Sutcliffe Development Theory
Notes relating to atmospheric development diagnosed using total thickness (TTHK) charts.
[ 1. This note was put together to give a 'flavour' of the ideas surrounding diagnosis of development from thickness charts. If you have come to this page as a first-year student of meteorology, then this is NOT for you! ]
[ 2. Inevitably, given the history of the times (1940's), the efforts of meteorologists on the 'winning' side has assumed dominance. However, it is clear that much work was done in Germany both before and during the Second World War, and this contribution should be remembered. If I find more in this, I will add it.]
The science of forecasting has come a long way since the days of ancient weather lore and a belief in the whims of the gods! During the mid-19th century, the birth of a scientific basis to weather forecasting was witnessed, with the use of the developing electric telegraph networks to exchange data, the discovery of 'laws' relating the wind field to the pressure distribution (Buys Ballot, 1857), and later in the century, the analysis of weather types associated with depressions and anticyclones (Abercromby, 1883).
During the 'Great War' of 1914-1918, as is well-known, the 'Norwegian' frontal & air-mass theories were thoroughly researched and once hostilities ceased, they were enthusiastically adopted & developed by the leading meteorological services around the world - indeed, perhaps a bit too enthusiastically, as they didn't necessarily apply to sub-tropical / tropical regions or indeed to all occasions in the mid-latitudes. Not only that, work on the dynamic basis for atmospheric development tended to be somewhat overshadowed.
Nevertheless, the work of Bjerknes (father & son), Bergeron & Solberg formed the basis of 'front-line' forecasting work up to and including the Second World War. To produce a 'PROG' of the weather 18 to 24 hours ahead involved use of empirical techniques which moved the fronts based on gradients across them, moved the lows (or highs) following continuity and rules based on flow patterns around these features, and in large part, experience of situations past: the upper air (even if it was available), didn't get much of a look-in in the process.
The foregoing might imply that somehow the 'upper air' was ignored! Not a bit; much work was undertaken using primitive kite and balloon ascents during the early part of the 20th century, and increasing air-flights produced more information. Meteorologists realised that to understand & predict the 'weather', they would need more information on & understanding of, air flow well above the surface. The problem was - lack of data!
The Second World War provided the impetus (and the data) for research into upper air patterns and their influence on the surface weather. As part of the procedure for analysis and forecasting of upper air patterns, thickness charts (partial and total) became the stock-in-trade of weather services (particularly for RAF Bomber Command, the USAAF & the Luftwaffe), and out of these charts came the ideas of development theory tied to forcing aloft - R.C. Sutcliffe in the United Kingdom had already researched this immediately prior to the outbreak of war (alongside the work of others, particularly Brunt & Petterssen), and by the latter half of the 1940's he had enough data to publish his seminal work in 1947 (see references below).
Sutcliffe showed that development (expressed in terms of relative divergence between 1000 and 500 hPa levels) can be diagnosed from total thickness charts: in crude terms, the equation can be expressed as . . . .
[div(500-1000)] = [LAT] + [STEER] + [DEV]
div(500-1000) represents the relative divergence through that column.
[LAT] .. the 'latitude' term
[STEER] .. the 'steering' term
[DEV] .. the 'developmental' term
the thermal wind (500-1000hPa) appears in each term, so the stronger the thermal wind (i.e. the tighter the thickness gradient), then the more effective is the vorticity-driven development that takes place.
The [LAT] term, which diagnoses the variation of the Coriolis parameter with latitude in the direction of the thermal wind - is generally small, but is important for example in the creation of the notorious 'Scandinavian High' in winter, and the significant areas of low pressure following cold outbreaks from the north over the west & central Mediterranean in late autumn / early winter.
The [STEER] term is proportional to the strength of the thermal wind and to the variation of surface vorticity in the direction of the thermal wind. This term is dominant when the pattern of surface vorticity is well marked & the thermal wind almost 'zonal' (or running west-to-east), i.e. immediately before distortion of the Polar Front undergoing wave-development. This confirms the subjective assessment that small-scale mid-latitude lows are 'driven' along coupled to the thickness gradient aloft - tighter gradient, swifter movement.
The remaining term [DEV] is proportional to the strength of the thermal wind and to its variation of vorticity along the flow. Curvature and shear of the thickness pattern contribute to the latter, and give rise to the 'standard' developmental patterns indicated below.
