Stratosphere and Polar Vortex Watch 2020

[bluearmy]

ec day 10 seems to be at least  a day ahead of gfs on its route to an ssw (if it arrives). Note the height of the vortex is 2862 dm at day 10 whereas at the moment it is 2808dm.  Losing intensity ........

 

[SqueakheartLW]

My favourite ensemble member on the 00z GFS today is Member 9

Here we have the peak of the warming event at +204 hours

The vortex weakens and begins to split here at +264 hours

The lobes are pushed away from the pole leaving a large warmer area right through the pole from Siberia to Europe by +312 hours

By +384 hours there is very little left of the polar vortex. One small area above the Pacific and an even weaker remnant above Southern Asia

Just imagine if that -40 m/s member came off. Since GFS Member 9 looked to be the best one by far I think that could be this one

Edited December 23, 2020 by SqueakheartLW

[Singularity]

Sure is a lot of spread among the GEFS, while GFS is now among the most suppressed for total warming and rate of progress toward a SSW.

Not sure what to make of that. GEFS were recently upgraded but I imagine they’re still short of GFS in the stratosphere?

Regardless, with ECM looking faster, that’s encouraging for achieving a SSW. Main question is, split or displacement?

The displacement GFS has now shown for two runs in a row leaves a bit much of a vortex over Greenland/Canada for my liking - but for all we know, this detail could well be nullified soon.

[bluearmy]

13 minutes ago, Singularity said:

Sure is a lot of spread among the GEFS, while GFS is now among the most suppressed for total warming and rate of progress toward a SSW.

Not sure what to make of that. GEFS were recently upgraded but I imagine they’re still short of GFS in the stratosphere?

Regardless, with ECM looking faster, that’s encouraging for achieving a SSW. Main question is, split or displacement?

The displacement GFS has now shown for two runs in a row leaves a bit much of a vortex over Greenland/Canada for my liking - but for all we know, this detail could well be nullified soon.

I’m a little confused by the displacement comments ...... surely those runs not showing a reversal are likely showing a displacement of the spv (they may well have gone on to split in any case) whereas those which do reverse are probably splitting the spv??

the last four gfs ops showed one split, two with stretched vortex centred Asian side and the 00z run with vortex beginning to stretch but centred to our north 

Edited December 23, 2020 by bluearmy

[Singularity]

Perhaps, but reversals do happen with displacements, albeit a bit less often than with splits.

That being what classifies a ‘major displacement SSW’ in some analysis.

True that some may actually be slower routes to splits - can be hard to tell if the ‘pinch’ will manifest.

My earlier remarks are mainly in light of some GEFS members being as soon as days 11-12 with a split. Fascinating possibilities afoot.

Edited December 23, 2020 by Singularity

[ALL ABOARD]

Hmmmm I make that one in six... 

 

[snowking]

This might be a good time to remind everyone that the Stratosphere charts on Meteociel will not show you whether there is a split in the polar vortex or not - the polar vortex is not just a chunk of cold air, you therefore need to view the geopotential height charts available from a couple of sources.

The easiest one to read is at over at wxcharts for the 10mb level at least: 

WXCHARTS

WXCHARTS.COM

---

A new weather forecast model data viewer for Europe and North America. Clickable forecast and ensemble plots, hourly GFS, ECMWF, ARPEGE and GEFS plots

It combines both the temperature and the geopotential heights, as well as labelling where the vortex (L) is, and where any geopotential waves (H) are. As you can see from it's latest run, we get a lot of pressure and a big squeeze on the vortex, but no split at 10mb. You can also view a similar chart on netweather I believe for the 30mb level for free, and from memory for the 10mb and 100mb levels if you have NW Extra.

However, it's worth reminding everyone again that the stratosphere and the polar vortex do not just exist at a single pressure level of 10mb (I think the focus is on this pressure level as we use this to define whether a true SSW has taken place or not), and therefore to get a true idea of what is happening you need a comprehensive view vertically of as much of the stratosphere as possible. The best tool for this is instantweathermaps here: 

GFS 50mb Geopotential Height -- Instant Weather Maps

WWW.INSTANTWEATHERMAPS.COM

This allows you to view a number of different layers of the stratosphere for each GFS Op run to try and get a true picture of how the vortex is shaping up (or not!). 

