Discussion: lecture course examination possible dates:

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Discussion lecture course examination possible
dates Thursday, 14 December (last day of
lecture) Another date in January (09 or 10
January)? Examination will last 1 hr 30 minutes
(but you may not need all of that time). Format
primarily multiple choice, a few short-answer
essay (two or three sentences) Must be
concluded before 15 January 2007
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Front a narrow zone of transition between air
masses of contrasting density, that is, air
masses of different temperatures or different
water vapor concentrations or both. Named by
the airmass that is advancing
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• Fronts are actually zones of transition, but
  sometimes the transition zone, called a frontal
  zone, can be quite sharp.
• The type of front depends on both the direction
  in which the air mass is moving and the
  characteristics of the air mass.
• There are four types of fronts that will be
  described stationary front, cold front, warm
  front, and occluded front.

When 2 different air masses come together,
interesting things can happen
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• Indentifying a front on a surface weather map or
  by your own weather observations
• Look for
• Sharp temperature changes over a relatively short
  distance
• Change in moisture content
• Rapid shifts in wind direction
• Pressure changes
• Clouds and precipitation patterns

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• Stationary front a nearly stationary narrow zone
  of transition between contrasting air masses
• winds blow parallel to the front but in
  opposite directions on the two sides of the front
• in the mid-latitudes, typically separates cold
  dense cP air from milder mP air
• often associated with a wide region of clouds
  and rain or snow on the cold side of the front.
• clouds and precipitation result from
  overrunning, as warm humid air flows upward over
  the cooler air mass, more or less along the
  frontal surface, cools through adiabatic
  expansion which triggers condensation and
  precipitation.

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• Cold front a narrow zone of transition between
  advancing relatively cold (dense) air and
  retreating relatively warm (less dense) air.
• over Europe, temperature contrast across a cold
  front is typically greater than that across
  stationary or warm front.
• cold frontal passage is associated with a sharp
  temperature drop in winter and a noticeable
  humidity drop in summer.

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• Some of the characteristics of cold fronts
  include the following
• steep slope
• faster movement / propogation than other fronts
• most violent weather among types of fronts
• move farthest while maintaining intensity
• tend to be associated with cirrus well ahead of
  the front, strong thunderstorms along and ahead
  of the front, and a broad area of clouds
  immediately behind the front (although fast
  moving fronts may be mostly clear behind the
  front).
• can be associated with squall lines (a line of
  strong thunderstorms parallel to and ahead of the
  front).
• usually bring cooler weather, clearing skies,
  and a sharp change in wind direction.

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The slope of a cold front is steeper (150 to
1100) than the slope of a warm front (1150)
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General weather characteristics of a cold front
Weather Feature Before frontal passage Region of front After frontal passage
Winds SE to SW gusty W to NW
Temperature warm sudden decrease steady cooling
Dew point high steady decreases steadily
Pressure falling steadily minimum rapid rise steady rise
Visibility fair to poor poor then improving good
Clouds Ci, Cs, Cb Cb Cu
Precip showers heavy precip clearing
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• Warm front a narrow zone of transition between
  advancing relatively warm (less dense) air and
  retreating relatively cold (dense) air.
• warm front is associated with a broad cloud and
  precipitation shield that may extent hundred of
  kilometers ahead of the surface front

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• Some of the characteristics of warm fronts
  include the following
• slope of a typical warm front is more gentle
  than cold fronts
• tend to move slowly.
• are typically less violent than cold fronts.
• although they can trigger thunderstorms, warm
  fronts are more likely to be associated with
  large regions of gentle ascent (stratiform clouds
  and light to moderate continuous rain).
• are usually preceded by cirrus first (1000 km
  ahead), then altostratus or altocumulus (500 km
  ahead), then stratus and possibly fog.
• behind the warm front, skies are relatively
  clear (but change gradually)

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• The type of frontal weather depends on the
  stability of the warmer air
• when warm air is stable, a frontal inversion may
  exist in the upper frontal region, a steady
  light-to-moderate rainfall or frontal fog is
  observed in the presence of nimbostratus or
  stratus clouds, respectively.
• when the warm air is unstable, brief periods of
  heavy rainfall are observed in the presence of
  cumulonimbus clouds.

