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11-01-2019, 14:10   #211
hatrickpatrick
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Very interesting to see Met Eireann officially acknowledge this. Seems to me based on this and the UKMO's live stream the other day, that experts are at least somewhat confident that downwelling is likely to occur.
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11-01-2019, 15:19   #212
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' The strat. vortex is currently weaker than all other years in the ERA interim record!!!
The zonal mean zonal wind at 10 hPa 60N is today (GFS analysis): -8.6 m/s
Weakest zonal wind at 10 hPa 60N in ERA interim record for todays date is: -8.4 m/s 2004
Strongest zonal wind at 10 hPa 60N in ERA interim record for todays date is: 66.8 m/s 2009 '

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12-01-2019, 17:44   #213
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There is still no sign of downwelling of note over the next 10 days. What we've seen now in terms of wind reversals in the strat and movement down to upper tropospheric levels (200 hPa and below) is about the extent of what we will see in the foreseeable. There are no indications of any trend towards a more retrogressive (blocking) largescale pattern, the 50 hPa ridging at 150W from about 6 days not showing any signs of amplifying and cutting off over the pole by day 10. From 100 hPa downwards the patter remains remarkably consistent, with a low over NE Canada/Greenland and a zonal pattern over the Atlantic showing up right down to 500 hPa.

For blocking towards the end of the month and into February we need to be seeing signs of reversals first at 100 hPa, then at 200, 300 and finally 500 hPa levels, but they are absent.

GFS 100 hPa Analysis, 5-day 10-day forecasts






ECMWF 100 hPa 10-day forecast


Seasonal zonal wind evolution and NASA 10-day forecasts


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12-01-2019, 20:00   #214
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Reading the posts above, are we heading for an anticlimax in terms of a proper wintry outbreak?
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12-01-2019, 21:42   #215
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Reading the posts above, are we heading for an anticlimax in terms of a proper wintry outbreak?
Far too early to say. SSW and PV are only possible factors to delivering a cold spell.
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13-01-2019, 13:13   #216
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There is still no sign of downwelling of note over the next 10 days. What we've seen now in terms of wind reversals in the strat and movement down to upper tropospheric levels (200 hPa and below) is about the extent of what we will see in the foreseeable. There are no indications of any trend towards a more retrogressive (blocking) largescale pattern, the 50 hPa ridging at 150W from about 6 days not showing any signs of amplifying and cutting off over the pole by day 10. From 100 hPa downwards the patter remains remarkably consistent, with a low over NE Canada/Greenland and a zonal pattern over the Atlantic showing up right down to 500 hPa.

For blocking towards the end of the month and into February we need to be seeing signs of reversals first at 100 hPa, then at 200, 300 and finally 500 hPa levels, but they are absent.
This excellent analysis yesterday really knocked my confidence in the any upcoming potential.
But I was buoyed again this morning with an excellent ECM run, and I read that we're finally starting to see some signs of reversal at 100 hPa.

I've no idea how to verify that, so I'd love if you could please take the time to check for me. Ta
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13-01-2019, 14:46   #217
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Zonal winds at 100hpa in postive, a certain deceleration but expected to stay well into positive.

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13-01-2019, 20:36   #218
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https://twitter.com/Forecas55175638/...38351150075904
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15-01-2019, 19:24   #219
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https://twitter.com/judah47/status/1085245537606139904
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16-01-2019, 20:45   #220
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https://twitter.com/WorldClimateSvc/...23852724641797

https://twitter.com/WorldClimateSvc/...30995989254145
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17-01-2019, 16:49   #221
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17-01-2019, 17:23   #222
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@Sryan, one area of stratospheric forecasting I'm still entirely ignorant on is what is meant by wave 1, wave 2, wave breaking etc when people discuss these forecasts. Without wanting to hijack the thread (I'll look up proper lessons on this after winter is over and the Night Walkers have been defeated), you and several others have mentioned previously that we may not see blocking results from this SSW event because unlike the Feb 2018 event, this one has been severely lacking in wave-2 activity.

Is this still your assessment, despite various signs that downwelling is occurring? Is the lack of wave-2 activity still a problem as we head into next phase of Winter? Someone on Twitter mentioned that there's been a wave 2 "pulse" in recent weeks, but will this be sustained enough to change the pattern?