What we previously saw from the outer stretches of the GFS was a top-down warming up between the 1-10mb levels pushing downwards, forcing a displacement of the polar vortex at these levels, followed by a split. Whilst we are still seeing the displacement up at these levels, we are no longer quite seeing the split (though this may follow afterwards):

There's a reason I've picked the 2mb level - it shows quite nicely why we can't just look at the temperature charts alone:

Looking at that temperature chart, if we are making the assumption that the vortex is represented just by the cold blue/purple temperatures, it looks like there has been a "split" - but as you can tell when comparing this to the geopotential heights chart above it, this is actually not the case. 

However, the second piece of the puzzle has been the bottom-up warming which has likely been triggered as a result of repeated +EAMT (East Asia mountain torque) events, which is helping to weaken the vortex and allow it to be pushed around by the geopotential "waves" (the H's we saw on the 10mb chart from wxcharts above) a bit more. To see how this is developing, we need to take a look at the 100mb level and then work our way up.

What we can see when doing that is that by +336, we have a wave pushing in from the Aleutians at 100mb starting to cause a split in the vortex:

By the end of the run, the split has become even more noticable:

But if we look up at the 50mb level at +336, we notice at this point we don't yet have a split:

We have to wait until the end of the run before we see the split occur here:

So this tells us the split is working its way up from the bottom of the stratosphere.

So a few takeaways here:

1. You can't use just the temperature charts alone to tell whether there is a displacement and/or split in the polar vortex in the stratosphere - for that you need to check the geopotential heights 

2. For our current event we are looking at a displacement of the vortex which further helps to weaken it, followed hopefully by a split.

3. We need to keep an eye on all levels of the stratosphere, not just the 10mb level - although of course just for beauty's sake to hit the definition of an SSW we all want to see a reversal at the 10mb level.

4. GFS has been a little less keen on the top down split in recent runs, though that's not to say it won't return again or indeed that it wouldn't happen a few frames after the +384 mark.

Hope that helps.

Edited December 23, 2020 by snowking

[Singularity]

Great insight on diagnosing the GFS runs, thanks. If only we could do that for GEFS runs too! Maybe there’s a way out there somewhere... but it might not be free.

Admittedly I’ve been focusing on the top-down pathway as that’s the one I associate with rapid downwelling if a split is achieved at around 10 mb... but thinking about it, it could well be that a bottom-up split development is a key aspect toward achieving an effective downwell to begin with. It’s also a reflection on the troposphere being more receptive.

The SSW process is still one of the most poorly resolved areas of meteorology - seems like every month or two I read papers that contradict earlier ones regarding what leads to what, when a SSW should be classified as major, whether minor displacements should even be considered a type of SSW to begin with... that sort of thing.
 

So, I try not to presume much when it comes to upcoming SSW possibilities... but that bottom-up process has raised my eyebrows as we saw that in Nov-Dec 2010, propagating upward from the Greenland area in conjunction with the impressive blocking high formations around there. I’m not sure it made it to 10 mb that time though - I believe a separate event then followed in January...?

[northwestsnow]

Thanks singularity and Kris for some excellent analysis...

Much goes over my head , but it sure sounds positive !

[bluearmy]

Remember that the bottom up split is less reliable because it relies on the trop being modelled correctly ......however, keep an eye on when that wave(s) is ejected upwards from the trop because if that’s in a reliable timeframe all systems are go 

would love to have access to eps strat data to see why they were so keen on an accelerated warming yesterday. 

 

[Catacol]

I’m going to take a slightly unscientific path - and judge that the vortex will split because of an accumulation of contextual factors. We have EAMT spiking now, and a forecast of another spike soon...possibly a very big spike. We have a semi permanent Ural ridge causing no end of long term harm, and a meridional Atlantic with a Greenland ridge adding more pressure. And the EPS in general has the North Pacific high very suppressed.

Modelling has the vortex skewing over to the atlantic side where wave 2 pressure is exerted according to yesterday’s ECM from around day 10. And when one considers the approximate shape of the upper and lower vortex as forecast, and the below average wind speeds holding it all together - I just think we don’t need to hang on every chart looking for that split. If wind speeds were higher and the core of the vortex more coherent then a displacement would seem a fair bet. Right now instinct just tells me it will split.

