In Depth Model Discussion, Analysis and Summaries

[summer blizzard]

The PNA is a reflection of surface pressure like the NAO, so straight away.

Models have now backed off the PNA change other than for a short time.

[snowlover2009]

Is the PNA going positive then? I assume that is the best thing, so introduce better heights, e.g. scandi high like you mentioned. If it continues negative, we will get a continuation of our current weather?

[Kentish Kiwi]

Ive been watching NOAA sst anomaly charts for the end of Sept/Oct and the latest available, the seas to our west should be cooling but are warming, a cool pool continues to develop way out accross the atlantic but what else I've noticed is that the seas around the southern tip of Greenland are indeed anomalously warm and continue to warm which is another positive point to note.

Would you mind explaining why this is a positive note? I note that the sea's surrounding southern Greenland were also anomalously warm at the end of last October and I take it this was a factor in promoting the intense and prolonged blocking in that area?

It's probably been explained many times in this thread or the SST one, but I don't really understand how/why warm anomalies such as that promote blocking... I would have thought increased warmth would lead to increased evaporation and therefore more rising air and lower pressure, in which case I was lead to believe that colder sea temperatures/negative anomalies would promote blocking. However this appears to be the opposite of the reality.

Is it because warmer seas in the north of the North Atlantic and cooler seas in the south lead to a smaller temperature contrast and therefore contribute to a weaker Jet Stream (by taking away the fuel that is the natural contrast in temperatures)?

Edited November 16, 2011 by Kentish Kiwi

[Snowmad79]

Haha I think your probably closer to the mark than I was!. Used to be under the impression that HP was stronger over or close to warm seas but as I've just found out, actually warmer seas weaken HP.

So what are we saying ? If cold sea anomalies are futher south in atlantic and Warmer seas encompass most of the northern part of the atlantic the Jet would either be weaker or tracking futher south due to lesser temp gradients ? which in turn allows any HP over Greenland to build southward even if it does appear to weaken it ?

[summer blizzard]

Both only partly right..

It is actually the couplet that is important.

If you have a warm anomaly above a cold anomaly then you get atmospheric divergence and high pressure (which is why we want this in the Atlantic). If you have a cold anomaly above a warm anomaly then you get convergence and a signal for low pressure.

In regards to the effect around Greenland, Greenland almost always has a surface high in place anyway which i assume nullifies the instability created by a warm anomaly.

At the moment i would say that the Atlantic is 50/50.

[Snowmad79]

Both only partly right..

It is actually the couplet that is important.

If you have a warm anomaly above a cold anomaly then you get atmospheric divergence and high pressure (which is why we want this in the Atlantic). If you have a cold anomaly above a warm anomaly then you get convergence and a signal for low pressure.

In regards to the effect around Greenland, Greenland almost always has a surface high in place anyway which i assume nullifies the instability created by a warm anomaly.

At the moment i would say that the Atlantic is 50/50.

I knew the current pattern over the atlantic was more favourable but now you have given me futher clarity on why, so thanks for that mate, much appreciated.

[sebastiaan1973]

Elsewhere I Found this interesting comment by Glacier Point. Due to the character of my question I put it in this more in depth topic.

That's the sort of timeframre we're dealing with here.

The modelling of the Pacific high is putting up a large, flat and somewhat southward displaced ridge. That's not conducive to a rapid upstream change displacing cold air out of the Arctic. What will happen as a result of this is that the Siberian High will begin to gain strength as the polar vortex is pulled west of its current position.

So in fact, we are going to phase 2 of Cohens snowatmospheric circulation linkage?

http://web.mit.edu/j...netal_GRL10.pdf

Cohen et al. [2007] put forth a conceptual model of a dynamical pathway consisting of a sixâ€step process starting with a rapid advance in Eurasian snow cover and culminating

in a negative surface AO. The six steps in sequential order are:

1) rapid advance of Siberian snow cover in October, (achieved, although the biggest gain was/is in november)

2) a strengthened Siberian high with colder than normal temperatures and higher sea level pressure (SLP) anomalies,

3) increased upward Eliassen Palm (EP) flux or the three

dimensional wave activity flux (WAF [Plumb, 1985]),

4) a stratospheric warming,

5) downward propagation of the associated height and wind anomalies from the stratosphere down to the surface and finally

6) a negative surface AO

Typically the cycle begins with the advance in snow cover in October and concludes with a lower tropospheric AO response beginning in mid to late January.

