[Stratosphere and Polar Vortex Watch 2020]

On 11/11/2021 at 20:55, Eagle Eye said:

We follow the warm air back to Asia

Direct quote

"There are a few clues that one can use to trace the origin of the descending warm air over the pole, but before looking at that, let's talk about that descending air in general. Of course the rapid descent of air in the stratosphere compresses and warms, but a key clue comes from knowing that it was already anomalously warm before it began to be compressed. What warmed it and where?

Thanks to satellite reanalysis data, we can in fact see exactly where the warm air came from before reaching the polar latitudes. As this animation so excellently displays, a wave of warm air moved up from
South Central Asia in the weeks prior to the SSW event that occurred in early January 2013. This wave in fact seems to almost "explode" up into the stratosphere, almost like the warming you'd get from a volcanic eruption (again, hence the reason for early theories on SSW's looking for a volcanic connection)."

Continuing with the analysis



This warm bubble rises due to the thermal wave that's over central Asia around the 21st December, the fact that this rises up so quickly indicates a very strong dynamical event which forced warm air up into the stratosphere from the troposphere.  It then moves northeast at 10 hPa stratospheric levels and higher. Here's different view of the thermal wave on December 28, 2013:

Here's another perspective on this wave that shows both its origin at lower latitudes, as well as it's progress toward the north. Remember, that this is occurring at the 10 hPa mid-stratosphere level:

This is a typical 'standard fingerprint' of an SSW, here's another one from an SSW in 2002 - 2003

Similar development in the thermal waves when it was developing at lower latitudes that year as well, there are similar waves before any SSW events as far as I know no example of it beginning in anywhere other than Asia, for example January 2009

 

When considering the development of these thermal waves at mid-stratospheric levels over Asia, there are several factors that must be considered. First, such of a rapid build-up of rising warm air can obviously not come from evaporation as normal adiabatic cooling would occur and we would see the exact opposite condition such as we find associated with the normal convective processes in thunderstorms or on a much larger scale in Hadley Cells.

Secondly, the consideration is that the rapidity necessary to bring such a large mass of warm air upward into the stratosphere. Some process must essentially "launch" this warm air up to the stratosphere, as we see it seems to "explode" into the stratosphere. The air has to be relatively warm to begin with (anomalously warm relatively to normal stratospheric temperatures at 10 hPa), for no matter how rapidly it is launched or how warm it was, it will expand and cool by some amount, no matter what. So in our search for the source of the thermal wave, we are looking for a source of warm air and a mechanism by which that air is launched rapidly upward into the stratosphere.

This is why some researchers suggested that volcanoes may be to blame for these waves though there isn't much evidence to support that.
 

Then we look for a High Positive Omega

The vertical rising or falling of air in the atmosphere is known as omega and is measured in Pascals per second (Pascal/s). A high positive omega over a geographic region would indicate that a significantly large mass of air was rapidly moving upward in that region, and likewise, a negative omega would indicate that a mass of air that was falling or moving downwards. 

In the formation of a thermal wave in the stratosphere over South Central Asia we would be looking for a region in the troposphere within the same rough region with an unusually high positive omega. It would have to be an area that displayed what would be much like vertical jet stream or plume of warm air moving rapidly upward in the troposphere, then through the tropopause, to form the thermal wave in the stratosphere.

"In looking at the omega reanalysis data for the period around the formation of the thermal wave that formed over Asia in December 2012, we find the following very interesting data that displays the average omega across the entire troposphere from 1000 hPa up to 100 hPa. This chart was further constrained to show the region of interest, namely South Central Asia from 60E to 110E longitude and 0 to 70N latitude: "

When looking at this reanalysis chart we see three regions of high positive omega. These regions represent areas where air was moving rapidly upwards across the troposphere averaged over the entire month. For the sake of simplicity, I chose to narrow my analysis down to the area which has the highest positive omega of the three, which was the one furthest north and west.  Here's a closer view of the omega in that region during December 2012:

Again, for a clearer understanding of omega when looking at the preceding diagram, you could imagine a stream of air moving right out of the centre of the red  area, outward from the model area. This stream would be flowing up through the entire depth of the troposphere during the month of December 2012. For the sake of comparison, here's the omega for the exact same region during the month of July 2012:

When we begin comparing the two, we see that the high omega over this region in December 2012 truly is at least a monthly anomaly and not a year-round feature of this region. So we at least now have a potential area for a vertical stream of air moving into the stratosphere to form the thermal wave in December 2012. 

