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Discussion: lecture course examination possible dates:

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Indentifying a front on a surface weather map or by your own weather observations ... over Europe, temperature contrast across a cold front is typically greater than ... – PowerPoint PPT presentation

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Title: Discussion: lecture course examination possible dates:


1
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

8
  • 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.

9
  • 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.

10
  • 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.

11
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 development Two
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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