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Appearance in Satellite Data

Deformation is a feature of a varying wind field, changing cloud formations and humid areas affected by it.

In satellite images Deformation Band is a cloud line which

  • elongates, becoming narrower and longer, usually keeping its orientation
  • thins out in the middle, eventually breaking
  • can be straight or slightly arched

Deformation zones are generated by opposing flows, where cloud systems of two synoptic or meso-scale cloud systems are brought closer to each other. A deformation zone needs a cyclonic and an anticyclonic circulation to form. As anticyclonic circulations tend to be rather dry, characteristic patterns for deformation zones can be best seen in water vapour image loops. After the deformation stage the cloudiness dissipates or merges with other cloudiness.

The deforming band elongates in the direction of the upper level flow. Often there is sinking motion on the poleward side of the cloud band. This sinking air can be seen spreading in both directions with the upper level wind field.

Typical appearances for Deformation Bands in different channels:

  • In VIS images grey, partly translucent fibres with well confined, even sharp, edges
  • In IR images grey or white narrow, fibrous cloud band with well confined edges
  • In WV images grey or white band, often with dark area on the poleward side
  • In RGB combination images the Deformation Cloud Band can be distinguished from the background and nearby cloud systems more clearly than in the single channel images:
    • In WV6.2-WV7.3; IR9.7-IR10.8; WV6.2 combination the cloud band is seen white over a blue or green background. The sinking air polewards of the cloud band is seen as a reddish band.

On the 18th of February there were remains of an occluded cloud band, that became a Deformation Band stretching from Poland to White Sea:

18 February 2009/00.00 UTC - Meteosat 8 IR 10.8 image
18 February 2009/00.00 UTC - Meteosat 8 WV 6.2 image
18 February 2009/00.00 UTC - Meteosat 8 WV6.2-WV7.3/IR9.7-IR10.8/WV6.2 combination image

The development of the Deformation Band can best be seen in an animation:

18 February 2009/00.00 - 07.00 UTC - Animation Meteosat 8 WV 6.2 image;

In this loop a Deformation Band stretching from Poland to White Sea gradually narrows and elongates while being stationary.

Appearance in AVHRR imagery

AVHRR images can be used to more accurately locate the Deformation Band, where upper level cirrus bands are best seen in the IR channels or a suitable combination image.

15 June 2004/11.56 UTC - NOAA 0.8 µm image
15 June 2004/11.56 UTC - NOAA 11 µm image
15 June 2004/11.56 UTC - NOAA RGB image (0.6, 0.85 and 11 µm)
15 June 2004/11.56 UTC - NOAA RGB image (1.6, 11 and 12 µm)

A Deformation Band, just north of the Alps, is seen in all the AVHRR channels as a long, and rather narrow band of clouds, mostly consisting of thin upper level cloudiness. The sharp southern edge of the cloudiness is clearly seen in all the channels. The 124 combination image clearly shows that the cloudiness is cirrus (bluish tones - see Basics for more explanation).

Meteorological Physical Background

Deformation in the wind field

In a changing wind field four different kind of variations can affect elements flowing in the field (such as clouds and humid areas): translation, divergence, vorticity and deformation. The correspondent changes in clouds and humid areas are:

Variation in the wind field Change to the cloud
Translation Location
Divergence Area
Vorticity Orientation
Deformation Shape

  • Translation shifts elements
  • Divergence either contracts (convergence) or expands (divergence) elements
  • Vorticity rotates elements either cyclonically (positive vorticity) or anticyclonically (negative vorticity)
  • Deformation changes the shape of the elements

Mathematically the deformation D can be written as a sum of two components, stretching and shearing deformation (Dst and Dsh) as follows:

D = √Dst2 + Dsh2 , where in a (x,y)-plane

Dst = ∂u/∂x - ∂v/∂y and
Dsh = ∂u/∂y + ∂v/∂x,

u being the wind component in x-direction, v in y-direction.

The effect of stretching and shearing terms can be represented schematically as follows:

Stretching deformation. Green lines and arrows show the wind field. Light green square is an element, which stretches into the darker green rectangle. The area of the element stays the same. Here x-axis is the axis of dilatation and y-axis is the axis of contraction.
Shearing deformation. The situation is the same as on the left, but now the axes of dilatation and contraction are at 45° angle to the x- and y-axes.

