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ATMO 251. Fronts and Frontogenesis , Part 2. Deformation. There are three important properties that a vector wind field can have Divergence (tendency to move apart) Vorticity (tendency to induce spin) Deformation (tendency to change shape). “Stretching deformation”. “Shearing deformation”.
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ATMO 251 Fronts and Frontogenesis, Part 2
Deformation • There are three important properties that a vector wind field can have • Divergence (tendency to move apart) • Vorticity (tendency to induce spin) • Deformation (tendency to change shape) “Stretching deformation” “Shearing deformation”
Let the second term be zero for now • Let du/dx equal -dv/dy • This means the divergence (du/dx+dv/dy) = 0
Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle – calm winds • An object anywhere in this field will be “deformed” by the winds
Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds H L L H
Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds
Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds
Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds
Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds Axis of dilatation (axis along which object is stretched)
700-mb heights and streamlines Axis of dilatation?
Water vapor loop \ \
Frontogenesis • Now, imagine that the pure deformation we looked at earlier is happening to a “box” with a temperature gradient – cold on one side and warm on the other • What happens to the temperature gradient?
If it’s warm on the west side and cold on the east side, the deformation makes the isotherms spread out – the temperature gradient gets weaker
BUT, if it’s cold on the north side and warm on the south side, the isotherms get closer together.
Frontogenesis • This has made the temperature gradient stronger, and is called “frontogenesis” – the creation of a front
Frontolysis • When the temperature gradient gets weaker, it is called “frontolysis” – the death of a front • As we just saw, the orientation of the temperature gradient in relation to the axis of dilatation determines whether deformation strengthens or weakens the gradient frontolysis frontogenesis
Frontogenesis • How else can frontogenesis/frontolysis occur? • Convergence/divergence • But at the synoptic scale, divergence is usually small, so deformation is more often important
Frontogenesis and thermal wind • Frontogenesis puts the atmosphere out of thermal wind balance – if the temperature gradient is changing, the wind shear must be changing too • If T gradient changes, P gradient also must change
Why does this all matter? • Frontogenesis actually causes rising and sinking motion! • This vertical motion doesn’t happen just because a front exists, but because the front is strengthening or weakening • Because the vertical motion acts to weaken the front, if we see a strong front with storms ahead of it, it’s almost certain that frontogenesis is going on • We don’t see it plotted on TV, but it is very common to see frontogenesis used in research and forecasting