Edited November 18, 2011 by sebastiaan1973

[summer blizzard]

It looks like the change to a +PNA will occur after all and the subsequent change for the UK in combination with a +AO will be a pretty much classically zonal pattern.

[sebastiaan1973]

I found another stratospheric temperature data set that goes back to 1950. This time I focused on the area 50-90N 90E-150W (basically Russia and surrounding areas) where the correlation is strongest.

I used 70mb stratospheric temps from the NCEP reanalysis:http://www.esrl.noaa.gov/psd/cgi-bin/data/timeseries/timeseries1.pl

Based on this I would give us at least a 75% chance of having a +AO in December+January. If the stratosphere remains near record cold for the next 2-3 weeks, our chances diminish even further. There has never been a significantly -AO January following a top 10 cold stratosphere.

November stratosphere

-The December AO was 1.09 higher following the 10 coldest than the 10 warmest.

-The January AO was 1.75 higher following the 10 coldest than the 10 warmest.

-The December AO was .63 higher following cooler than average than warmer than average

-The January AO was 1.71 higher following cooler than average than warmer than average

November + December stratosphere

-The December AO was 1.82 higher following the 10 coldest than the 10 warmest

-The January AO was 2.81 higher following the 10 coldest than the 10 warmest

-The December AO was .72 higher following cooler than average than warmer than average

-The January AO was 1.41 higher following cooler than average than warmer than average

- 8/10 coldest had +AO Dec, only 1 below -.1

- 8/10 coldest had +AO Jan, none below -.2

-9/10 warmest had -AO Dec, none above +.2

-9/10 warmest had -AO Jan, none above +.3

For reference this is what this Oct 15 - Nov 15 looked like this year:

http://img257.imageshack.us/img257/1192/compday1088223617732022.gif

Which matches very well to the November stratosphere preceding +AOs. The following is a correlation between the November stratosphere temperature and the subsequent January AO.

http://img824.imageshack.us/img824/7527/1088223617732022148.gif

In addition, if we look higher in the stratosphere at 20mb the spatial pattern of anomalies 10/15-11/15 has matched that of other Novembers preceding +AO December and Januaries. The similarity is astonishing. The current stratosphere is in a classic "pre +AO pattern."

10/15-11/15 20mb temps this year:

http://img812.imageshack.us/img812/1192/compday1088223617732022.gif

November 20mb temp correlation to subsequent AO:

http://img825.imageshack.us/img825/8510/1088223617732022352.gif

http://www.americanwx.com/bb/index.php/topic/29714-negative-nao-coming/page__st__20

[chionomaniac]

I have read through the American weather forum thoughts on the AO and have to agree somewhat with the opinion that the AO is likely to remain positive through December and much of January. The indications suggesting opposite are too few and far between. Even the high snowfall late on into October over Siberia may be too late to induce stratospheric feedback.

I read numerous times in the model thread that the models are only accurate to around 6-7 days and anything could spring out after that. However, the cold stratosphere is not going to suddenly change in this timeframe and northern blocking is going to be extremely unlikely until stratospheric conditions do change. The state of the GWO and MJO are going to hold court where exactly troughs and ridges are positioned and I suspect it will take some time before we see a drastic change.

For those looking at every FI option for anything other than amusement for the coming weeks are going to be left very disappointed when any cold option disappears in the reliable time frame.

I suspect that it will be well into the new year before we see that -10ºC 850 isotherm travel further south than the far north of Scotland.

January could hold some promise but we will need to see the turnaround in stratospheric conditions before then.

c

[damianslaw]

I have read through the American weather forum thoughts on the AO and have to agree somewhat with the opinion that the AO is likely to remain positive through December and much of January. The indications suggesting opposite are too few and far between. Even the high snowfall late on into October over Siberia may be too late to induce stratospheric feedback.

I read numerous times in the model thread that the models are only accurate to around 6-7 days and anything could spring out after that. However, the cold stratosphere is not going to suddenly change in this timeframe and northern blocking is going to be extremely unlikely until stratospheric conditions do change. The state of the GWO and MJO are going to hold court where exactly troughs and ridges are positioned and I suspect it will take some time before we see a drastic change.

