Interitus

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  1. There is just far too much importance being applied to the QBO. The time-latitude chart below shows the zonal wind anomalies at the 30mb level from Jan 2014 to end of March 2017 - The phases of the QBO can seen in the tropical anomalies on the left hand side, and the seasonal winter stratospheric vortex development in high latitudes on the right. There is no transfer of QBO anomalies towards the midlatitudes, with the anomalies effectively meridionally confined below 20°N. The reason is that meridional wind flow is very low, in the region of less than a couple of m/s at most. The Holton-Tan relationship describes a link between a weaker winter time vortex with an easterly QBO but this is not because of a transfer of momentum, it is because of the effect of the critical line i.e. zero or easterly flow, on planetary waves generated from the troposphere. These are prevented from moving equatorwards and are directed more towards the pole so more effectively weakening the vortex. The effect of QBO phase can then be seen in stratospheric northern annular mode (NAM) and thence in the tropospheric AO/NAO. This is restricted to wintertime however, as planetary wave propagation is greatly limited by summer polar easterly flow, requiring the presence of the vortex westerlies from autumn onwards. This also affects the major stratospheric meridional motion - the Brewer-Dobson circulation is generated by planetary waves during the winter half of the year, and does provide a northward flow which as it descends adiabatically warms the polar stratosphere significantly above what would be expected from winter radiative cooling. But it is largely above the 30mb level and the velocity is very, very low - tracers such as tropical ozone show that the age of polar stratospheric air may be 4 or 5 years.
  2. Tied in to possibility of active strat final warming coupling with troposphere; here is this morning's GFS (from weatheriscool) showing potential downward progression of zonal wind anomalies -
  3. For a while the GFS has been forecasting an 'active' final warming - active in the sense that it there is a dynamic wave-1 forced baroclinic vortex to produce above average temperatures / wind reversal, rather than purely 'passive' warming as the vortex dissipates through radiative relaxation from increasing insolation. Indeed, some ensemble members forecast wind reversal greater than summer values and the stronger this warming the less likely a weak westerly will return this spring. Generally an effect on the troposphere is unclear, though a couple of runs have suggested a S-T coupling as shown in the downward progression of zonal wind anomaly in the right hand chart below, from 29/03 06z
  4. The above was posted in March 2014 and indeed it went on to become the warmest annual CET on record. With this year now nearing the end of March ahead of target, provisionally requiring 1.39°C for the rest of the year compared to the record annual anomaly of 1.44°C, it may be time to keep an eye on this.
  5. Good reply, but just a couple of thoughts. The height of cumulonimbus clouds are restricted by the tropopause, where the cloud spreads out forming the characteristic anvil shape. There can be a degree of overshoot, but cumulonimbus don't reach 49,000 feet over the UK so lowering the estimated distance. However on the other hand, at night in particular, high cloud layers can reflect the light from the lightning which may otherwise be over the horizon.
  6. This would appear to be a misperception which has happened frequently recently when a seemingly unremarkable month is much warmer than average. According to the MetO Hadcet pages to March 28th, the provisional minimum anomaly is 2.3°C - http://www.metoffice.gov.uk/hadobs/hadcet/cet_info_min.html But the maximum anomaly is 2.7°C - http://www.metoffice.gov.uk/hadobs/hadcet/cet_info_max.html
  7. The finalised MERRA-2 figures give an SSW on 01/02/17 by just -0.07 m/s! 26/02/17 was +0.8 m/s
  8. It depends on how a final warming is defined, but only 3 times in the MERRA data since 1979 has the 10mb 60°N wind become easterly in March and remained so onwards for the rest of the summer. Last year was the earliest on 05/03, a whole two weeks earlier than 19/03/86, then 24/03/85. Further to the previous post that the average 10mb 60°N wind speed becomes easterly on 12th April, the actual average date is 17th April. The latest are 09/05/09 (after record SSW in Jan), 10/05/01 and finally 13/05/81. Looking at the analogues, the suspicion is that this year it will probably be at least a month away, towards the end of April.
  9. Is it possible to embed a full twitter conversation made up of a number of tweets with images, as displayed on twitter itself when just the using the link?
  10. The average date of 10mb wind reversal in the MERRA-2 reanalysis data is 12th April. Research shows that sometimes the final warming occurs first in the mid-stratosphere eg 10mb and other years in the upper stratosphere and working down. Hardiman et al. categorise the FW by which level they occur at first - 1mb or 10mb, and suggest that this has implications for tropospheric effects and forecasting - http://onlinelibrary.wiley.com/doi/10.1029/2011JD015914/epdf
  11. Yes, possibly. Where it does become interesting is in the future when composites and research make use of SSW events, usually little thought is given to the actual details, just whether it was a 60°N 10mb reversal.
  12. Following the 10mb 60°N zonal wind reversal criteria, there was an SSW on 1st February according to the ECM analysis at the time (?? m/s) and the MERRA2 reanalysis (-0.63 m/s), but wind speed just fell short on NCEP/NCAR reanalysis, with reversal at 62.5°N (-0.89 m/s) but not 60°N (+1.48 m/s). This was forecast reasonably well. A follow-up 2nd wind reversal or separate SSW - arbitrary criteria based typically on being at least 20 days apart - has been repeatedly forecast but hasn't happened. However, not sure that this is all down to poor modelling or just a bit of luck. With the event in the image above, on the 26th February the ECM analysis was 0.53 m/s, MERRA2 was 0.72 m/s and NCEP/NCAR 1.87 m/s. Fine margins.
  13. But the next day - Hard to make any meaningful predictions with such variable output, but it also raises questions about what would be assumed to be downward propagation. How does this manifest itself, and is that what is being witnessed in the second image or is it just coincident with tropospheric variability giving polar height rises for the trop and lower strat?