Unfortunately this is an area I've tried repeatedly to get my head around and failed, so for the time being I must rely on your and others' expertise to interpret this aspect of the event
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17-01-2019, 18:36   #223
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@Sryan, one area of stratospheric forecasting I'm still entirely ignorant on is what is meant by wave 1, wave 2, wave breaking etc when people discuss these forecasts. Without wanting to hijack the thread (I'll look up proper lessons on this after winter is over and the Night Walkers have been defeated), you and several others have mentioned previously that we may not see blocking results from this SSW event because unlike the Feb 2018 event, this one has been severely lacking in wave-2 activity.

Is this still your assessment, despite various signs that downwelling is occurring? Is the lack of wave-2 activity still a problem as we head into next phase of Winter? Someone on Twitter mentioned that there's been a wave 2 "pulse" in recent weeks, but will this be sustained enough to change the pattern?

Unfortunately this is an area I've tried repeatedly to get my head around and failed, so for the time being I must rely on your and others' expertise to interpret this aspect of the event
Well to explain this question, I must start from the bottom on the basis of what is a sudden stratospheric warming.

A sudden stratospheric warming is a dynamic event that occurs in the stratosphere with a sudden rise of tens of kelvins in temperature. These warming events are the result of natural weather patterns and large-scale pressure systems like the Alaskan High producing atmospheric waves called Rossby waves (also known as planetary waves). These waves are propagated or travel upward into the stratosphere and often result in displacing the Polar Vortex away from the North Pole, along with a deceleration in the zonal winds. If continuous disturbance occurs via higher amplitude of atmospheric waves, further warming may take place.

Meanwhile,

Wavenumbers are defined by the number of troughs and ridges in a wavelength which stretches in a full circle at a given latitude around earth. They're usually identified on maps like 500mb geopotential heights. The different wavenumbers vary on the number of troughs and ridges along with amplitude of the waves.

I, myself, is still learning on the matters of wavenumbers and only have a very amateur understanding of them. However, you will find some great resources online here and there on this matter. Just make sure you search for the correct thing like rossby or planetary waves, zonal wavenumbers etc.

Thank you for the compliments but I would not put myself in the same league as GL, Cohen, Simon Lee, Butler etc.
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18-01-2019, 19:44   #224
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18-01-2019, 19:56   #225
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As sryan has said, wave-1 and 2 refer to the zonal wavenumber (number of tropospheric planetary waves present along a line of latitude). Wave 1 means one very large wave extending right around the globe, so on a long timescale. Wave 2 is obviously two waves, so of smaller scales in space and time. Wave 2 can be superimposed on wave 1, the same way the FM signal is superimposed on the carrier signal frequency. Then there are higher wavenumbers, referring to smaller and smaller disturbances, but the effects of these are less pronounced. The further north you go (higher latitude), the smaller the latitude circumference is, so the smaller a wave needs to be to a certain wavenumber compared to if it were further south (larger latitude circumference, see below).



For each wavenumber there is a critical zonal windspeed, above which the wave can not propagate vertically ("break") into the middle atmosphere (stratosphere and mesosphere). When we have a low-wavenumber stationary wave (say wave-1), a moderate westerly wind below the critical speed (and fairly consistent with height) can allow upward breaking of the wave into the stratosphere, where it will disrupt the polar night jet (the strong westerly jet way up in the stratosphere). When this happens, adiabatic heating occurs due to subsidence of the air.

I like to think of it all like having a bucket of water and stirring it vigourously anticlockwise (anticyclonically) with a stick to form an even vortex. Looking down from above, this mimics looking down on the northern hemisphere, the north Pole in the centre. The lowest level of water in the centre represents the lowest geopotential height of the cold polar night (polar vortex) sitting over the Pole. The outer edge of liquid near the bucket wall is the westerly polar night jet at lower latitudes.

Now if you stick a paddle down into the vortex, it will disrupt the flow of water, just like an upward-propagating tropospheric wave disrupts the polar night jet. You can imagine how the vortex of water will now split towards the centre (Pole), and the whole spin of water will slow down. This is like the polar vortex slowing down and even reversing, causing the sudden stratospheric warming.

If you quickly start the water spinning again then the disruption will be minimised and the effects will not propagate down to the bottom of the bucket. Likewise, the effects of the polar vortex split propagate downwards into the troposhere, but a lot of factors determine if and how much this happens. In this case the vertical wind profile has not really played ball to allow it to properly happen, and the vortex has already recovered up in the mesosphere and upper stratosphere, eventually extending down through the whole layer in the next few days.

If you want to get your teeth into more theory then Holton's book is the best out there. I spent €90 on the hardback edition several years ago, but now it's available for free as a pdf below.

http://danida.vnu.edu.vn/cpis/files/...%20Edition.pdf
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Last edited by Gaoth Laidir; 18-01-2019 at 20:12.
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