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General weather characteristics of a warm front
Weather Feature Before frontal passage Region of front After frontal passage
Winds NE to E variable S to SE
Temperature cool, slowly warming steady rise warmer
Dew point steady rise steady increases, then steady
Pressure usually falling levels off slight rise, followed by fall
Visibility poor improving fair
Clouds Ci, Cs, As, Ns, St, fog stratus Clearing with scattered Sc
Precip light to moderate, can be SN or RA drizzle or nothing usually none
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Occluded front (occlusion) a narrow zone of
transition formed when a cold front overtakes a
warm front.
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• Dry Line
• boundary that separates moist air mass from a
  dry air mass
• also called Dew Point Front
• most commonly found just east of the Rocky
  Mountains rare east of the Mississippi River
• common in TX, NM, OK, KS, and NE in spring and
  summer

Warm, moist air Southeast winds
Hot, dry air Gusty southwest winds
Rocky Mountains
Dry Line
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States like Texas, New Mexico, Oklahoma, Kansas,
and Nebraska frequently experience dry lines in
the spring and summer. Dry lines are extremely
rare east of the Mississippi River.
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How do fronts form?
1
2
4
3
5
6
7
8
9
10
11
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Terms 1, 5, 9 Diabatic Terms Terms 2, 3, 6, 7
Horizontal Deformation Terms Terms 10 and 11
Vertical Deformation Terms Terms 4 and 8 Tilting
Terms Term 12 Vertical Divergence Terms
Three-Dimensional Frontogenesis Equation
Bluestein (Synoptic-Dynamic Met. In
Mid-Latitudes, vol. II, 1993)
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Assumptions to Simplify the Three-Dimensional
Frontogenesis Equation
y
?
? 1
x
? 2

• y axis is set normal to the frontal zone, with
  y increasing towards the cold air (note y
  might not always be normal to the isentropes)
• x axis is parallel to the frontal zone
• Neglect vertical and horizontal diffusion
  effects

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Simplified Form of the Frontogenesis Equation
A B C
D
Term A Shear term Term B Confluence term Term
C Tilting term Term D Diabatic
Heating/Cooling term
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Frontogenesis Shear Term
Shearing Advection changes orientation of
isotherms
Carlson, 1991 Mid-Latitude Weather Systems
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Frontogenesis Confluence Term
Cold advection to the north
Warm advection to the south
Carlson, 1991 Mid-Latitude Weather Systems
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Why are cold fronts typically stronger than warm
fronts? Look at the shear and confluence terms
near cold and warm fronts
Shear and confluence terms oppose one another
near warm fronts
Shear and confluence terms tend to work together
near cold fronts
Carlson (Mid-latitude Weather Systems, 1991)
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Frontogenesis Tilting Term
Adiabatic cooling to north and warming to south
increases horizontal thermal gradient
Carlson, 1991 Mid-Latitude Weather Systems
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Frontogenesis Diabatic Heating/Cooling Term
frontogenesis
T constant
T increases
frontolysis
T increases
T constant
Carlson, 1991 Mid-Latitude Weather Systems
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Frontogenesis/Frontolysis with Deformation with
No Diabatic Effects or Tilting Effects
where
and
ß angle between the isentropes and the axis of
dilatation
Petterssen (1968)
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MID-LATITUDE CYCLONES

• the cause of most of the stormy weather in the
  northern hemisphere, especially during the winter
  season
• Understanding their structure and evolution is
  crucial for predicting significant weather
  phenomena such as blizzards, flooding rains, and
  severe weather.

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• Mid-latitude (or frontal) cyclones
• large traveling atmospheric cyclonic storms up
  to 2000 kilometers in diameter with centers of
  low atmospheric pressure
• located between 30 degrees and 60 degrees
  latitude (since the continental United States is
  located in this latitude belt, these cyclones
  impact the weather in the U.S.)

• form along the polar front
• an intense system may have a surface pressure as
  low as 970 millibars
• normally, individual frontal cyclones exist for
  about 3 to 10 days moving in a generally west to
  east direction
• precise movement of this weather system is
  controlled by the orientation of the polar jet
  stream in the upper troposphere
• commonly travels about 1200 kilometers in one day

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How many mid-latitude cyclones can you identify
from this satellite image?
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How many mid-latitude cyclones can you identify
from this satellite image?
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What causes mid-latitude cyclones to form?
surface
Upper-air
Surface extra-tropical (i.e., non-hurricane)
cyclones are directly coupled with the
upper-levels. Typically an upper-level trough,
and its associated super-geostrophic wind
maximum, move over a surface temperature
gradient.
Remember our equation for relative vorticity
(spin) generation? Notice the 2nd term
baroclinic term. The stationary front /
temperature boundary provides the necessary
baroclinic energy for the surface cyclone to
develop.
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Stages of mid-latitude cyclone developmentTwo
models of development
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Mid-latitude cyclone model application 6 June
1944(Petterssens forecast)
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• Mid-latitude cyclones are "deep" pressure
  systems extending from the surface to the
  tropopause
• A surface low-pressure system grows if there is
  vertical wind shear (winds increasing with
  height) and thermal instability (convection).
• The factors that lead to lowering of the
  pressure at the surface are

• Diverging airflow at high altitudes
• Inflow of warm, moist air at low and mid levels.
  
• Latent heat release caused by convection in the
  warm air mass sector of the growing storm system.

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