

[franksingleton]

………….
To me though, the Jetstream is separate from the troposphere;
………..

Sorry, Cruiser2B, The jet stream is part of the troposphere. You (at least, I) cannot separate them or think of them as distinct entities. I say, again, that you have to think about the whole atmosphere. In fact, the models now take stratospheric effects into account. After all, roughly 10% of the mass of the atmosphere is above the tropopause near the equator and 30% or more near the poles.

The idea that lows are steered by the jet stream is really one of the rules of thumb that we used to use ore-computer. If I did not have a computer model available, I would use the same idea. Pre-dating the jet steering concept, we used to say that lows move in the direction of the warm sector isobars.

See http://weather.mailasail.com/w/uploads/Franks-Weather/wave-depression.png, from my book although a similar diagram has no doubt appeared in other books. This is schematic of the relationship between jet streams and frontal lows.

A frontal wave starts to form in association with a small ripple in the jet stream. Don’t ask me which comes first. I cannot say. As the jets stream ripple moves along the jet, so does the surface low. As the low deepens due to a combination of dynamical and latent heat effects, the jet stream becomes increasingly distorted. This is a two way process. As the jet stream becomes increasingly distorted the low usually turns polewards.

In my book, I take the same line as KellysEye. Don’t worry over much about the theory. Take the forecasts from the professionals as your best guidance. Concentrate on knowing what they say in synoptic chart and text form. Apply your own knowledge and experience for local, short term weather (or employ Simon). My only difference from KellysEye is that I would not bother too much with jet stream charts. That information, along with everything else about the atmosphere that can be measured and modelled will be built into the output. Who am I to think that I can do any better?

 

[Fascadale]

Whats the best way to do this?

I've done the RYA courses, I've looked at a few websites but I'm still a bit confused.

Recommendations would be welcome for good weather books to read etc.

(Or maybe being confused is something I just have to accept)

Have you made your choice yet?

No decision as yet but I am enjoying the thread.

Many thanks for all the contributions and suggestions.

There was some truly spectacular weather to watch from the north coast of Ardnamurchan over the last weekend: made all the more enjoyable by my tiny understanding of what was going on.

 

[franksingleton]

No decision as yet but I am enjoying the thread.

Many thanks for all the contributions and suggestions.

There was some truly spectacular weather to watch from the north coast of Ardnamurchan over the last weekend: made all the more enjoyable by my tiny understanding of what was going on.

A measure of the improvement in forecasting has been the warnings given for the current windy and rainy period. Similarly, the October storms over southern England were warned about 6 or 7 days ahead. Also the weather that led to the North Sea tidal surges. In recent years, snow, that most difficult weather for the UK, has also been well forecast. As a former Met Office senior forecaster, 10 years in the job, I cannot help but be very impressed.

 

[Cruiser2B]

The idea that lows are steered by the jet stream is really one of the rules of thumb that we used to use ore-computer.

When I looked at upper air charts the rule of thumb was that the path of the low could be predicted, but I never heard that the jet stream steered the low.

A frontal wave starts to form in association with a small ripple in the jet stream. Don’t ask me which comes first. I cannot say.

You have a tendency to speak in riddles Frank. If you can't say which comes first, how is it you can say the jet stream steers the low?

As the jets stream ripple moves along the jet, so does the surface low. As the low deepens due to a combination of dynamical and latent heat effects, the jet stream becomes increasingly distorted. This is a two way process. As the jet stream becomes increasingly distorted the low usually turns polewards.

Or we could look at it being that Jetstreams form over the boundary between two airmasses. Coincidentally, this is where frontal lows also form - "frontal" of course refers to the boundary between two airmasses. As the low 'sucks' the warm air into the cold in the familiar "shark fin" shape, that boundary develops an acute bend - which you refer to as a distortion in the Jetstream. Would you not agree that at these "distortions" the jet often weakens, dissipates, or splits and even after it's gone the low could carry on along the boundary until it occludes?

 

[franksingleton]

When I looked at upper air charts the rule of thumb was that the path of the low could be predicted, but I never heard that the jet stream steered the low.

It seems that my background, length and depth of experience in meteorology somewhat exceeds yours.

You have a tendency to speak in riddles Frank. If you can't say which comes first, how is it you can say the jet stream steers the low?

