Download Surface Cyclone Structure

Cyclone Structure and Fronts
AOS 101 – Section 302
Ross A. Lazear
April 24, 2007
Background on Cyclones . . .
A cyclone is:
• An area of low pressure around
which the winds flow counterclockwise in the northern
hemisphere, and clockwise in the
southern hemisphere
• Hurricane (tropical cyclone)
• Midlatitude cyclone
• Today, we’ll focus on midlatitude,
or extra-tropical cyclones, which
have a life cycle and frontal
structures. Hurricanes, which we’ll
talk about later, have no fronts.
http://www.wunderground.com/hurricane/history/iop4_sat.jpg
Background on Cyclones . . .
Remember from the beginning of the semester…
Midlatitude cyclones are crucial in maintaining a temperature
equilibrium on our planet. This is because in the northern
hemisphere . . .
• . . . They advect warm air northward
• . . . And they advect cold air southward
• This helps reduce the radiative disequilibrium on our planet!
Background on Cyclones . . .
We’ve already discussed that
friction near the surface of the
earth causes winds to converge
near cyclone centers (low
pressure), and spin cyclonically
(positive vorticity).
As a result, there are generally
more clouds and precipitation
near a cyclone center, as the IR
satellite image to the right
suggests.
L
Background on Cyclones . . .
The figure to the right
represents a typical
midlatitude cyclone:
• Cold, dry air is advected
eastward behind the cold
front
• Warm, moist air is advected
north behind the warm front
• The fronts move in the
direction the “teeth” point
Background on Cyclones . . .
http://earthstorm.ocs.ou.edu/images/Fronts.gif
More on fronts in a bit…
Locating a Cyclone
1. Find the region of
lowest sea level
pressure
2. Find the center of the
cyclonic (counterclockwise) circulation
Locating a Cyclone
1. Find the region of
lowest sea level
pressure
L
2. Find the center of the
cyclonic (counterclockwise) circulation
Locating a Cyclone
3. Find where the clouds
appear to be moving
counter-clockwise
around a point
(northern hemisphere)
L
http://www.wunderground.com/hurricane/history/iop4_sat.jpg
Fronts
Cold Front
• A transition zone where a cold air mass replaces a warm air mass
• Drawn as a blue line with blue triangles pointing in the direction of
the front’s movement
Fronts
Cold Front
• Cold air is more dense than warm air.
• As the dense, cold air moves into the warm air region, it forces the
warm air to rapidly rise just ahead of the cold front.
• This results in deep convective clouds, occasionally producing
strong to severe thunderstorms (depending on how unstable the
atmosphere ahead of the cold front is).
• Often, the precipitation along a cold front is a very narrow line of
thunderstorms
Fronts
Warm Front
• A transition zone where a warm air mass replaces a cold air mass
• Drawn as a red line with red half-circles pointing in the direction of
the front’s movement
Fronts
Warm Front
• Again, warm air is less dense than cold air.
• As the warm air moves north, it slides up the gently sloping warm
front.
• Because warm fronts have a less steep slope than cold fronts, the
precipitation associated with warm fronts is more “stratiform” (less
convective), but generally covers a greater area.
Fronts
Occluded Front
• A region where a faster moving
cold front has caught up to a slower
moving warm front.
• Generally occurs near the end of
the life of a cyclone
• Drawn with a purple line with
alternating semicircles and triangles
• Cold front overrides the warm
front, and generally heavy
“stratiform” precipitation exists
Fronts
• Front is stalled
• No movement of the
temperature gradient
• But, there is still convergence
of winds, and forcing for ascent
(and often precipitation) in the
vicinity of a stationary front.
• Drawn as alternating segments
of red semicircles and blue
triangles, pointing in opposite
directions
Locating Fronts
Fronts are associated with . . .
• Strong temperature gradients
• Positive vorticity
• Lower pressure
• Regions of convergence of the winds
• Often precipitation and clouds (regions of ascent)
Locating Fronts
Here, the winds are
rapidly changing
counterclockwise across
this temperature gradient.
The winds are blowing
warm air from the south.
This is a warm front.
Locating Fronts
In this case, the winds are
also rapidly changing
counterclockwise across
this temperature gradient,
indicating positive
vorticity.
The winds are blowing
cold air from the
northwest.
This is a cold front.
Locating Fronts
Locate the cyclone and fronts in these
surface observations:
To find the cyclone,
• Find the center of cyclonic
circulation
To find the fronts,
• Find large temperature gradients
• Identify regions of wind shifts
• Look for specific temperature
advection (warm/cold)
• Look for kinks in the isobars
(regions of slightly lower pressure)
Locating Fronts
Locate the cyclone and fronts in these
surface observations:
To find the cyclone,
• Find the center of cyclonic
circulation
To find the fronts,
• Find large temperature gradients
• Identify regions of wind shifts
• Look for specific temperature
advection (warm/cold)
• Look for kinks in the isobars
(regions of slightly lower pressure)
The Life-Cycle of a Midlatitude Cyclone
A mid-latitude cyclone is born
in a region where there is a
strong temperature gradient,
such as an old stationary front
(or, a polar front).
The region is associated with
low pressure. Northward
moving warm air, and
southward moving warm air act
to bend the temperature
gradient and form warm and
cold fronts.
The Life-Cycle of a Midlatitude Cyclone
This is the “mature” stage of
a cyclone’s life.
The cold front and warm
front are well developed, and
the pressure at the cyclone
center (L) has decreased to
its minimum in the life-cycle.
Generally, the most intense
precipitation will occur at
this point in a cyclone’s life.
The Life-Cycle of a Midlatitude Cyclone
This is the “occluded” stage of a
cyclone’s life.
The cold front has caught up to the
warm front, and the low pressure
center becomes displaced from the
warm air mass.
As a result, the cyclone begins to
weaken (and the pressure begins to
increase).
Generally, this occurs when the
low is “vertically stacked,” or
when the surface low is directly
under the upper low. We’ll talk
much more about this next week.
http://www.fas.org/irp/imint/docs/rst/Sect14/of_schem2.jpg
Precipitation Around a Cyclone and its Fronts
To the right is a major cyclone
that affected the central U.S. on
November 10, 1998.
Around the cold front, the
precipitation is more intense, but
there is less areal coverage.
North of the warm front, the
precipitation distribution is more
“stratiform”: Widespread and
less intense.
http://weather.unisys.com
Precipitation Around a Cyclone and its Fronts
Again, in this radar and surface
pressure distribution from
December 1, 2006, the
precipitation along the cold front
is much more compact and
stronger.
North of the warm front, the
precipitation is much more
stratiform.
Also note the kink in the isobars
along the cold front!