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)
Another example
700-mb temperature, heights, winds
700-mb heights and streamlines
Axis of dilatation?
Water vapor loop
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
Frontogenesis and thermal wind
Frontogenesis and thermal wind
Frontogenesis and thermal wind
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
950-mb winds and potential temperature
White contours = frontogenesis
White contours = frontogenesis