For those looking at every FI option for anything other than amusement for the coming weeks are going to be left very disappointed when any cold option disappears in the reliable time frame.

I suspect that it will be well into the new year before we see that -10ºC 850 isotherm travel further south than the far north of Scotland.

January could hold some promise but we will need to see the turnaround in stratospheric conditions before then.

c

Interesting thoughts - perhaps the second half of the winter will hold much more cold and snowy prospects than the first half, who knows early Spring could hold a surprise or two.

Can I ask what the state of the stratosphere was in mid december 2009? Don't recall it being overly warm?

The position of ridge and troughs will dictate where cold and snow may occur, the northern half of the country could still see plenty of snow at higher levels and temporarily lower levels thanks to cold polar maritime air at times - also northerlies can and do often occur on the back of deep depressions even when we have positive NAO and AO - but they tend to be shortlived affairs.

[chionomaniac]

Interesting thoughts - perhaps the second half of the winter will hold much more cold and snowy prospects than the first half, who knows early Spring could hold a surprise or two.

Can I ask what the state of the stratosphere was in mid december 2009? Don't recall it being overly warm?

The position of ridge and troughs will dictate where cold and snow may occur, the northern half of the country could still see plenty of snow at higher levels and temporarily lower levels thanks to cold polar maritime air at times - also northerlies can and do often occur on the back of deep depressions even when we have positive NAO and AO - but they tend to be shortlived affairs.

The stratosphere in 2009 was warm right through until Mid December.

http://acdb-ext.gsfc.nasa.gov/Data_services/met/metdata/annual/merra/t60_90n_30_2009_merra.pdf

It was this warmth that set things up right through until the end of January before the colder stratospheric conditions influenced. Note the lag here.

[sebastiaan1973]

http://acdb-ext.gsfc..._2000_merra.pdf -> during november and december, but no 'winter weather' for us. As always lots of work has to be done before colder weather can reach us. “Nothing ever exists entirely alone; everything is in relation to everything else.†Edited November 20, 2011 by sebastiaan1973

[damianslaw]

The stratosphere in 2009 was warm right through until Mid December.

http://acdb-ext.gsfc..._2009_merra.pdf

It was this warmth that set things up right through until the end of January before the colder stratospheric conditions influenced. Note the lag here.

Thanks for the reply - I wasn't sure but obviously the warm strat had a major impact in terms of delivering winter cold that year despite an unfavourable ENSO - strong el nino..

Are there any examples of cold strats and cold winters? I guess in such cases other factors have come into play i.e. favourable ENSO.. it appears you need either a favourable ENSO or warm strat for cold weather - alas for the time being we have neither but the ENSO will become more favourable as winter wears on even of the strat doesn't - but I think there is an increasing likelihood of a warm up by the end of dec which could propogate downwards to deliver a cold end to the winter. There is also the solar factor - which in the past two winters was also very favourable for cold, alas it is less so this winter.. As a cold lover I hope we see some warming preety soon..

Edited November 20, 2011 by damianslaw

[chionomaniac]

Thanks for the reply - I wasn't sure but obviously the warm strat had a major impact in terms of delivering winter cold that year despite an unfavourable ENSO - strong el nino..

Are there any examples of cold strats and cold winters? I guess in such cases other factors have come into play i.e. favourable ENSO.. it appears you need either a favourable ENSO or warm strat for cold weather - alas for the time being we have neither but the ENSO will become more favourable as winter wears on even of the strat doesn't - but I think there is an increasing likelihood of a warm up by the end of dec which could propogate downwards to deliver a cold end to the winter. There is also the solar factor - which in the past two winters was also very favourable for cold, alas it is less so this winter.. As a cold lover I hope we see some warming preety soon..

I think looking at the cold strat question to cold winters, one would need to analyse each winter in detail, and make sure that any cold wasn't preceded by a minor warming or event in the stratosphere. Certainly since I have been monitoring the strat I have failed to see high latitude blocking following a cold stratosphere. Every time the stratosphere has cooled the tropospheric response has been predictable and yet for the reverse it is so hit and miss.

I am still waiting to be proved wrong, and actually wouldn't mind being so ( the snow lover in me overrides everything) but can't see it happening.

