On Wednesday, 30 October 2013 21:16:37 UTC, Bernard Burton wrote:
"Will Hand" wrote in message ... From the excellent blog by John Mason of ukweatherworld.co.uk http://www.geologywales.co.uk/storms/autumn13b.htm (near the end after some lovely photos). Will -- http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm Will Hand (Haytor, Devon, 1017 feet asl) --------------------------------------------- Some insight into the structure of the 28th Oct low crossing southern UK can be gained from an analysis of the thickness budget over the low centre and its change with time. Changes in surface pressure, or of that at any level in the atmosphere, come about as a result of changes in the thermal structure of the atmosphere above. Falling pressure at a level is a result of warming, or an increase of mean thickness, in the atmosphere above that level, while rising pressure results from falling thickness above. To this end, upper air analysis charts for 12 hour intervals between 27/1200 and 28/1200 and for 1000, 500, 300, 100 and 30 mbar were used, obtained from the University of Wyoming weather site: http://weather.uwyo.edu/upperair/ The table below shows the change in thickness over the low centre at 12 hour intervals between 27/1200 and 28/1200 for the following constant pressure levels in the troposphere, 500 to 1000 mbar (A), 300 to 500 mbar (B) and for the stratosphere, 100 to 300 mbar (C), 30 to 100 mbar (D) and above 30 mbar (E). Change in thickness for levels A to E, . 27/12 to 28/00 A -6, B -5, C +10, D 0, E +11 28/00 to 28/12 A -4, B -2, C +9, D +1 E +8 Total change A -10, B -7, C +19, D +1, E,+19 Net change +decametres22 Tropospheric change -17, stratospheric change +39 Change in surface pressure at low centre, -26 mbar, (993 to 967 mbar). Equivalent change in 1000 mbar height, -22 dam (-6 to -28 dam), brought about by the net change in thickness of +22 dam. What is important to note is that the changes in thickness over the low come about by different means at the various levels in the atmosphere above. In the troposphere, where the total thickness between 300 and 1000 mbar falls by 17 dam, advection and dynamics is moving a thermal tongue or wave towards the ENE. Mass ascent through the troposphere will be releasing latent heat, though most of this will be in the precipitation shield ahead of the surface low, but the thickness budget shows that heat is being lost throughout the troposphere over the low, implying advection of cooler air in the thermal tongue, probably plus physical processes arising from the precipitation process, and by radiation losses in the column. By the time the low was over the North Sea, the surface warm tongue was occluding out, and probably accounts for the modest but relatively larger fall in level A than B in this period as cooler lower tropospheric air advected over the centre. So, what leads to the continuing fall in surface pressure in the face of tropospheric cooling over the low? The answer lies in the stratosphere, where in the lower stratosphere (level C) we see a change of +19 dam, associated with the approach of the upper trough and its development, and also change of +19 dam in the atmosphere above 30 mbar (level E). This latter comes about entirely as a result of the advection of the tropospheric warm tongue across the ESE thickness gradient above 30 mbar, from lower to higher thickness values. Although a much more devastating storm could have evolved with the given situation, should the trough in the upper troposphere have been more dynamically active, causing a greater rise in thickness in level C, in this instance much of the change in surface pressure arises jointly from the rapid translation of the tropospheric warm tongue beneath a pre-existing thermal gradient in the upper atmosphere, and a more modestly active trough in the upper troposphere as it advanced towards the surface low from an upwind direction. -- Bernard Burton Wokingham Berkshire. Weather data and satellite images at: http://www..woksat.info/wwp.html

Thanks for those most interesting figures, Bernard, but there is one thing that puzzles me.
A thickness value for "above 30 mb" (E) is given but no upper limit to the layer is given. Surely a thickness must be specified between two defined pressure levels. The figure quoted appears to have been obtained by subtraction from the indicated total change. Could you clarify this?

Tudor Hughes, Warlingham, Surrey.