Skipping a bit here as this is beginning to get quite long explaining an SSW in clear effect

Direct quote
 

"SSW's are marked by rapidly descending air over the polar region.  This air warms as it descends but we've seen that it was warmer than normal to begin with. Through reanalysis, we have seen that the air travelled to the high latitudes as a stratospheric thermal wave at 10 hPa. In looking at the origins of this thermal wave we find high omega areas just south of the Taklamakan Desert in the Kunlun Shan and Altun Mountains. When prevailing north to south winter winds blowing across the Taklamakan desert (and including possible energy from jet stream winds) are directed toward the Tibetan Plateau under the right conditions, the perfect topological lifting conditions occur, allowing the warmer air from the desert to ascend rapidly into the stratosphere where it builds into a thermal planetary wave of warmer air that is carried north by prevailing upper level winds.

Upon reaching the higher latitudes of the Arctic, the warmer air begins to descend rapidly, warming even more but also carrying enough momentum during the descent to drag mesospheric air down into the stratosphere as well. This intrusion of mesospheric air during SSW events has been well documented, but is outside the scope of my research. Suffice to say it has been measured by many researchers and can be identified through its unique chemical signature. One excellent recent source for more information on this is Kvissel et al., 2012. As illustrated earlier, the rapidly descending air in the stratosphere over the Arctic has effects or teleconnections that reach some 9,000 km away to the equator. Here we see that the momentum of the polar descending air pulls the air in the stratosphere up at the equator, such that this air cools and expands and creates a temporary temperature and pressure anomaly.

Again, here's another example of this from the large SSW event that occurred around January 21, 2009 where, in this equatorial stratosphere profile of temperature, you can readily see the effect of air being drawn upward into the stratosphere: "

 

Other Causal Elements of SSW's: The Winter Tropopause

I have only briefly mentioned the tropopause so far but it shouldn't be taken out of any discussion of the full dynamics of SSW's. The intrusion of rapidly ascending warm air into the stratosphere from the troposphere means that this stream must penetrate through the tropopause as it ascends.

There are also some topological circumstances with location of the Taklamakan desert next to the Tibetan Plateau. The desert provides the ideal source of warm air to be "launched" upward and the rapid and steep mountain Kunlun Shan and Altun Mountains are the perfect launch pad for this 'launch' as it were. With the winter winds prevailing from the desert toward the mountains in this region, it only takes the right set of circumstances to initiate the precursor events to an SSW. 

"We must however add the additional factors of the lower winter tropospheric height and the fact that the Kunlun Shan and Altun Mountains are at an ideal position for penetrating the tropopause with warm air from below as the tropopause drops down rapidly between 30 and 40 degrees, as seen in this simple illustration (B. Geerts and E. Linacre, 1997):"

 

Not only does the tropopause lower in the winter, it also drops down dramatically just at the point where the Kunlun Shan and Altun Mountains ranges are located and therefore this is ready to provide the necessary launch platform for warm air from the Taklamakan desert. It is quite possible that had these mountain ranges been located four to five hundred hundred kilometres further south where the tropopause is higher, that the character of NH SSW's and NH winters might be completely different, that is why such small things can have such massive changes across an entire Hemisphere's Winter for quite a few years.

 

"Investigating numerous SSW events using reanalysis data has shown that in every event that I could find reliable data on, the initial locus of the warm air has come from South Central Asia. I investigated one potential source of this warm air among several-- namely at the point where the Taklamakan Desert meets the Tibetan Plateau in the Kunlun Shan and Altun Mountain ranges. 

