Download Norwegian Cyclone Model Open Wave Cyclone Mature Cyclone

11/15/16
Baroclinic instability and
cyclogenesis
•  As mentioned before, nature hates gradients
and tends to work in such a way as to decrease
the gradient.
•  The temperature gradient btwn the polar and
midlat. air masses tends to be strongest near the
surface (why?)
•  Both a strong sharp surface temperature gradient
(surface front) and a strong well-defined jet are
preferred regions of baroclinic instability. (they
usually go together).
Open Wave Cyclone
Norwegian Cyclone Model
A kink forms in the front, as warm
air starts to move N and the cold air
S. This is cyclogenesis (the birth of
a cyclone) and is also referred to as
At first convergence occurs along a a frontal wave. The pressure begins
to drop in the center, forming a low
quasi-stationary (inactive) front
Mature Cyclone and Dissipation
Triple point
Cyclonic winds develop as the
low deepens. Convergence into
the low causes upward motion and
precipitation. There is also vertical
motion associated with each front
The faster moving cold front
catches up with the warm front,
squeezing the warm sector into a
smaller space. Wind speeds
increase as the central pressure
drops
Three Cyclones in Different Stages
of Development
Here are three cyclones in
different stages that have
formed along the POLAR
FRONT.
Note that the polar front
does not have to be
oriented E-W as in the
idealized model and often
it is not, as seen in low #1
here.
Note that though Low #3 is the strongest, it is no longer deepening.
In the mature phase, the cyclone forms
an occluded front with the cold front
catching up with and undercutting the
warm front. The triple point is a likely
place for a secondary low to form
Finally, the cyclone is cut off
from the energy of the
temperature gradient and it
slowly dissipates, well into
the cold air region
Idealized vertical structure
Note: 1) divergence at 300 mb
above the low and convergence
above the high.
2) Cut off lows at surface have
open (not cut-off) wave patterns
above them at 500 mb.
3) At 500 mb cold air is moving
over low
4) The axis of the developing low
tilts westward with height.
5) Vertically-stacked lows are no
longer growing, but dissipating.
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Four Frontal Types
occluded
stationary
warm
cold
Warm and Stationary fronts
STATIONARY FRONTS do not move, i.e. the front stays in
the same place, but warm air can still overrun cold air. WARM
fronts and STATIONARY fronts have similar weather, but
WARM front displaces cold air at the surface.
Note the shallow angle of the warm front
Occluded fronts
Cold occlusion
The colder air wedges
under the cool air near
the center of the
cyclone.
The cold occlusion is
like a cold front (is a
cold front actually) but
since the temperature
difference is less, the
weather is usually less
dramatic.
Frontal types
•  Note: the type of front is determined by type
of air moving into an area. A cold front
brings cold air, a warm front, warmer air
•  Both warm and cold fronts act such as to lift
air.
•  Since the warm air is less dense, it is the
warmer air mass that is always lifted.
Cold fronts
Cold fronts have a sharper, steeper nose, therefore a small zone of
lifting and smaller zone of clouds/precip, along and just behind the
front.
Warm fronts have a broader extent, and are not as steep. So... they
have a broader band of precip, which is often well ahead of the
front.
Phrontal Phacts!
•  Both warm and cold fronts can produce snow and/or icing
conditions depending on the season and the location
relative to the front.
•  The lifting along cold fronts is typically stronger and more
likely to result in convection.
•  Strong, fast moving cold fronts often produce squall lines
of organized convection (thunderstorms) sometimes with
hail and tornadoes along or ahead of the front.
•  Stationary fronts often occur when cold air is dammed up
against the terrain and warm air overruns the cold air. This
is called cold air damming and can produce icing
conditions.
•  Remember that there must be some appreciable amount of
moisture in the lifted air before clouds and precip occur.
Otherwise we refer to a dry frontal passage.
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More Phrontal Phacts!
•  Pressure typically rises dramatically after cold frontal
passage, especially in the earlier stages of cyclone
development.
•  Wind often changes direction by 45° or so after frontal
passage. This and the pressure change are often used by
forecasters (rather than temperature changes) to determine
front location.
•  Frontal passage can be very hard to detect in mountainous
terrain. Often the upper-air cyclone passes over mtns, and
the surface conditions reform on the other side. Other
times, the storm just “dies”.
•  A developing extratropical cyclone tilts westward with
height. A mature or decaying E.C. is usually stacked, i.e.
the low pressure center at each height is directly above the
surface low.
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