An interesting read about The Greenland Blocking High of December 2010

http://www.islandnet.com/~see/weathe...1/alm11jan.htm

"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