This combination provides an ideal topographically forced "launching pad" for warm air to be lifted up through the lowered winter tropopause into the stratosphere. This air is then advected as a thermal wave toward the northern latitudes where it eventually descends, warms further, and creates an SSW event. This diagram illustrates the overall dynamics of this entire process:

 

In addition to the extensive reanalysis data, there appears to be tangential corroborating evidence in support of this overall dynamic, such as the high level of dust in Greenland ice cores that has an origin point in the Taklamakan desert and Tarim Basin. This dust could be part of the thermal wave that descends over the Arctic during the SSW and is eventually deposited in Greenland ice.

Future research could focus on gathering mid-stratosphere air samples at 10 hPa during one of the thermal wave episodes, just prior to the SSW event. A high level of dust inside that thermal wave that could be identified through isotopic analysis as coming from the Taklamakan Desert and Tarim Basin would be strong evidence of the origin of the warm air. Other avenues of research should include the analysis of other regions of South Central Asia that also show high positive omega values in the days leading up to the formation of the 10 hPa thermal wave. It is quite possible that several high positive omega areas could be involved simultaneously or each during different SSW events. 

Finally, a close analysis of the regions involved in high positive omega events should be conducted to see what meteorological conditions exist to create the wind field energy necessary for topographic lifting. In addition to pressure zones and prevailing winds, energy from the lowering of the tropopause and the jet stream during winter could be an important source of energy for high positive omega events and the precursors to SSW's."

A lot of this has been direct quote's because I feel it better to quote than try changing it and explaining it further as it was already efficiently explained

This "theory" first did the rounds a few years ago and unfortunately it is as wrong now as it was then.

To start with, the Taklamakan desert and Tamir basin are not a source of warm air in winter, it is typically bitterly cold with temperatures below freezing.

For sake of argument and assuming anomalous warmth, a bigger issue is that positive omega represents sinking air - negative omega is rising air.

But the biggest problem is that it fails to grasp the stratification and stability of the atmosphere. Ironically it is dismissive of moist processes which actually go a long way to overcome the stability, but also the tropopause which magnifies the scale of the problem, and also that adiabatic cooling would continue up to 10mb.

A convenient way to examine it is with consideration of potential temperature to compare the density of air at different altitudes. Theoretically, parcels of air with equal potential temperature are equally buoyant and interchangeable. Indeed for much of the atmosphere this holds true with isolines of potential temperature forming natural flow pathways - see isentropic analysis.

At the 10mb level in the stratosphere, potential temperature might be in excess of 850K - this would be the temperature an air parcel here would have if brought down to 1000mb. Conversely, an air parcel from near the surface would have to be much more than anomalously warm to remain buoyant at 10mb, rather hundreds of degrees C.

Studies of large forest fires have shown estimated surface temperature perturbations of 60C initiating large pyro-cumulonimbus clouds but the significance of latent heat release from adiabatic processes was 3 times that of the sensible heating (ie the fire). This is able to overshoot into the lower strat like regular storms but further uplift is capped by the tropopause. Examination of volcanic eruption plumes shows only the largest would be capable of reaching above the middle stratosphere. In any circumstance it must also be remembered that air parcels in reality are subject to substantial mixing and entrainment of the ambient air.

So there is no 'launch pad' producing a plume of warm air to 10mb (even the driest desert air would produce clouds as evidence), mountain uplift creates air parcels which rapidly become denser than the environment and negatively buoyant, returning to their initial level - or actually oscillating above and below this level ie orographic gravity waves.

[February 2021 C.E.T. and EWP forecast contests]

29 minutes ago, I remember Atlantic 252 said:

Just shows that when members say 'models underestimate the cold block' is normally untrue, the Atlantic will find a way in, often quicker than NW members predict

Breakdowns can sometimes show a tendency to be delayed with cold hanging on, but what's amusing is that when these charts appear is the common insistence is that they are wrong and the colder output is more likely, rather than the writing's on the wall.

[Model output discussion 10/02/21]

Talk of a March easterly and 'downwelling' - where is this downwelling from?