In the schematics above it can be seen that a col (saddle surface) and a confluent zone are required for the deformation to occur.

In the satellite images the stretching and shearing deformation can be pictured as follows:

Stretching deformation. A cloud band elongates uniformly. This is typical for occluded cloud bands.
Shearing deformation. Different parts of a cloud band elongate in opposite directions. This can be seen e.g. in Cold Fronts.

The col surface can also be half of a saddle, which leads to the formation of a "mushroom pattern" (a curved cloud band):

28 November 2004/06.00 UTC - Meteosat 8 IR 10.8 image; red solid: positive stretching deformation 300 hPa, red dashed: negative stretching deformation 300 hPa
28 November 2004/06.00 UTC - Meteosat 8 IR 10.8 image; magenta solid: positive shear deformation 300 hPa, magenta dashed: negative shear deformation 300 hPa
28 November 2004/06.00 UTC - Meteosat 8 IR 10.8 image; brown: total deformation 300 hPa

Typical locations for Deformation Bands

Deformation zones are generated by opposing flows. Typical situations in which deformation occurs, are those in which the cloud bands of two synoptic or meso-scale cloud systems are brought close to each other. Once they start merging, the elongation of the cloud band becomes obvious.

The location of deformation is most often a col (see sketches before). The maximum of a deformation field is nearly always in the confluent area of the wind field, on the colder side of it. At the saddle point the deformation is non-existent.

The most likely location is the area near the pole ward boundary of Warm Front and Occlusion cloud bands. Often a cold front approaches this cloud area from the north. The cloud bands merge, then they start to elongate. In the end the cloud band breaks somewhere in the middle of the band, and the rest of the cloudiness dissipates.

Deformation on a col with a Cold Front and an occluded front merging
Part of warm frontal cloudiness breaks off

Deformation zones are seen as linear structures with diverging moisture parcels along the line. The intensity of the deformation zone is proportional to the contrast in moisture gradient across the zone.

The moisture gradient is typically sharpest pole wards of the Deformation Band. Dry sinking air coming from the north is advected towards the axis of dilatation, which can be seen as a dark area in WV imagery. The contrast between dry and moist air is enhanced by the transportation of moist air from the south by the moist upper level conveyor belt.

28 November 2004/06.00 UTC - Meteosat 8 WV 6.2 image; blue solid: relative humidity 500 hPa > 70%, blue dashed: relative humidity 500 hPa < 70%, green: streamlines 300 hPa

Deformation Bands and Frontogenesis

Deformation is a primary factor in frontogenesis and frontolysis.

The full frontogenesis function Ft consists of three components: Ft = Fh + Fv + Fq, where Ft is frontogenesis caused by horizontal wind field, Fv frontogenesis by vertical motions and Fq frontogenesis by diabatic heating. The contribution from Fh is necessary for the formation of synoptic scale fronts. It can be expressed as:

deformation_band

where Dtot is the total deformation, D is the horizontal wind divergence and b the angle between the axis of dilatation and the isentropes.

Frontogenesis can occur when cos2b > 0, which means that 2b < 90° or b < 45°. Frontolysis can occur when 45° < b < 90°.
Frontogenesis is strongest in the confluence zone, and zero in the saddle point, where there is no wind.

Frontogenesis
Frontolysis

The effect of vorticity on the deformation field is shown the following diagram:

25 March 2004/06.00 UTC - Meteosat 8 IR 10.8 image; red: wind 500 hPa, magenta: equivalent potential temperature 500 hPa
25 March 2004/18.00 UTC - Meteosat 8 IR 10.8 image; red: wind 500 hPa, magenta: equivalent potential temperature 500 hPa

Deformation Bands and Jet Fibres

Deformation Bands have a somewhat similar appearance to Jet Fibres towards the end their life cycle (see Jet Fibres ). Their origins and often also the locations with respect to jet streams, however, are different. The similarities and differences can be briefly listed as follows:

  • Jet Fibres occur only in the vicinity of an upper level jet. Deformation Bands extends to areas with weak winds, usually a col.
  • Jet fibres associated with a Cold Front occur only on the cold side of the front, whereas Deformation Bands are more likely on the warm side of a Cold Front.
  • Jet Fibres have a typical life cycle ranging from 8 hours up to 24 hours. Deformation Bands have shorter life-cycles, due to the strong sinking motion that takes place at the poleward side of the cloud band. Typical life cycle for Deformation Band cloudiness is 6-9 hours.
  • Both Jet Fibres and Deformation Bands have a dark stripe in WV imagery along the cold side of the cloudiness due to sinking motion at upper and middle troposphere.
  • Jet Fibres experience deformation when they are brought into a deforming wind field. They should in this case be still classified as Jet Fibres due to their origin.

