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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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An interesting read about The Greenland Blocking High of December 2010
http://www.islandnet.com/~see/weathe...1/alm11jan.htm |
#2
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"Teignmouth" wrote in message
... An interesting read about The Greenland Blocking High of December 2010 http://www.islandnet.com/~see/weathe...1/alm11jan.htm Quotes from the article above. "A blocking high is defined as an anticyclone (a cell of high surface pressure) that remains stationary or nearly so and blocks the normal movement of migratory cyclones across the latitudes. The warm air of a blocking high extends high into the troposphere (increasing the heights of the various pressure surfaces) and thus alters the upper air wind flows to travel around the high" "Meanwhile, under the Greenland High, which reached a maximum central barometer pressure of at least 1068 mb (31.54 inches) on 15 December, temperatures in southern Greenland rose to around 4oC (40oF). A ship plying the North Atlantic under the High reported an air temperature of 15.6oC (60oF)". While I found the article quite interesting, there are a couple of issues I would wish to challenge. In the 1st para (above) from the article, a blocking high as defined there, is just a manifestation of the blocking pattern in the upper troposphere. It is this pattern that steers cyclonic developments around the high, due to the jet stream that is generated by the high pressure aloft. It is not the warm air of the blocking high that alters the wind flows to travel around the high, but the horizontal temperature contrast in the upper troposphere/lower stratosphere, juxtaposing cold air over the blocking high below with relatively warm air to its north. Centres of high pressure at any level in the atmosphere are overlain by anomalously cold air, and centres of low by anomalously warm air, but it is important to understand that these anomalies do not have to be in the troposphere. In the case cited by the author, a quick check of the ascents for southern Greenland for the period 13th to 15th December 2010 shows that the rise in surface pressure is linked to cooling in the stratosphere, mostly above 30 mbar, and also in the 100 to 300 mbar layer. A thickness budget calculation on the data form 04270 and 04360, both in southern Greenland, is shown below. Note that a decrease in thickness with raise the pressure (heights) below, and increasing thickness will lower them. All values are 24 hour thickness change (except 100mb height change) in Decametres. 04270 1000 ht 300/1000 100/300 30/100 above 30 13/00 to 14/00 +18 +9 -13 -2 -12 14/00 to 15/00 +10 +18 -6 -3 -19 04360 13/00 to 14/00 +2 -4 +11 +1 -10 14/00 to 15/00 +14 +16 -18 +3 -15 Station 04360 is on the east coast of Greenland, and in the first period experienced a rise in thickness in the lower stratosphere, 100/300 layer, as an upper trough crossed, which also advected some colder air in the lower troposphere at that time. The important thing to notice is that over the 2 day period the atmosphere above 300 mb had cooled by 55 dam at 04270 and by 28 dam at 04360. Without the warm advection in the troposphere mentioned in the article, the 1000 mbar height would have risen by those amounts, instead of 28 and 16 dam resp. In fact, the increase in thickness in the troposphere over the same period amounted to 27 dam at 03270 and 12 dam at 04360. This increase in tropospheric thickness, far from causing the blocking high, was reducing its strength. In para 2 above, I would take issue with the quote that the Greenland high reached at least 1068 mbar. I have pointed out elsewhere, that these extreme high values over the Greenland plateau are an illusion brought about by reducing the pressure at the surface over the plateau to sea level. The resulting msl pressure will depend only on the value of temperature chosen to assign to the fictitious layer between the plateau and sea level and has little physical meaning. The author also quotes a pretty useless fact, that a ship under the high had a temperature of 15.6C. Without being more exact about its actual location, that is a meaningless quote. -- Bernard Burton Wokingham Berkshire. Weather data and satellite images at: http://www.woksat.info/wwp.html |
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