[blitzen]

I read this thread with interest nearly every day for future signs of any cold. I am however confused as most of it is way over my head. I have a question for the more informed: Why isn't the unusually cold strat' affecting Alaska's ability to currently be experiencing such extremely cold temperatures that we have all been reading about lately? I am sure there is a simple explanation but I am curious to know why it is.

[Paul]

Here's the latest strat temp map for your info

[chionomaniac]

I read this thread with interest nearly every day for future signs of any cold. I am however confused as most of it is way over my head. I have a question for the more informed: Why isn't the unusually cold strat' affecting Alaska's ability to currently be experiencing such extremely cold temperatures that we have all been reading about lately? I am sure there is a simple explanation but I am curious to know why it is.

The simple explanation is latitude.

The majority of Alaska is situated north of 60ºN whereas the UK is situated south of 60ºN.

So generally speaking Alaska is on the cold side of the polar front jet and if cold is being kept in more northerly latitudes then there is every chance that the bottled up cold is being kept there as part of the La Nina pattern.

http://www.theworldmapsphotos.com/wp-content/uploads/2011/09/World-Map-With-Latitude.jpg

Edited November 20, 2011 by chionomaniac

[blitzen]

The simple explanation is latitude

http://www.theworldm...th-Latitude.jpg

Thanks for that Chiono. It was really confusing me as I thought 'all for one and one for all' so to speak! Ironic really when you view your link, that there isn't that much difference in latitude. We really are a good way north to have the climate that we have!

[summer blizzard]

Here's the latest strat temp map for your info

Wow, never seen that chart before.

Is that available from Netweather Extra? Also how often is it updated?

Thanks for that Chiono. It was really confusing me as I thought 'all for one and one for all' so to speak! Ironic really when you view your link, that there isn't that much difference in latitude. We really are a good way north to have the climate that we have!

I believe that Alaska is also fairly high.

Basically you should think of Alaska as being Scotland but with only a warm sea to the south rather than all round.

Chino, what phases of the MJO would we have to get into to prompt high pressure over the UK?

[pottyprof]

Wow, never seen that chart before.

Is that available from Netweather Extra? Also how often is it updated?

Yep. Look for the stratospheric temps (30 hpa) in the drop down.

As far as I'm aware it updates every run of the GFS.

[summer blizzard]

Yep. Look for the stratospheric temps (30 hpa) in the drop down.

As far as I'm aware it updates every run of the GFS.

Seriously tempted to join once my bank balance is healthy in January.

Especially now i can even get to watch the GFS version of stratospheric warnings (looks much better than the ECWMF).

[Isolated Frost]

It's a shame that this thread isn't as popular as it should be. I have some questions, which hopefully someone could answer in very layman terms

What is frictional torque (I think i've grasped mountain torque)

What do the phases of global angular momentum indicate?

What does the QBO signal?

What is angular momentum flux convergence (or surface flux in general)?

And a few other notes on angular momentum, and it's general influence on our weather, and the rest of the world. I think I understand the cells, the stratosphere, the jet etc. but would appreciate if anyone could answer, or add anything similar. Thanks a lot, IF

[lorenzo]

I agree IF - I enjoy looking at the science of what is beyond the base models and looking around for further guidance.

A lot of the material I find on line are academic papers that really need a user friendly summary.

I appreciate everyone has pressure on time - it would be great to have net weather technical guides. This may in turn lift the model threads to a new level.

[Isolated Frost]

Yes lorenzo, I would love a simple explanation on them factors, so I can begin to learn about the science of the models, rather than read them without knowing what's next or how this happens

GP's signature is in my opinion the best around and it would be my motto, I'd like to learn the stuff and then really get into it. This is why the stratospheric thread and in depth thread are now my bookmarks.

I'm only 14 and so I may be slightly impatient, but I really do thrive to learn about some of the stuff that GP, phil.n.warks, chiono etc. know, and I hope you don't mind me asking these questions, but I fear that I do not have the knowledge to understand some topics in advanced discussions.

I hope the mods don't mind, but to my original question (which I would be totally grateful if anyone could answer), I have a following two. I have a brief knowledge on the brewer-dobson circulation, but would anyone care to explain what it is, and how it influences us? And also (sorry!), what is a wave breaking event (I gather that it maybe stratospherically based?).

Thanks loads and I admire those that have took the effort to learn and share their knowledge on net-wx