Transcript Understanding Fronts

Understanding Fronts
A 3-D grasp on fronts and frontal
movements and cyclones
Air Masses
► An
air mass is a defined as a large body of
air with very similar characteristics.
► Generally speaking, air masses are generally
defined by temperature and dewpoints
(moisture parameters.)
► Air masses are named based on the source
region of the air mass itself
Types of Air Masses
►
Continental Polar, “cold and dry”
 Originates closer to the Poles over
land-locked regions.
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Continental Tropical, “warm and dry”
 Originates closer to the Tropics over
land-locked regions.
►
Maritime Polar, “cold and damp”
 Originates closer to the Poles over
water.
►
Maritime Tropical, “warm and humid”
 Originates closer to the Tropics over
water.
►
Arctic, “very cold”
 Originates in the very cold land-locked
areas
Analyzing Air Masses
► An
air mass is most easily identified by
comparing it to other air masses.
► Air masses can be modified with time, most
notably by days of sunshine or lack thereof.
► Fronts are the dividing line between air
masses so understanding air masses, means
understanding where fronts are located.
What is a Front?
► Definition:
A narrow transition zone, or boundary,
between disparate synoptic scale air masses
whose primary discontinuity is density. It is
synoptic scale along the length of the front but
mesoscale across the front itself.
► Commonly associated with ..
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Moisture gradient
Temperature gradient
Wind shift
Pressure Trough
Convergent boundary
Rules for finding fronts
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Look for a strong temperature gradient. The front is
located on the warm side of the sharpest gradient.
Likewise, look for a strong dewpoint gradient. The front is
located on the moist side of the sharpest gradient.
Generally found in a pressure trough – look for three hour
pressure changes. Fronts will show a decrease in pressure
followed by a rapid increase in pressure after the frontal
passage.
Look for a sharp change in wind direction. A cyclonic shear
in the wind direction usually indicates a frontal passage
Check weather and cloud patterns that are usually
associated with different kinds of fronts (more later on
this.)
Types of Fronts
►
Cold
 Noted by cold air advancing
and displacing warmer air that
exists.
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Warm
 Noted by cold air retreating
from an area.
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Stationary
 While differing air masses exist
along a boundary, little
movement is analyzed of the air
masses.
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Occluded
 A complicated process where
the surface low becomes
completely surrounded by
cooler/cold air. Occlusion
processes can be a “cool type”
or “cold type” (more later.)
Cold Front
► Marked
on a map with a blue line and blue
triangles pointing towards the warm air.
► Slopes of 1/50 to 1/150
► Associated with cumulus & cumulonimbus
clouds ahead of the front in the warm air,
producing showers and thunderstorms.
Cold Front
► Simple
3-D idea:
http://www.physicalgeography.net/fundamentals/7r.html
Cold Front
http://www.free-online-private-pilot-ground-school.com/images/cold-front.gif
Cold Front
► An
animation:
Warm Front
► Marked
on a map by a red line with red
semi-circles pointed towards the cool air (in
the direction the warm air is retreating to.)
► Slope ranges from 1/100 to 1/300.
► Generally associated with stratus type
clouds, overcast skies, fog, and general rain
or snow.
Warm Front
► Simple
3-D idea:
http://www.physicalgeography.net/fundamentals/7r.html
Warm Front
http://www.free-online-private-pilot-ground-school.com/images/warm-front.gif
Warm Front
► An
animation:
Stationary Front
► Marked
by alternating blue lines & blue
triangles (pointed in the direction of the
warmer air) and red lines & red semi-circles
(pointed in the direction of the cooler air)
► Usually noted as *quasi*-stationary as it is
rarely ever completely stationary. It tends to
meander a bit.
Occluded Front
► Marked
by a purple line with alternating
purple triangles and purple semi-circles, all
pointing in the direction of the frontal
movement.
► There are two general types of occlusions,
cool-type and cold-type. Examples to follow.
Occluded Front
► Simple
3-D idea:
http://www.physicalgeography.net/fundamentals/7r.html
Occluded Front
http://www.free-online-private-pilot-ground-school.com/images/occluded-front.gif
Cyclones: Putting it all together
► There
are two types of cyclones,
tropical/warm core and extratropical/cold
core.
► Extratropical cyclones are characterized by
having differing air masses frontal
movements where tropical cyclones do not.
► We will concern ourselves with extratropical
cyclones in this presentation.
