Interitus

Members
  • Content count

    1,825
  • Joined

  • Last visited

Community Reputation

1,616 Exceptional

About Interitus

  • Birthday

Recent Profile Visitors

15,975 profile views
  1. Last year was a large major SSW, but because the winds didn't return to westerly it doesn't count as a major midwinter warming but as a final warming instead. Not sure what a 'normal' event is, but this forecast 2nd SSW of the winter is dragging on for ages and is even maybe not a certainty having vanished within 10-days on yesterday's ECM, and not on last night's GFS 0z as plotted on the weatheriscool site. Bear in mind that the GFS at one point had a reversal with a split vortex for as early as the 14th February to generally by the 18/19th for a number of runs.
  2. Thanks for your reply, it wasn't picking holes but rather genuine curiosity. I was always under the impression that meteorology was essentially a scientific endeavour, though on these forums it does at times tend towards a more creative pursuit! Thus what is widely accepted or taken for granted here may not be the actual truth. The interchangeable use of the term 'pv' for either strat/trop polar vortex can be ambiguous (not to mention normally meaning potential vorticity in 'serious' meteorological circles) but blurring the distinction also causes problems when defining the notable characteristics of winter 2015/6 which will be examined in a moment. But firstly now it is clear by pv you were referring to the stratosphere, how one defines a record weak or strong vortex is not trivial. Thank you for the link, it is an interesting read and there is no need to question the veracity of their findings and it provides convincing empirical evidence, though it does raise a few points. What time period? The study covers November and December. An anomalously weak, and maybe more so a strong vortex is unlikely to last for a full two months, or even one month, and also how well the anomaly coincides with the time period will skew the results. What height range to examine? The use of the microwave limb sounder and ERA interim extends to the mesosphere well above the 10mb of the NCEP/NCAR (which they include) and MERRA (not included) reanalyses. The data are essentially similar though there are differences. Also is the 'meat' of the vortex eg. 50 or 30 mb more important than higher up? What area to examine? They looked at 60-80°N which may or may not capture the whole vortex. Also zonal means by their very nature blur the details and some sort of 2d moment analysis could be used, though a stronger unperturbed vortex is likely to be fairly zonally symmetric, concentric about the pole. What metrics to include? They looked at a combination of temperature and wind speed, but maybe could've looked a some measures of potential vorticity or geopotential perhaps (though of course these are all linked by quasi-geostrophic theory). The strongest average MERRA November wind at 60°N was in 1990 with 25.04 m/s (ranked 1st at 10, 30, 50, 70, 100mb, 2nd at 150mb). 2015 was 2nd at 22.84 m/s. Interestingly, the 10mb speeds were 1990 39.31 m/s vs 2015 38.61 m/s, however very different in NCEP/NCAR - 1990 34.85 m/s vs 2015 37.86 m/s - tricky to be definitive with such large discrepancies. There's a selection of temperature metrics that could be used, but averaged over 6 levels mentioned above, 2012 was coldest at 80°, between 60-90°N and 55-75°N. 1982 at 90°N and 2015 had lowest absolute minimum 50-90°N. Combined rankings - not very scientific and unweighted - gives 2015, 2011, 1982 & 2012 (joint 3rd). December seems more clear cut, 2015 was coldest at 90°N, 80°N and absolute, but 2nd to 1980 for 60-90°N polar cap. Wind speed average was 1st with 30.56 m/s average for all levels ahead of 1986 with 29.86 m/s. However, while 1986 was strongest at 70, 100 and 150mb, 2015 was strongest at 10, 30 and 50mb - obviously the higher wind speed with altitude skews the averages so is not totally conclusive. Combined rankings - 2015, 1980, 1994. Regarding weak November vortices (ie 2016), from MERRA data, overall 2009 was weakest by temperature, 2000 by wind speed (weakest for bottom 4 levels, 2016 weakest at 10 and 30mb). Combining rankings gives 