Ill crawl back into my hole and hide for a bit if it doesn’t come to pass! - but if/when it does then it’s all about those spinning daughter vortex shards. Where will they end up? That’s a toughie.......

 

[snowking]

40 minutes ago, bluearmy said:

Remember that the bottom up split is less reliable because it relies on the trop being modelled correctly ......however, keep an eye on when that wave(s) is ejected upwards from the trop because if that’s in a reliable timeframe all systems are go 

would love to have access to eps strat data to see why they were so keen on an accelerated warming yesterday. 

 

Yes ^^ absolutely this, thanks Nick - I should have added this into the original post. We should expect to see as much variation in the lower stratospheric forecast as we will in the troposphere given that the bottom-up action is tropospheric led.

[Kirkcaldy Weather]

If you click on the tweet you can see the interesting discussion about the possible warming event and some older similar patterns that resulted in warming events in the past 

 

Edited December 23, 2020 by Kirkcaldy Weather

[Catacol]

Well - Eric has plumped for a displacement. His composite for this kind of displacement looks good....but it is a slight blow today for a guy of his undoubted knowledge not to plump for a split. Insufficient North Pacific ridging the culprit.

[Mike Poole]

I don’t see how this cannot be not just a split but a total dismemberment.  

The vortex is in tatters in different ways at different levels of the atmosphere.  This plot sums up the forecast position at T384:

You can’t just push a thing like that to one side, which is what a displacement would be!  

My thoughts are a little delay on the split - strat forecasts maybe have more variability due to a lot of the drivers being trop driven this time, but the same split is showing at the end of the GFS 12z:

Just not quite there yet.  

Edited December 23, 2020 by Mike Poole

[Vikos]

Well, EC is on track...

That 60°N is close to be negative

 

But a split... wait and see

Ozone looks good to me, but a bit more would be fine!

 

Edited December 23, 2020 by Vikos

[chionomaniac]

Not convinced by displacement only.  There will definitely be some wave 2 activity. Unusual to see the primary source of this is Atlantic based.  I guess if primary source of wave 2 activity was pacific based then that would be more concerning.  But it isn’t. So the fact that wave 2 activity is Atlantic trop based can only be a good thing no mattEr what eventually happens further up. It’s a case of needing to focus on the sources of the disruption rather than the actual disruption itself.

[Nick F]

Strong signal from 12z EPS for a reversal ...

 

[chionomaniac]

I had a big discussion earlier on Twitter about using 65N. After reading the papers suggested I think we need to concentrate on 60N

[Singularity]

There is, I believe, such a thing as a wave-1 dominated vortex split.

It happens when you have a huge wave-1 and just a little wave-2. The dominant ridge shoves the vortex against the weak one and it’s like bringing a plank down on a sharp rock edge. 

Also... as far as I can tell from literature, wave-2 diagnostics are tied to the usual source which is an Aleutian Low, East Asia High combo. Not sure if this is truly the case but if it is, Atlantic sourced wave-2 via Greenland will be missed.

[chionomaniac]

19 minutes ago, Singularity said:

There is, I believe, such a thing as a wave-1 dominated vortex split.

It happens when you have a huge wave-1 and just a little wave-2. The dominant ridge shoves the vortex against the weak one and it’s like bringing a plank down on a sharp rock edge. 

Also... as far as I can tell from literature, wave-2 diagnostics are tied to the usual source which is an Aleutian Low, East Asia High combo. Not sure if this is truly the case but if it is, Atlantic sourced wave-2 via Greenland will be missed.

Yep. Hence without the Pacific strong source, the Greenland block is all that really matters in this case. Normally the pinch point is thumb in Pacific, broken little finger Atlantic area ( often Scandinavia based). Opposite now

[Allseasons-Si]

GEPS goes for a split at 10 hpa.

gfs at384.

 

[snowking]

Yesterday’s 12z EC op going all in on a top down reversal:

And with flux just starting to turn poleward, this is only the beginning

[bluearmy]

The 10hpa temp anoms on ec (gem similar) v different to gfs at day 10 

    

[georgiedre]

21 minutes ago, bluearmy said:

The 10hpa temp anoms on ec (gem similar) v different to gfs at day 10 

    

When will we get an idea on what will be a likely outcome for the UK.  Whats your pennies worth?