With developing low pressure at the surface, horizontal convergence at low-levels implies upward motion through the troposphere, and divergence aloft (in the region of the tropopause). The converse applies for developing high cells. The circulations are indicated on this classic diagram:-
The work by Sutcliffe (and others) can be summed up neatly in a graphical format; there are four basic patterns of thickness isopleths (indicated below). With each pattern, there is associated a major and minor (vorticity-driven) surface development area.
Chart showing developmental regions associated with thickness (500-1000 hPa) patterns.
C: cyclonic development (circled = major / most effective forcing)
A: anticyclonic development (circled = major / most effective forcing)
Green-dashed lines: approximate thermal ridge / trough axes
Magenta arrows: direction / proportionate strength of thermal wind
For more on all these matters, see the references below.
'A Contribution to the Problem of Development', R C Sutcliffe; QJRMetS/RMetS, 73, 1947
'The Theory & Use of Upper Air Thickness Patterns in Forecasting', R C Sutcliffe & D Forsdyke; QJRMetS/RMetS, 76, 1950
'Weather Map', Meteorological Office/HMSO, 1956
'The Meteorological Glossary', Meteorological Office/HMSO, 1972
'Dynamical meteorology: some milestones', B W Atkinson; RMetS (in "Dynamical Meteorology, An introductory selection"), 1981
Selected glossary of terms:
Buys Ballot's Law: as originally formulated: " if you stand with your back to the wind (in the northern hemisphere), then low pressure lies on your left-hand side ". This gives rise to the standard patterns (in the Northern Hemisphere) of anticlockwise winds circulating around a low pressure area, and clockwise motion around an area of high pressure (reverse for the Southern Hemisphere).
Convergence: When air flows in such a way that the area occupied by a particular 'group' of air particles lessens ('drawing together'), the pattern is said to be convergent. Convergence in the atmosphere is associated with vertical motion, and hence development (or weakening) of weather systems. For example, convergent flow near the surface is coupled to, and may be the primary cause of, upward motion, leading to cloud formation/shower initiation etc.
Coriolis parameter: as a consequence of Earth's rotation, air moving across its surface appears to be deflected relative to an observer standing on the surface. The 'deflection' is to the right of movement in the northern hemisphere, to the left in the southern hemisphere. (also known as the Coriolis acceleration, or deflection)
Divergence: When air flows in such a way that the area occupied by a particular 'group' of air particles grows ('spreads apart'), the pattern is said to be divergent. Divergence in the atmosphere is also (along with convergence/q.v.) associated with vertical motion, and hence development (or weakening) of weather systems, depending upon the level where the divergence is dominant in a particular atmospheric column. For example, divergent flow aloft is coupled to, and may be the primary cause of, upward motion, leading to widespread cloud formation/cyclogenesis etc.
Norwegian model: The classical idea of a travelling wave depression on the polar front running forward (usually west-to-east) and deepening, with the cold front moving faster than the warm front, thus 'occluding' the warm sector, with the parent low slowing / turning to the left (in northern hemisphere), and filling up.
Thermal wind: a theoretical (vector difference) wind that relates the magnitude of the horizontal temperature gradient in a defined layer to the real winds that blow at the top and base of that layer. The speed of the thermal wind is proportional to the temperature gradient.
Thickness (or Relative Topography): The difference in height between two layers in the upper air. The most commonly used being the thickness between 500 mbar (or hPa) and 1000 mbar (or hPa), and normally expressed in dekametres. The larger the value of thickness, the warmer the column of air.
Tropopause: the (usually) abrupt change from falling temperatures with height in the tropopause, to near-uniform, or rising temperatures in the stratosphere.
Troposphere: lowest layer of the atmosphere, with an average depth of 16 to 18 km around the equator, 9 to 12 km temperate latitudes and well below 9 km much of the time in arctic regions. There is a general fall of temperature with height (i.e. a positive lapse rate), with an average value of some 6.5 degC / km (or 2 degC / 1000ft).
Vorticity: a measure of the 'spin' of a portion of a fluid - in our case, of atmospheric particles. Vorticity in a cyclonic sense is designated 'positive', and in an anticyclonic sense is designated 'negative'. In synoptic meteorology, we often only consider vorticity in a horizontal plane - i.e. the 'spin' behaviour of air particles as they move along in the atmospheric flow as depicted on classical 'weather maps'.
Zonal: A predominantly west-to-east airflow is termed zonal (and an east-to-west airflow is negative zonal). The strength of the flow in any sector may be expressed in terms of a zonal index given by the difference in average contour height between two latitude circles through the sector.