The example below illustrates a case showing Jet Fibres and Deformation Band over the Atlantic Ocean. The Deformation Band is within weak, but confluent upper level flow, whereas the Jet Fibres, more to the south, are clearly associated with a polar front jet streak.

19 April 2004/18.00 UTC - Meteosat 8 IR 10.8 image; yellow: isotachs 300 hPa; yellow arrows: wind 300 hPa

Key Parameters

  • Wind vectors or streamlines at 300 and 500 hPa
    Wind vectors, preferably streamlines, show the presence of a col (saddle surface) at the area of deformation, with confluence on both sides of the col
  • Total deformation at 300 and 500 hPa
    Total deformation maximum found in the vicinity of the col, with maximum values in the confluent zone on the cold side, where the jet is tightest
  • Upper/middle tropospheric humidity
    High relative humidity gradient at the axis of dilation
  • Omega at 700 hPa
    Weak or moderate sinking motion at, and on the cold side of the deformation zone

Wind/streamlines 300/500 hPa

28 November 2004/06.00 UTC - Meteosat 8 IR 10.8 image; yellow: streamlines 300 hPa

Total deformation 300 hPa

28 November 2004/06.00 UTC - Meteosat IR image; brown: total deformation 300 hPa

Humidity 500 hPa

28 November 2004/06.00 UTC - Meteosat 8 WV 6.2 image; blue solid: relative humidity 500 hPa > 70%, blue dashed: relative humidity 500 hPa < 70%

Omega 700 hPa

28 November 2004/06.00 UTC - Meteosat IR 8 10.8 image; yellow solid: sinking motion, yellow dashed: rising motion

Typical Appearance In Vertical Cross Sections

As Deformation Bands occur either in the vicinity of frontal clouds or outside frontal zones, the vertical cross sections, especially the isentropes, may vary substantially from case to case.

  • Total deformation
    Total deformation maximum at the level of maximum winds (jet stream level), on the pole ward side of the band.
  • Upper/middle tropospheric humidity
    High upper and middle tropospheric relative humidity gradient at the axis of dilation.
  • Vertical motion
    Weak or moderate sinking motion in middle and upper levels of the troposphere, within and towards the cold side of the Deformation Band.

28 November 2004/06.00 UTC - Meteosat IR image; position of vertical cross section indicated

Total deformation

28 November 2004/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), brown: total deformation

Upper/middle tropospheric humidity

28 November 2004/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity

Vertical Motion

28 November 2004/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion

Weather Events

A Deformation Band occurs when there is deformation of the cloudiness at middle or upper levels in the troposphere. There are no weather events at the surface associated with a Deformation Band. The example below shows a Deformation Band over Western Europe.

Parameter Description
Precipitation
  • No precipitation associated with Deformation Band. Existing rainbands tend to weaken at deformation areas.
  • Precipitation from low level clouds not dependent on deformation at upper levels
Temperature
  • No significant change
Wind (incl. gusts)
  • No significant change
Other relevant information
  • Upper/middle level cloudiness dispersing
  • High level cloudiness in the form of fibres


10 October 2004/18.00 UTC - Meteosat IR image; green: weather events

References

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  • BLUESTEIN H.B. (1992): Synoptic-Dynamic Meteorology in Midlatitudes, Vol 1: Principles of Kinematics and Dynamics. Oxford Univ. Press.
  • CARLSON T. N. (1998): Mid-Latitude Weather Systems, American Meteorological Society.
  • HOLTON J.R. (1979): An Introduction to Dynamic Meteorology. 2nd Ed., Academic Press, New York.
  • KURZ M (1998).: Synoptic Meteorology; Deutscher Wetterdienst.
  • PALMEN E., NEWTON C.W (1969):Atmospheric Circulation Systems. Academic Press.
  • SAUCIER W.J (1955): Principles of Meteorological Analysis, Chapter 10. Univ. of Chicago Press, Chicago, Illinois.