2009, 2016, but then 1996 ahead of 2000. Incidentally, the preponderance of more recent years may possibly have a link to sea ice.... or it may not! Turning to December 2015 in particular, it undeniably featured a very strong stratosphere vortex, but then is there a link to the weather conditions experienced? On the face of it an AO ranking 9th of 67 years (since 1950) would suggest a strong troposphere vortex, perhaps tying in to the Antarctic circumpolar wind analogy locking cold into the Arctic. However, this is a contradiction with the notion of the jet powering towards the Arctic which requires some sort of compensatory flow southwards. This can be seen in this plot of 500mb meridional wind anomaly - Southerly flow into western Europe and the US are clear but so is northerlies for Eastern Siberia, the northwest Atlantic, and from Russia into the Middle East and North Africa. The effects such as record European warmth can be seen below, but also some substantially colder though largely 'empty' regions so not receiving mention - Full details from https://www.ncdc.noaa.gov/sotc/global/201512 Working down through the pressure levels, the NCEP/NCAR data shows that the December 60°N zonal wind was 3rd strongest (of 69 years from 1948) at 100mb, 18th at 250mb, 35th at 500mb, 33rd at 850mb and 36th at 1000mb - so the strength of the stratosphere doesn't seem to have been experienced lower down. Also, the NAO was a relatively modest 0.48 for such a mild December in western Europe (and wet for UK) ranking 23 of 67. The reason appears to be due to anomalous meridional circulation, which is one of the hallmark effects of ice loss theory. edit: with a bit more time a better ranking of monthly values based upon combined standard deviation of wind and temperature anomalies (T80° or T60-90°) gives - Strongest November, T80 - 1990, 2015, 1982 - T6090 - 1990, 2015, 2011 Weakest November, T80 - 2000, 2016, 2009 - T6090 - 2009, 2016, 2000 Strongest December, T80 - 2015, 1980, 2011 - T6090 - 2015, 1980, 2011 Weakest December (for interest's sake), T80 - 1987, 1998, 2000 - T6090 - 1987, 2000, 1998
  3. Just on these two points, how have they been determined?
  4. Red is west, the stronger easterly phase is blue.
  5. Woah, a vacuum with negative pressure! Scary stuff!
  6. This is a classic easterly! Watch how the cold air mass from eastern Europe and beyond, crosses the UK today, easily seen on the meteociel 850 ThetaE chart, as highlighted - In two days time, the air mass is over the mid-Atlantic - And by next Tuesday it reaches Newfoundland, Canada! - The trajectory plot at that time shows that for St John's, air just above 850mb (1300 metres) crosses Scotland today, after coming from northern Ukraine - air a little higher at 1350 metres has come from the steppes of southern Siberia, near Kazakhstan! - The upshot is that for northern Newfoundland, the temperatures go from below -30°C this weekend, to positive territory with heavy rain and sleet into Wednesday next week - Damn these mild easterlies! (Just a bit of fun )
  7. Some great theories above, but it looks like it is the GFS which is starting to smell the coffee with a renewed wave 1 attack like the ECM, rather than the sequence of wave 2 splits which it pulled out of nowhere for a number of runs at day 10 onwards. The ECM or GEOS/MERRA have not shown any great wave 2 amplitude, indeed it is below 'average'. Of course there are theories of vortex splitting from exciting the resonance of the barotropic mode which can occur with relatively low wave 2 forcing, but I have no idea if the GFS can handle this. On the subject of wave number it can be seen from the reanalysis data that wave 1 is dominant for nearly all SSW regardless of whether it is a split or displacement, only 4 in the MERRA data have dominant 10mb wave 2 for the fortnight prior to reversal. This is confirmed by this interesting recorded presentation from the 2015 AMS conference which examines this in more detail and makes the surprising suggestion that wave 1 can actually transform into wave 2 in the stratosphere as they describe - https://ams.confex.com/ams/95Annual/webprogram/Paper256393.html - click on "Recorded Presentation" link to view. With regards