Fronts and Surface Cyclone Structure

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Fronts and Surface Cyclone Structure

• AOS 101 Discussions 301/303
• April 21st / April 23rd, 2008

Discussion Leader Brian Miretzky
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Background on Cyclones . . .

• A cyclone is
• An area of low pressure around which the winds
  flow counter-clockwise in the northern
  hemisphere, and clockwise in the southern
  hemisphere
• Hurricane (tropical cyclone)
• Midlatitude cyclone
• Today, well focus on midlatitude, or
  extra-tropical cyclones, which have a life cycle
  and frontal structures. Hurricanes, which well
  talk about later, have no fronts.

http//www.wunderground.com/hurricane/history/iop4
_sat.jpg
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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!

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Background on Cyclones . . .
Weve already discussed that friction near the
surface of the earth causes winds to converge
near cyclone centers (low pressure), and have yet
to discuss that they 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
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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

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Background on Fronts

• Discussed the example of a polar front last week
• Definition - boundary, transition zone between
  two different air masses
• the two air masses have different densities.
  Frequently, they are characterized by different
  temperatures and moisture contents
• front has horizontal and vertical extent
• frontal boundary/zone can be 1-100 km wide!!
• types of synoptic-scale fronts
• stationary fronts
• cold fronts
• warm fronts
• occluded fronts

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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 fronts movement

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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

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Common (I.E not every time) Characteristics
Associated with Cold Fronts
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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 fronts movement
• The TEMPERATURE CONTRAST ALONG WARM FRONTS IS
  GENERALLY LESS DIFFUSE (DISTINCT) (I.E. THE TEMP
  GRADIENT IS LESS)

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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.

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Common Characteristics Associated with Warm Fronts
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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

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Cold Occlusion (The type most associated with
mid-latitude cyclones)

• cold front "lifts" the warm front up and over the
  very cold air
• Associated weather is similar to a warm front as
  the occluded front approaches
• once the front has passed, the associated weather
  is similar to a cold front
• vertical structure is often difficult to observe

http//apollo.lsc.vsc.edu/classes/met130/notes/cha
pter11/index.html
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Warm Occlusion

• cold air behind cold front is not dense enough to
  lift cold air ahead of warm front
• cold front rides up and over the warm front
• upper-level cold front reached station before
  surface warm occlusion

http//apollo.lsc.vsc.edu/classes/met130/notes/cha
pter11/index.html
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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

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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)

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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.
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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.
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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)

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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)

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Polar Front Theory - Development and Evolution of
a Wave CycloneAlso, referred to as Norwegian
Cyclone Model (NCM)

• The wave cyclone (often called a frontal wave)
  develops along the polar front
• when a large temperature gradient exists across
  the polar front - the atmosphere contains a large
  amount of Available Potential Energy (Remember
  the greater the temperature difference on the
  Skew-T corresponded to a large Convective
  Available Potential Energy)

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NCM cont.

• (b) - An instability (kink) forms in the polar
  front. This instability is the incipient cyclone

http//apollo.lsc.vsc.edu/classes/met130/notes/cha
pter12/index.html
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NCM cont.

• (c)- A fully-developed "wave cyclone" is seen
  12-24 hours from its inception. It consists of
• a warm front moving to the northeast
• a cold front moving to the southeast
• region between warm and cold fronts is the "warm
  sector"
• the central low pressure (low, which is deepening
  with time)
• overrunning of warm air over the warm front
• cold air surging southward behind the cold front
• wide-spread precip. ahead of the warm front
• narrow band of precip. along the cold front
• Wind speeds continue to get stronger as the low
  deepens - the Available Potential Energy (APE) is
  being converted to Kinetic Energy (KE)
• The production of clouds and precip. also
  generates energy for the storm as Latent Heat is
  released

http//apollo.lsc.vsc.edu/classes/met130/notes/cha
pter12/index.html
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NCM cont.

• (d) - As the cold front moves swiftly eastward,
  the systems starts to occlude.
• Storm is most intense at this stage
• have an occluded front trailing out from the
  surface low
• triple point/occlusion - is where the cold, warm
  and occluded fronts all intersect

http//apollo.lsc.vsc.edu/classes/met130/notes/cha
pter12/index.html
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Final Stage

• (e) - the warm sector diminishes in size as the
  systems further occludes.
• The storm has used most all of its energy and
  dissipates
• All of the APE has been utilized and the KE has
  dissipated into turbulence- cloud/precip
  production has diminished
• The warm sector air has been lifted upward
• The cold air is at the surface - stable
  situation.
• The temperature contrast which drove this whole
  situation from the surface perspective is no
  longer near the center of the wave of low
  pressure

http//apollo.lsc.vsc.edu/classes/met130/notes/cha
pter12/index.html
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Another view
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Another View
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Weather associated with a typical late fall to
early spring mid-latitude cyclone
Figure courtesy of Jon Martin
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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
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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!
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Locating a Cyclone

• Find the region of lowest sea level pressure
• Find the center of the cyclonic
  (counter-clockwise) circulation

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Locating a Cyclone

• Find the region of lowest sea level pressure
• Find the center of the cyclonic
  (counter-clockwise) circulation

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Intro to the vertical structure of mid-latitude
cyclones

• Up until now we have been looking only at surface
  development, but what about features like the jet
  stream and upper level troughs and ridges.
• Mass continuity
• Can not have convergence in the same column at
  both upper levels and lower levels for more then
  a brief moment since it would not satisfy mass
  continuity,
• But can have a low over a low since it will take
  time for the low to fill in i.e. rise to a great
  enough pressure where it is no longer low as
  compared to its surroundings

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Pressure Changes
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The End is Near

• More to come next week on the vertical structure
  of cyclones
• Quiz 2 next week on all discussion lectures
  after spring break.
• Homework today is first part to case study- more
  information to come soon (i.e. Friday by email
  and posting on my webpage)
• Remember for contouring use all knowledge you
  have gained through the semester and see me if
  you have any questions