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03jtrickey

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    January 2004 thundersnow

    in Historic Weather

    Posted · Edited by 03jtrickey

    I remember this event well...I was a chorister at Gloucester Cathedral, and we had just begun our evening rehearsal when the choirmaster told us that the heavy rain that had initially been falling had started to turn to sleet, and soon afterwards there were blizzard-like conditions outside. Frustratingly, I missed seeing most of the snow falling due to being indoors singing the service, which was a shame. Later, outside the cathedral, paved surfaces were completely glazed with sheet ice where the rain had frozen, beneath 1-2 inches of fresh snow. The snow was slightly deeper at home in Cheltenham. It was probably one of the best snow events from my somewhat snow-starved childhood.

    Stratosphere Temperature Watch 2012/2013

    in Spring Weather Discussion

    Posted · Edited by 03jtrickey

    Another thing 03jtrickey, did you try adding a lag time between NAO/AO and CET? I found that the CET maximum correlation on average to be 2 days after NAO/AO.

    Good point - I will add that to my list of things to test out. Thanks Posted Image

    As I went to test that suggestion out I noticed a mistake in my AO/NAO/CET correlations (I'll blame it on copying formulas in Excel late at night). The updated values are as follows and affect the correlations between the CET and AO 60 days after displacement:

    Posted Image

    Using these corrected values, the correlation between CET and AO increases for both displacements and split vortex events after SSW onset, and in both cases the AO is more strongly correlated with CET than the NAO after onset. Sorry about that.

    • Like 1

    Stratosphere Temperature Watch 2012/2013

    in Spring Weather Discussion

    Posted

    Just to add a caveat to the above mean CET anomaly values and emphasise the variability in the data...here are graphs for displacements and splits with the upper and lower quartiles of the CET anomalies plotted for each day.

    The interquartile range has a tendency to increase every time there is is a strongly negative CET anomaly. This might indicate that the mean is being forced down at these points by just a few of the events, so while there could be strongly negative values at these points, it is by no means a certainty.

    Posted Image

    • Like 1
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    Stratosphere Temperature Watch 2012/2013

    in Spring Weather Discussion

    Posted · Edited by 03jtrickey

    I have been doing some analysis today of the relationships between Central England Temperature (CET), NAO and AO in the 60 days before and after vortex displacements and splits.

    Here are some results:

    Posted Image

    Posted Image

    Some initial comments:

    Mean CET anomalies are relatively lower in days 60-30 prior to splits compared to displacements.

    There is a large peak in the CET anomaly prior to both displacement and split vortex events, but for displacement events this occurs 10 days earlier and is followed ~10 days later by a negative anomaly just before SSW onset (10hPa wind reversal).

    Both displacements and splits are, in the mean, followed by swings between positive and negative CET anomalies. The displacement event mean CET anomalies are almost the inverse of those for the split vortex events from ~days -5 to day 25 (i.e. the alternating pattern is shifted slightly earlier for displacements), but both splits and displacements have their strongest negative CET anomaly around day 40. I'm not sure what the exact onset of the previous SSW was (anyone?) but first few days of Jan +40 days would take us up to ~10th+ of February which would tie in well with the expectations of others on this forum.

    With regard to the AO and NAO, people have been commenting recently that the NAO in particular hasn't correlated well with the cold we have been getting. I did some simple r^2 correlation calculations for mean values of NAO against AO, CET against NAO and CET against AO in the periods before and after both types of SSW.

    In the case of vortex displacements, the daily AO values appear to become much less correlated with the NAO after displacement, and neither index is strongly correlated with CET temperature. In the case of vortex splits, AO also becomes less correlated with NAO after displacement, but the AO correlates quite a lot more highly with CET than the NAO does. Could this suggest that the AO is more important for post-split-SSW cold? (NB the correlations might increase if they were taken over longer than daily time periods...)

    Investigating the cause of these patterns would be interesting & I have yet to think about how they may relate to the precursors identified by Martius/Cohen and Jones. I will also probably do some more detailed analysis at some stage, to look at individual years, test correlations for different time periods, and look at the relative frequency of different anomaly values etc. However I would be interested to hear your thoughts (and please let me know if you see anything that is obviously erroneous) Posted Image

    Methods:

    Dates of SSWs analysed are from Cohen and Jones (2012) and only events with no overlap between the onset +/-60 day periods have been included. The dates used are therefore as follows:

    Displacements: 30/11/1958, 16/01/1960, 08/12/1965, 02/01/1970, 29/02/1980, 04/12/1981, 24/02/1984, 23/01/1987, 16/12/2000, 02/01/2002, 07/01/2004, 24/02/2007, 22/02/2008

    Splits: 30/01/1958, 23/03/1965, 24/02/1966, 08/01/1968, 02/02/1973, 22/02/1979, 02/01/1985, 22/02/1989, 18/01/2003, 21/01/2006, 24/01/2009, 09/02/2010

    The Daily CET anomalies were calculated relative to the 11-point binomially smoothed climate normals for 1961-1990 from Parker (2009) "Anomalies of Central England Temperature Classified by Air Source".

    NAO/AO data is from the NOAA website (ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.ao.index.b500101.current.ascii and ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.nao.index.b500101.current.ascii).

    • Like 10

    Technical Teleconnective Papers

    in Spring Weather Discussion

    Posted · Edited by 03jtrickey

    I've got to write three extended essays for my Geography degree...and I can write about pretty much anything. One of my modules was on seasonal prediction so I'm going to attempt something along the lines of "Will improved knowledge of stratosphere-troposphere interactions increase the skill of seasonal forecasts for UK winters?". I've really enjoyed following the stratosphere thread this winter (many thanks to chionomaniac & GP) so I thought I'd do something that's topical and that I'm interested in!

    I've just come across a very recent paper published online on 13 January 2013 which may be of interest.

    http://www.nature.co...l/ngeo1698.html

    "Enhanced seasonal forecast skill following stratospheric sudden warmings

    Advances in seasonal forecasting have brought widespread socio-economic benefits. However, seasonal forecast skill in the extratropics is relatively modest1, prompting the seasonal forecasting community to search for additional sources of predictability2, 3. For over a decade it has been suggested that knowledge of the state of the stratosphere can act as a source of enhanced seasonal predictability; long-lived circulation anomalies in the lower stratosphere that follow stratospheric sudden warmings are associated with circulation anomalies in the troposphere that can last up to two months4, 5. Here, we show by performing retrospective ensemble model forecasts that such enhanced predictability can be realized in a dynamical seasonal forecast system with a good representation of the stratosphere. When initialized at the onset date of stratospheric sudden warmings, the model forecasts faithfully reproduce the observed mean tropospheric conditions in the months following the stratospheric sudden warmings. Compared with an equivalent set of forecasts that are not initialized during stratospheric sudden warmings, we document enhanced forecast skill for atmospheric circulation patterns, surface temperatures over northern Russia and eastern Canada and North Atlantic precipitation. We suggest that seasonal forecast systems initialized during stratospheric sudden warmings are likely to yield significantly greater forecast skill in some regions."

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