SSW-MJO(-SSW?) and in reference to this 10hpa temperature anomaly animation doing the rounds - Yes, at the time of the stratospheric warming in the Arctic, meridional circulation creates a commensurate cooling over the tropics which, theory goes, enhances tropical convection. Except this is at 10mb, there is no convection at this level. The cooling needs to be in the lower strat/upper troposphere (see eg. Kodera (2006) paper Influence of stratospheric sudden warming on the equatorial troposphere) and as can be seen in the plot below, cool equatorial anomalies only feature lower down at the end of run, at around the time of the projected MJO peak amplitude (with possible uplift showing in cool mid-trop anoms). Factor in propagation time for any extratropical waves, then more for vertical propagation and a wave 1 displacement is probably already under way.
  8. Not to say that it will be incorrect, but regarding an SSW induced outlook a note of caution -
  9. He's a bit behind the game considering I posted this on the mod thread 4 days ago - https://www.netweather.tv/forum/topic/87058-model-output-discussion-25th-jan-the-final-third-of-winter-beckons/?page=15#comment-3534630 edit: It's interesting that the Attard GFS forecast animation linked in that post still shows a coupling between the stratospheric 10mb and tropospheric 500mb anticyclone as they move from Alaska into eastern Siberia - http://www.atmos.albany.edu/student/hattard/realtime/dt_500/DT_500.php In light of Cohen's comment about classic downward propagation of weak polar vortex to surface, it is worth considering what mechanism may be at play here, and what is meant by 'classic'. The 'classic' reviews do show some quick response but also more generally over a period of weeks for propagation of anomalous zonal wind, based upon composites of numerous events which blur the mechanisms behind this. This leads to the rather vague 'will it/won't it propagate?' and 'what will be the effects?' kind of questions. At the present time, a better explanation might be found in the work of the likes of, for example, Harnik, Shaw and Perlwitz, with a quick response through downward wave reflection/coupling at the actual time it is occurring.
  10. OK, by identifying 10mb zonal wind minima as the lowest wind speed over a 21 day period (central date +/- 10 days) and then finding the maximum wind speed within the following fortnight, there is a correlation of only 0.06 between wind increase and monthly QBO i.e. no significant relationship. The average increase by month is - Oct 9.83 m/s Nov 16.78 m/s Dec 22.47 m/s Jan 21.50 m/s Feb 20.34 m/s Mar 12.48 m/s Apr 5.00 m/s This displays a clear seasonal signal and the reason should be fairly obvious. The wind speed might be expected to be a balance between temperatures recovering post-warming and the amount of further perturbation and wave activity. Hence the largest increases are found during the period of shortest days when the stratosphere is at its coldest, and with the greatest chance of radiative cooling to help temperatures recover. The GFS 06z forecast of 27 m/s for 15/02/16 is (subject to fluctuation) slightly above average for February or winter average of 21.38 m/s but within 1 s.d. (29.56 m/s) and a lot less than record of 42.13 m/s just last year on 09/02/16 (from 11.53 to 53.66 m/s, QBO 6.77) or 2nd place 40.34 m/s from 04/12/81 (-3.08 to 37.26 m/s, QBO -12.31)
  11. Just to recap the progress of the warming from earlier posts above, with an initial wave 1 followed by the main wave 1, they can be roughly identified in the NCEP blocking chart below as highlighted. It is interesting that the initial Pacific wave 1 was well forecast to day 10 and even noticeable at day 12. However, the Atlantic based wave 1 which has initiated the SSW was poorly forecast beyond day 7, thus perhaps confirming suspicion of Atlantic zonal bias in this instance, with ramifications for the SSW forecast and also obviously for the weather in the UK/Europe in general as the ongoing blocking persists - Though to be fair, if yesterday is confirmed as a reversal, the GFS chart below from 19/01 was pretty accurate -
  12. Except with the Berlin ECM it was on the analysis chart, so it was yesterday!