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boyblue

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    The World's Glaciers

    in Climate Change

    Posted

    ‘There are known knowns. There are things we know that we know. There are known unknowns. That is to say, there are things that we now know we don't know. But there are also unknown unknowns. There are things we do not know we don't know.’

     

    I'll leave it at that.

    • Like 1

    In Depth Model Discussion and Summaries

    in Spring Weather Discussion

    Posted

    hopefully this will copy

    Links for snow information guides

     

    Posted Yesterday, 23:59

    http://forum.netweather.tv/topic/78787-model-output-discussion-17th-dec2013-12z-onwards/page-6

    Seeing as we are discussing the chances of snow, here is some supplementary info regarding various parameters and their relationships with the likelihood of snow... (all from the Met Office, for educational purposes)..

    Probability of snow________________90%____70%___50%___30%___10%

    Surface temp ( C ) _________________+0.3____+1.2___+1.6___+2.3____+3.9

    Height of 0C isotherm agl (hPa)______12______25____35_____45______61

    1000-500hPa thickness (gpm) ______5180___5238__5258___5292____5334 ______ (the "528 line" falls around 40%)

    (mutually exclusive)

    Based on WHEN precip begins, as mentioned above latent cooling of the atmosphere will lower the freezing level once the snow starts falling/melting.

    The wet-bulb freezing level is very important as well...

      Mainly snow Readily turns to snow   Mainly rain   Snow very rare

    Height of theta-w freezing level agl < 1000ft (~300m)               <2000ft (~600m)             >= 2000ft       >= 3000ft  (~1000m)

    It's also important to watch out for cold surface air undercutting warm air.

    A general rule of thumb for the UK is that an 850hpa theta-w of 2C or below can allow for snowfall down to penetrate down to all elevations (with a large number of caveats obviously). Heavy instability and persistent precip with an initial wet-bulb temperature level of as high as +3C can allow snow to reach the surface.

     

    Because of the marginality evaporative cooling can help to deliver a surprise, heres a good article by Philip Eden about this:

     

    The best way to explain what happens in the latter case is to look at an example: the best recent occurrence was on Wednesday 14 January 2004 when a shallow depression tracked eastward across Wales and the Midlands. Ahead of the area of rain and snow, temperatures stood at 5 or 6°C, so why did the rain - turn so readily to snow? The clue was the absence of much wind. Crucial also was the heaviness of the rain. In the UK, nearly all the rain that falls starts off as snow at cloud-level. This is because the temperature normally decreases with height above the Earth's surface. When the air is saturated, according to the laws of physics the temperature drops by 0.5°C for every 100 metres above the ground. Thus on that Wednesday morning the temperature was around zero about 1000 metres above places like Coventry and Northampton an d at that altitude the falling snowflakes would begin to melt.

     

    However, a large amount of energy is needed to melt snow, and high above England on a dull January day the only source of energy is the air itself. This energy is known as 'latent heat' specifically, the latent heat of fusion. Thus the melting snowflakes absorbed heat from the surrounding air, causing the temperature to drop, in turn allowing the falling snow to penetrate lower and lower until it eventually reached the ground. The heavier the precipitation the more energy is absorbed and the more rapid the temperature will drop, but this can only happen when there is no wind to mix the cooled air with "uncooled" air arriving from elsewhere.

     

    This mechanism produced even bigger surprise snowfalls in central and southern England in late-January 2001 and early-January 1994, so forecasters should always be on the lookout for it.

     

    The January 1994 event was a big surprise as I was living in Kingston at the time, the key as PE says is to have little wind to stop too much mixing and heavy precip.

     

    This quote taken from a History of Weather in Epsom and Ewell:

     

    The 6th of January started with heavy rain which turned to sleet and then snow. Some flakes measured 3 inches across and lay up to 6 inches deep in places.

     

    And the archive chart you just wouldn't have ever thought that this could produce such a surprise for the south!

     

     

     

     Steve Murr, on 17 Dec 2013 - 22:47, said:

    yes mate - its not the only parameter to measure-  however 1290 is usually the benchmark -

     

    Probability:............................90%.....70%.....50%.....30%.....10%

    850-1000 hPa(gpm)..............1279.....1287....1293....1297....1302 (un-adjusted)

     

    If we could bet 1293 then that's 50/50- obviously the scaling on the NAE would be better at intervals of 5 instead of 10- however at least it shows roughly where the risk is-

    Theres also an adjustment for height-  so elevated areas can see snow in higher thicknesses.

     

    S

     

    Net Wx Guide information

     

    http://forum.netweather.tv/topic/27989-how-to-try-and-forecast-snow/

    Thanks John, I really enjoyed reading this article & appreciate the work gone into producing this write-up.

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