Sometimes have to face the fact that tropospheric disruptions may follow an SSW but might not actually be directly caused by one.

[North West Regional Discussion Feb 2021 onwards]

24 minutes ago, iand61 said:

Some in the town obviously have a thing for inviting a couple of friendly sheep to illegal lock ins but I think a camel might be pushing it a bit

just think how much it would drink

Illegal flock ins.

[Weathercast.co.uk website]

On 28/01/2021 at 15:59, h2005__uk__ said:

Unfortunately that doesn't seem to have all stations - at least it doesn't have my two local ones (Andrewsfield 03684 and Writtle 99127).

I can't believe the MetOffice is still so bad at providing this basic info free of charge.

Andrewsfield data is available quite widely as it is a SYNOP site so is carried by Ogimet for example - enter the WMO number in this form -

Form to get daily summaries from synop reports

WWW.OGIMET.COM

Hourly data can be found by clicking on the dates.

Skylink offers a wider range of sites, some that weren't available on weathercast. Not convinced by the accuracy of all the stations but a potentially useful and interesting resource -

SkyLinkWeather Aviation - Latest Weather for London Stansted

SKYLINKWEATHER.COM

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Current weather for the UK, Europe, Australia and New Zealand

Alas no Writtle...... but for that try weatherobs, fantastic site, possibly better than weathercast for viewing observations, which includes the additional Met Office sites as well as the standard WMO SYNOP and METAR locations - 

Weatherobs

WWW.WEATHEROBS.COM

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Global weather observations (METAR, SYNOP, BUOY, SHIP, TAF, CWOP) decoded to enable display of latest temperatures, weather, snow depths, precipitation, wind and wave conditions and more

 

[Yorkshire and E England regional discussion]

4 hours ago, Winter Cold said:

Wow it's cold out there, really low dew point too:

Why is your wet bulb temperature warmer than the dry bulb?

[North West Regional Discussion Feb 2021 onwards]

12 hours ago, SP1986 said:

That's interesting, everything further south looks incredible marginal on that FAX chart. Even the 528 DAM zone is north of most of the NW at that point. I would have thought the cold air would be thoroughly entrenched everywhere by that point (12pm Sunday). 

The thing is the 528 DAM is deceptive, it is representative of the temperature between 1000-500 mb but despite mention of 'deep cold', it is actually concentrated in the lower levels of the atmosphere. Temperatures up towards 500mb are not especially cold relatively speaking - indeed it soon becomes a westerly flow above the easterly lower down.

The repercussions of this means it's cold near the surface, which is want you want for snow, however with regards to shower activity it is largely developing below the 800mb level generated by surface forcing, there is limited deep convection.

Taking this precipitation chart for Monday as an example -

The black line in the middle from roughly Liverpool out into the North Sea is represented in the cross section below -

This shows vertical velocity - vertical air movement. Following the direction of wind flow from the east on the right hand side and comparing to the precipitation chart, there is gentle convection (max at about 700mb) over the North Sea producing showers, shown by the upwards arrows on the chart. Moving leftwards (towards the west) the showers are enhanced on meeting the Lincolnshire Wolds but the uplift is only up to about 800mb, the air is quite stable and the topographic wave forms a descending air at 750mb reducing shower activity inland.

Moving further westwards, upon approaching the Pennines, there is strong uplift below 850mb, producing heavier showers, but the topographic wave and strong descent caps development to below 800mb and effectively kills showers to the west of the Pennines, on the left hand side of the chart.

One way this flow pattern may breakdown is suggested by this run on Tuesday evening is a convergence zone (sometimes misleadingly described as a streamer).

[North West Regional Discussion Feb 2021 onwards]

Just now, Wardlegacy said:

Id have to check my records but on the slopes of the Pennines we had already got a covering of snow at that point from showery snow. You do seem to want to argue rather a lot, are you OK?

It's a discussion forum, disagreement should be permitted (unless in the MOD thread, obvs.)

