by FMI
variation in the wind field | change to the cloud |
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Translation | location |
Divergence | area | Vorticity | orientation |
Deformation | shape |
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.
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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.
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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.
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Shearing deformation. Different parts of a cloud band elongate in opposite directions. This can be seen e.g. in Cold Fronts.
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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
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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
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28 November 2004/06.00 UTC - Meteosat 8 IR 10.8 image; brown: total deformation 300 hPa
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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
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Part of warm frontal cloudiness breaks off
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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
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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:
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
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Frontolysis
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25 March 2004/06.00 UTC - Meteosat 8 IR 10.8 image; red: wind 500 hPa, magenta: equivalent potential temperature 500 hPa
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25 March 2004/18.00 UTC - Meteosat 8 IR 10.8 image; red: wind 500 hPa, magenta: equivalent potential temperature 500 hPa
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