I do not speak in riddles. Observations do no show which occurs first, the frontal wave depression at the surface or the ripple in the jet stream. That is because they occur in conjunction.

Au contraire, you are unwilling to think of the atmosphere as an entity rather than being composed of distinct parts. That colours your thinking.

NWP models seek to simulate the atmosphere. Of course, they cannot and will not do so precisely because the computer power is infinitesimal compared to the atmosphere.

However, models do look at the entire atmosphere. It is significant that early ECMWF models used to calculate at 16 levels up to the lower stratosphere. They now do so at 137 levels up to about 80 km. That is in order to improve the calculation of how each level interacts with the rest. The more levels and the finer the horizontal grid, the better will the models simulate the real atmosphere. The improvements since the early 1980s is a clear demonstration of the effect of increased resolution.

 

[Cruiser2B]

It seems that my background, length and depth of experience in meteorology somewhat exceeds yours.

Well then rather than bragging about it, why don't you just answer the question.
Models all have to work on certain assumptions - if a model assumes that the Jetstream has a steering effect on the airmass below it, it is only because it has been programmed to assume that. One would think the programmer(s) would have had to have an inkling of how that process works in order to tell the model what to do. I just want to know by what means it is assumed that the Jetstream moves a giant lobe of a Polar airmass hundreds or thousands of miles south, for instance?
I'm not sure how you got the idea that I don't see the atmosphere as an entity? Even as an entity, it can have moving parts that may or may not have an effect on the other moving parts. A complex machine has many cogs - if one presumes to understand the machine, then one should be able to say how the movement of one particular cog affects the movement of another particular cog.

 

[franksingleton]

Well then rather than bragging about it, why don't you just answer the question.

If I am not answering your questions it because you are asking the wrong questions. You seem not to be prepared try to understand how the atmosphere works,

Models all have to work on certain assumptions - if a model assumes that the Jet stream has a steering effect on the airmassuu below it, it is only because it has been programmed to assume that.

This shows that you do not understand how numerical weather prediction models work.

One would think the programmer(s) would have had to have an inkling of how that process works in order to tell the model what to do. I just want to know by what means it is assumed that the Jetstream moves a giant lobe of a Polar airmass hundreds or thousands of miles south, for instance?

Whatever you might think, models do not have such built-in statistical relationships. They use the equations of fluid-dynamics and thermo-dynamics as applied to the atmosphere. Basically, these deal with

• The Navier-Stokes equations ie the equations that relates forces to accelerations ins a fluid
• Conservation of mass – equations of continuity.
• Conservation of water
• Thermodynamic equations
• Boyle’s law

Of course, these take into account topographic forcing, calculations of heat transfer due to convection, conduction, radiation and latent heat.

Your use of the word “programmers” has a pejorative ring. The formulations of the equations, the way that they handle all the various heat inputs, analyse the data etc are handled by some very clever mathematical physicists. The less demanding scientifically but still vital task of coding into computer code is done by “programmers.”

I'm not sure how you got the idea that I don't see the atmosphere as an entity? Even as an entity, it can have moving parts that may or may not have an effect on the other moving parts. A complex machine has many cogs - if one presumes to understand the machine, then one should be able to say how the movement of one particular cog affects the movement of another particular cog.

This is where you hoist yourself with your own petard. To try to draw an analogue between a machine and a fluid is nonsense. I really do have to suggest that you try putting numerical Weather Prediction into Wikipedia.

 

[Cruiser2B]

Frank - the question was very simple: By what physical process(es) does the Jetstream 'steer' a low, or how does a Jetstream move a frontal boundary?

The question wasn't actually directed at you, and as you seem incapable of answering it, please feel free to not respond.
I do wonder if your responses give a glimpse of what one could expect to read in your book. Do you gaze down from your ivory tower, sneering haughtily at those who are not meteorologists? "Gaze not upon the sacred runes, peons! The seers will give you the forecast; that's all you need to know. The model knows all."

 

[franksingleton]

Frank - the question was very simple: By what physical process(es) does the Jetstream 'steer' a low, or how does a Jetstream move a frontal boundary?
The question wasn't actually directed at you, and as you seem incapable of answering it, please feel free to not respond.