[North West Regional Discussion Feb 2021 onwards]

18 minutes ago, Wardlegacy said:

Depends where you were in the region

The notable March 2013 snowfall chiefly between 22-24th, a report from the Met Office -

https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/weather/learn-about/uk-past-events/interesting/2013/snow-and-low-temperatures-late-march-2013---met-office.pdf

[North West Regional Discussion Feb 2021 onwards]

40 minutes ago, chris78 said:

That is what you are inferring, it is not what people are saying.   Christ mate its a weather forum, the models are showing snow, and you are making out we cant be interested.   Plus if I get a 2cm of snow (I wont)  or if someone in Blackburn gets 8cm (they might)  Why should either of us give a lemon that someone else has 2 foot.    That is a very mean spirited way to look at it.    If I win a million on the lottery should I not be happy because someone else won 10? 

It's not what has been inferred at all, in the same way that it's not been made out the people can't be interested.

It has been regularly posted to ignore the precipitation and snow depth charts. Of course showers are parameterised in NWP output, and it isn't always precise, but equally they won't be a million miles out, that's just wishful thinking, unless some trough or other feature etc. materialised.

Think your analogy was wrong, you're winning the tenner 

[North West Regional Discussion Feb 2021 onwards]

23 minutes ago, iand61 said:

I can’t speak for the rest of the region but the March 2013 easterly certainly floated a few boats around these parts as did the 2018 one although I’d gladly miss out on the heavy disruptive snow part of the next few days if it means that my 85 year old Mother in Law can get her COVID vaccination and we can start to get some normality back into our lives.

However, March 2013 was a frontal system.

[North West Regional Discussion Feb 2021 onwards]

1 minute ago, Dexter said:

Simply not true I'm afraid. I can't speak for Western parts of the region having not lived here that long, but I can assure you from personal experience that Eastern parts of the region can and have seen significant amount of snow from an Easterly, several inches on occasions.

Ok, lol, and on these occasions what were depths like in other parts of the country? It's almost like people are insinuating that this area is a match for other areas for snow showers in an easterly, it beggars belief!

[North West Regional Discussion Feb 2021 onwards]

59 minutes ago, chris78 said:

For some in the region easterlies are great.     Now for me out on the West a big fall is incredibly unlikely, so for anyone living in those areas your get real comment would apply to every single snow forecast ever made/   i think lots of us here look at others in the N.West predicting ONLY 8cm of snow with disappointment and think...you dojnt your born!  

People are bigging it up, it's *possible* that there could be widespread notable depths, but the balance of probability is that it is unlikely and it will pale in to insignificance compared to other regions which will be worse affected. Without some frontal activity, that is the reality of the situation for most parts of this region away from the Pennines.

[North West Regional Discussion Feb 2021 onwards]

3 hours ago, cheshire snow said:

Indeed

And on the basis of 40 years local knowledge of the area and a very unstable Easterly flow i would like to up my prediction but will stay within the rules of the game the myth of this area been bad for snow from the east was put to bed long before the Beast of 2018 people in this thread looking at PPN charts and taking as gospel then more fool you I am not ramping this up and if I am wrong then so be it I will hold my hands up and take it on the chin.

Transition day is Saturday then we are locked in to cold spell also breakdown is delayed again on this morning charts so could well be still in this cold spell into the next weekend as well.

Enjoy your snow its on its way.

C.S

 

Most of the North west typically gets naff all from an easterly, BFTE was no different. Sure, maybe a few cm here and there, and if that floats your boat then fair enough, but it's not the foot plus that some charts are touting for eg the eastern side of the Pennines or East Anglia.

[North West Regional Discussion 30 December 2020 onwards]

9 minutes ago, Chris.R said:

Because the inversion dropped lower in the atmosphere and became thinner probably due to evaporative cooling so freezing level fell. WRF was showing this could happen last night.

Indeed, thanks for saving me from explaining it 

The deceptive thing is how far advanced the precipitation is from the front at the surface - so the milder air is not at the back edge of the snow/rain.  There is quite a sharp temperature contrast at the surface eg. North Wales 8°C at Capel Curig, 1°C at Lake Vyrnwy 29 miles and 4°C at Rhyl 23 miles distant; in Herefordshire 9°C at Hereford, 4°C at Shobdon 15 miles away. But the rainfall has long cleared these areas.

