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ATMO 251 Fronts and Frontogenesis, Part 2

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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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Let the second term be zero for now Let du/dx equal -dv/dy – This means the divergence (du/dx+dv/dy) = 0

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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

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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 L HL H

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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

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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

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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

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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)

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Another example

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700-mb temperature, heights, winds

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700-mb heights and streamlines Axis of dilatation?

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Water vapor loop

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\ \

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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?

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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

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BUT, if it’s cold on the north side and warm on the south side, the isotherms get closer together.

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Frontogenesis This has made the temperature gradient stronger, and is called “frontogenesis” – the creation of a front

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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

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Frontogenesis How else can frontogenesis/frontolysis occur? Convergence/divergence But at the synoptic scale, divergence is usually small, so deformation is more often important

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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

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Frontogenesis and thermal wind

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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

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950-mb winds and potential temperature

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White contours = frontogenesis

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