Your original query was about steering of lows by the jet-stream. I tried to explain that that was an old rule of thumb based on experience and dated back from the 1950s. In those pre-computer days forecasters could only use empirical rules. That was one of several

The point that I have been trying to make is that there is no direct answer to your question. An answer would have to explain the detailed physical processes that occur through the whole depth of the atmosphere. I could not do that. Anyone who attempted to do so would be writing a major thesis.


Simon did not reply to you, nor has he commented or objected to anything that I have said.

I do wonder if your responses give a glimpse of what one could expect to read in your book.

As regards my book, as I said in an earlier post, I have tried to avoid theory as far as possible. I concentrate on the practical aspects of using forecasts. My stance is that the user should take forecasts produced by professional organisations and add value through expeminence and commonsense. Knowing about the jet stream does not help me as a sailor and I do not suggest that the sailor should try to apply such ideas.

Do you gaze down from your ivory tower, sneering haughtily at those who are not meteorologists? "Gaze not upon the sacred runes, peons! The seers will give you the forecast; that's all you need to know. The model knows all."

You are little short of being insulting. I do not look down or sneer at those whose knowledge of meteorology is not as deep as mine. There are many who know far more than I do. There are many subjects where I know little.

When I see incorrect statements being made or questions asked, I try to respond on the basis of fact. You made some assertions that were incorrect. I stated facts. If I made incprrect statements on a subject about which I was not an expert, I would not and could not object to being corrected. I might query an expert. I would not have the temerity to refute his statements or accuse him/her of talking down to me.

 

[Cruiser2B]

I do not look down or sneer at those whose knowledge of meteorology is not as deep as mine.

Oh really!?!?

If I am not answering your questions it because you are asking the wrong questions. You seem not to be prepared try to understand how the atmosphere works,

This shows that you do not understand how numerical weather prediction models work.

This is where you hoist yourself with your own petard. To try to draw an analogue between a machine and a fluid is nonsense. I really do have to suggest that you try putting numerical Weather Prediction into Wikipedia

It seems that my background, length and depth of experience in meteorology somewhat exceeds yours.

I do not object to being corrected. I do object to being insulted. I did query an expert, but got you. I tried to make the best of it, but you failed to explain to me how an expert can say 'the Jetstream steers surface lows' if it is instead a complex interplay between innumerable factors and we don't really know what drives what.

If it's just a rule of thumb from "back in the day", then it is as I said, the Jetstream is an indicator of the low's path; it does not necessarily steer the low.
If the Jetstream is thought to steer the low, then an expert should be able to explain the basic physical process, just as the rest of the physical processes in meteorology are "dumbed down" for us simple sailors. There's no need to go into numerical weather prediction or how little I know about it.

 

[franksingleton]

Cruiser2B. I am clearly wasting my time.

 

[Cruiser2B]

franksingleton. We have something in common; you are wasting my time, too.

 

[GHA]

Cruiser2B. I am clearly wasting my time.

Hope I speak for many, but many thanks for actually taking the time to post on here. Someone who actually knows what he is talking about and takes the time to share is a very rare pearl on an Internet forum. Please feel appreciated and keep it up

 

[franksingleton]

Hope I speak for many, but many thanks for actually taking the time to post on here. Someone who actually knows what he is talking about and takes the time to share is a very rare pearl on an Internet forum. Please feel appreciated and keep it up

Thank you for that. I do, actually like a forthright discussion. There are many problems in trying to explain weather to lay people whether they are scientists or not. Largely that is because meteorology is both fascinating and frustrating. There are few absolutes and few, if any, “rules” that are of any real use to a sailor. That is contrary to what many think or would like to think.

The processes that drive the atmosphere are, basically, old fashioned classical physics of the heat, light, sound variety. They should be understandable to anyone capable of understanding UK 6th form or US High School Senior Grade physics. The difficulty comes in understanding and, even more, trying to explain the many and diverse interactions that take place. I cannot claim to understand them all, let alone explain them.

It really comes down to Donald Rumsfeld’s known knowns, there are a few of those; his known unknowns, there are many of those; and his unknown unknowns, maybe there are still many of those. After nearly 60 years experience in meteorology, I have come to the conclusion that the more that I know, the more I realise how much there is yet to know.

I will probably be accused of talking in riddles, again, but I hope that you get the message.