[North West Regional Discussion 30 December 2020 onwards]

3 minutes ago, Interitus said:

Heavy dumping of NO in Warrington, expecting some more in the next week.

Actually, to be fair there was sleet for a couple of hours - though more rain than snow. And then at roughly around 5am there was about half an hour of heavy wet snow, oddly as the front mostly passed through and was more likely to expect rain. Left the merest patches on grass and at bottom of windscreens where it slid down and accumulated.

[North West Regional Discussion 30 December 2020 onwards]

Heavy dumping of NO in Warrington, expecting some more in the next week.

[Weathercast.co.uk website]

Disappointing, could be the end of 9 years of data downloaded for hundreds of UK stations.

Weathercast was a decent site, been replaced by utter garbage at the moment.

[North West Regional Discussion 30 December 2020 onwards]

Rain and hail overnight left nothing on the ground in south Warrington, just started snowing now, got 20 mins to leave a covering for 9am, lol.

[Stratosphere and Polar Vortex Watch 2020]

2 hours ago, Vikos said:

 

Temperatures in the NH are dropping fast, that is an outcome of those stratospheric disturbances taking their influences.

Looks more like temperatures in the forecast period reverting to climatology.

[Stratosphere and Polar Vortex Watch 2020]

On 16/01/2021 at 07:40, sebastiaan1973 said:

Why is it ironic? I try to understand/ learn

Sorry for late reply. You posted charts which take their inspiration from these classic images -

Tim Dunkerton was one of the authors of these, but sadly didn't respond to the Cohen tweet with anything as insightful regarding the current situation going forward.

[Stratosphere and Polar Vortex Watch 2020]

On 17/01/2021 at 17:54, Frostbite1980 said:

No disrespect to Marco but he has been saying about the SSW possibly causing cold air into the uk etc when it first started and it didnt work out like that so i will take this tweet as a pinch of salt....no idea comes to mind 

Maybe not quite no idea, but little idea with regards to details - and this probably goes for all tweets and analyses by whomever.

 

17 hours ago, Bricriu said:

Not me. I got stuck at WAF. Wave forcing??

Wave Activity Flux - which is essentially wave forcing, propagating up through the atmosphere.

[Stratosphere and Polar Vortex Watch 2020]

9 hours ago, sebastiaan1973 said:

Well. Some time ago, I posted the study by Domeisen et al. There are different types of effect of SSWs related to the regime five days before the SSW onset and five days after.

It is my guess, Cohen tweet points in that direction, we see a dipping effect of the 'European Blocking' type.  If this is true, we can expect more 'dipping' in february.

The role of North Atlantic–European weather regimes in the surface impact of sudden stratospheric warming events

WCD.COPERNICUS.ORG

---

<p><strong class="journal-contentHeaderColor">Abstract.</strong> Sudden stratospheric warming (SSW) events can significantly impact tropospheric weather for a period of several weeks, in...

 

The use of 'dripping' paint charts in association with the Cohen tweet is ironic as one of the replies was from Tim Dunkerton (and characteristically slightly off the wall) -

Incidentally, the charts Cohen posted don't really show the expanded geopotential heights at the end of January that he was alluding to, oddly.

[Stratosphere and Polar Vortex Watch 2020]

8 hours ago, Catacol said:

But - I'd prefer this 100% over a year without a SSW which is much more predictable, largely because I'm not convinced in our modern climate that cold weather of any real depth can arrive in the UK without strat forcing and ideally a full on SSW.

December 2010.

[Stratosphere and Polar Vortex Watch 2020]

2 minutes ago, bluearmy said:

I get the context - but 2021 jan now has two dates with the lowest zonal flow at 10hpa 60N ..... as jan 21  progresses more of these clearly won’t happen! 

Well, exactly - date records are interesting but not necessarily particularly significant.