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uk.sci.weather (UK Weather) (uk.sci.weather) For the discussion of daily weather events, chiefly affecting the UK and adjacent parts of Europe, both past and predicted. The discussion is open to all, but contributions on a practical scientific level are encouraged. |
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#1
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.... and another routine question that pops up about this time of year
(and in this situation) relates to thickness (500 - 1000 hPa variety) *extremes*. This is covered in the FAQ he- http://weatherfaqs.org.uk/node/67 and by following the link at the bottom, you find a diagram giving an 'idea' of the extremes at a selection of locations for each month. Note the caveats at the bottom. The latest GFS run (12Z as I write this) out to T+180 has a large area sub-512 dam across Wales, most of Scotland and large area of north & central England. This would appear to be nudging the lower boundary of those figures for November ... and the series would have included the cold spells during November 1985 which, in some respects, is an approximate analogue to the current set-up, though note that in that month the *whole* month was cold, not just the latter third. However, this is a long way off! Experience would suggest that models tend to 'over-egg the pudding' particularly by 'over-advecting' model cold air downwind - and of course we *are* dealing with the 'model' atmosphere not the real one! Tricky thing - reality. Martin. -- Martin Rowley West Moors, East Dorset (UK): 17m (56ft) amsl Lat: 50.82N Long: 01.88W NGR: SU 082 023 |
#2
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On 21/11/10 21:40, Martin Rowley wrote:
... and another routine question that pops up about this time of year (and in this situation) relates to thickness (500 - 1000 hPa variety) *extremes*. This is covered in the FAQ he- http://weatherfaqs.org.uk/node/67 and by following the link at the bottom, you find a diagram giving an 'idea' of the extremes at a selection of locations for each month. Note the caveats at the bottom. The latest GFS run (12Z as I write this) out to T+180 has a large area sub-512 dam across Wales, most of Scotland and large area of north& central England. This would appear to be nudging the lower boundary of those figures for November ... and the series would have included the cold spells during November 1985 which, in some respects, is an approximate analogue to the current set-up, though note that in that month the *whole* month was cold, not just the latter third. However, this is a long way off! Experience would suggest that models tend to 'over-egg the pudding' particularly by 'over-advecting' model cold air downwind - and of course we *are* dealing with the 'model' atmosphere not the real one! Tricky thing - reality. Martin. Out of interest, typically what surface temperatures, daytime and nighttime would you expect for, say, a 510dam thickness assuming clear skies? Can't remember experiencing a thickness that low before. |
#3
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"Martin Rowley" wrote in
: The latest GFS run (12Z as I write this) out to T+180 has a large area sub-512 dam across Wales, most of Scotland and large area of north & central England. This would appear to be nudging the lower boundary of those figures for November ... and the series would have included the cold spells during November 1985 which, in some respects, is an approximate analogue to the current set-up, though note that in that month the *whole* month was cold, not just the latter third. Martin I've gone back to my thickness study I stuck on here last month - top 10 lowest thicknesses I could find from the NCEP reanalysis (which I realise will have its errors, and I've only looked from 1960-2000) is - and these are daily mean thickness for a gridpoint in west/central England: Date TT500 22/11/1965 517.2 19/11/1971 519.2 19/11/1962 519.4 21/11/1993 519.4 29/11/1969 519.7 23/11/1993 520.7 28/11/1985 520.8 22/11/1993 520.8 28/11/1980 521.2 25/11/1978 521.3 Richard |
#4
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"Adam Lea" wrote ...
snip Out of interest, typically what surface temperatures, daytime and nighttime would you expect for, say, a 510dam thickness assuming clear skies? Can't remember experiencing a thickness that low before. .... In the January 1987 severe cold spell, when thickness values dropped below 500 dam (probably, briefly, around 495dam), then widespread day maxima below -6degC were recorded across southern Britain .. (12th). I developed a diagram he- http://weatherfaqs.org.uk/node/155 which attempted to relate theoretical maxima to total thickness; this was based on various papers written back in the 1950s & 1960s when this method of forecasting extremes was all that was used ... things have moved on, and in particular, we now use algorithms based on vertical slices of the atmosphere below 1500 m, and in these extreme cases (i.e. very low thickness values), the lowest few tens of metres of troposphere often becomes 'uncoupled' from the general air mass and both maxima and minima can depart wildly from any relationship developed on a layer meaned over 5km. See also the notes at the bottom of the referenced/linked page. On the continent last winter (09/10), a cold pool [defined by the 500-1000 hPa thickness] of ~510dam drifted across southern Poland and parts of Germany (Saxony, Thuringia etc.) 18th/19th December 2009: remembering that this area is in the middle of a continent, and was already well-chilled/snow-covered, the typical day maxima were -14 and night minima -17 (over the lower ground). In our part of the world, given the more 'maritime' character of our weather, I wouldn't expect anything like such low values, at least not for the maxima; minima, as pointed out above, behave completely differently, especially over snow-covered surfaces. Martin. -- Martin Rowley West Moors, East Dorset (UK): 17m (56ft) amsl Lat: 50.82N Long: 01.88W NGR: SU 082 023 |
#5
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"Richard Dixon" wrote ...
I've gone back to my thickness study I stuck on here last month - top 10 lowest thicknesses I could find from the NCEP reanalysis (which I realise will have its errors, and I've only looked from 1960-2000) is - and these are daily mean thickness for a gridpoint in west/central England: Date TT500 22/11/1965 517.2 19/11/1971 519.2 19/11/1962 519.4 21/11/1993 519.4 29/11/1969 519.7 23/11/1993 520.7 28/11/1985 520.8 22/11/1993 520.8 28/11/1980 521.2 25/11/1978 521.3 Richard .... interesting, thanks Richard: *if* the GFS output is correct (still indicating sub-512 dam), then we'll be testing all of those values come this weekend! Martin. -- Martin Rowley West Moors, East Dorset (UK): 17m (56ft) amsl Lat: 50.82N Long: 01.88W NGR: SU 082 023 |
#6
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?"Dave Cornwell" wrote in message
... I can vouch for the havoc caused by the 520 thickness at exactly this time of year in 1993. I was due to fly from Heathrow I hadn't realised that the thickness was so high then! I remember the snow well - it started bucketing down at school and of course any hope of us lot concentrating was dashed - especially as the French teacher kept looking out of the window too! Eventually a Tannoy announcement was made that all pupils who live on the Isle of Sheppey were to go home immediately and buses were on their way. As I waited for the bus the snow was dripping from the rose bushes planted at the front of the school and one of my friends reckoned it was because they were photosynthesising and warming up! (He went on to get 4 As at A-level, must have been a blip that day). Back home there was around 3 inches of slushy snow and by the end of it we had just under 6 inches - which froze solid overnight. The snow stayed in the ditches by the fields for a good week or so before finally melting. 850s seem to have been around -10C (briefly) and my end of the (9 mile by 5) island got far more snow than the other end simply because of the onshore wind... SSTs were around 13C but all it did was add fuel to the clouds. This time around? Who can say, but I'm expecting to see at least some snowfall despite slightly higher 850s and a slightly cooler sea. |
#7
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"Richard Dixon" wrote ...
I've gone back to my thickness study I stuck on here last month - top 10 lowest thicknesses I could find from the NCEP reanalysis (which I realise will have its errors, and I've only looked from 1960-2000) is - and these are daily mean thickness for a gridpoint in west/central England: Date TT500 22/11/1965 517.2 19/11/1971 519.2 19/11/1962 519.4 21/11/1993 519.4 29/11/1969 519.7 23/11/1993 520.7 28/11/1985 520.8 22/11/1993 520.8 28/11/1980 521.2 25/11/1978 521.3 Richard .... have just looked at the soundings from yesterday (27th November, nominal time 12Z), and at Lerwick [Shetland] the total thickness (500-1000 hPa) was 507 dam, and at Albermarle [Northumberland] it was 511 dam. Using the data published in the FAQ (reference elsewhere in the thread) then these would appear to fall *outside* the minimum values implied from that series for November (but note caveats regarding period used etc.). Following Richard's post (above), although we don't have an explicit value for 'west/central' England (somewhere like the old Aughton), I reckon we've gone below even the lowest value he quoted (517 dam November 1965), with Watnall/Nottingham (east Midlands) 517 dam at 27/00Z and 513 dam at 28/00Z; the coldest air would have passed over them late yesterday. I made a comment that reality doesn't always accord with model at the range of the forecast ( roughly a week ahead ). I am very happy to note that the model (GFS in this case) played a blinder & I should have had more faith! A superb indication on the 'depth' of the cold air (in terms of how cold) and where the cold air would be. Martin. -- Martin Rowley West Moors, East Dorset (UK): 17m (56ft) amsl Lat: 50.82N Long: 01.88W NGR: SU 082 023 |
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