Cyclones: Putting it all together
► Typically,
cyclones are represented by using the
Norwegian Cyclone model. It is simplistic, but a
good way to start when understanding surface
lows and cyclones.
► Students must begin to understand the four
dimensional view of a cyclone
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North-South
East-West
Up-Down
Time
The cyclone
Cyclones: Norwegian Model
► Step
1: A subtle boundary exists
Surface View
Images source: http://www.srh.noaa.gov/jetstream//synoptic/cyclone.htm
3-D View
Cyclones: Norwegian Model
► Step
2: A wave forms on the boundary
Surface View
Images source: http://www.srh.noaa.gov/jetstream//synoptic/cyclone.htm
3-D View
Cyclones: Norwegian Model
► Step
3: Cyclone becomes mature
Surface View
Images source: http://www.srh.noaa.gov/jetstream//synoptic/cyclone.htm
3-D View
Cyclones: Norwegian Model
► Step
4: The occlusion process
Surface View
Images source: http://www.srh.noaa.gov/jetstream//synoptic/cyclone.htm
3-D View
Cyclones: Norwegian Model
► Step
5: Cold air dominates, near the end.
Surface View
Images source: http://www.srh.noaa.gov/jetstream//synoptic/cyclone.htm
3-D View
Cyclone: On satellite
Cyclone
Cyclone
► Radar
& Satellite view of a cyclone
Occlusion
Occlusion
Warm Front
Warm Front
Cold Front
Cold Front
A cyclone centered just south of Wichita, KS. Fronts well developed
The cyclone six hours later, near Kansas City. A mature cyclone
at this point with well developed fronts. Do you see them?
Another six hours, the cyclone (low) is beginning to occlude.
The surface low is being surrounded by the colder air.
Another six hours, the cyclone (low) is now fully occluded.
The surface low is completely within the colder air mass.
Another six hours, the cold front has surged well east while
the center of the cyclone has remained over the Midwest.
Animation of a cyclone
Review: Finding the fronts
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Rule number one for
finding fronts was to
look for a strong
temperature gradient
or change in
temperature.
Do you see one here?
Would you agree that
the air mass in the
Dakotas is not the
same as the one that
exists near Kentucky?
Review: Finding the fronts
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Rule number two for
finding fronts was to
look for a strong
dewpoint gradient, or
change.
Do you see one here?
If so, is it roughly in
the same location as
the temperature
gradient in the
previous slide?
Review: Finding the fronts
Rule number three for
finding fronts was to
look for a strong
pressure gradient, or
change. Strong falls
followed by strong
rises.
► Rule number four is to
look for a strong wind
shift (the yellow
vectors.)
► Do you see these
here?
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Note: in this image, the dashed blue lines represent
pressure rises while the solid blue lines represent
pressure falls. The numbers represent millibars per
three hours of fall or rise. The yellow vectors point
in the direction the wind is blowing towards.
Review: Finding the fronts
► The
last rule is to check cloud and weather
patterns.
Review: Finding the fronts
Every cyclone presents
unique characteristics.
► While the satellite and radar
didn’t show the front well,
the temperature, wind, and
moisture analyses provided
a clear cut picture of where
this front should be
analyzed.
► The rules for finding fronts
should not imply that all the
rules need to be met. It
means that you need to use
all those tools to find a
front. Often, fronts are very
subtle and only one rule
might be useful.
► Did you get it right? 
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Advanced tools: Soundings
From the same date as the
previous images, this is the
sounding from Omaha at
6pm.
► The sounding shows well
the slope of the cold front.
Notice the cold air has
settled in very well from the
surface to around 880mb,
and notice the wind shift to
go along with that.
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Advanced tools: Soundings
From the same date as the
previous images, this is the
sounding from Lincoln, IL at
6pm.
► The sounding is taken
before the front has
passed, within the warm
sector (where a maritime
tropical air mass exists.)
► Notice the difference
between the low levels of
the Omaha soundings vs.
this sounding, including the
low level winds.
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Advanced: Cross Section
Riverton
North Platte
Omaha
Davenport
Rapid City
Aberdeen
Chanhassen
Lincoln
Gaylord
White Lake
Advanced: Using Profilers
Advanced: Using forecast LI
Advanced: Forecast Precip & Thicks
The slope of the front
Temperatures at the surface with frontal features overlaid.
The slope of the front
Temperatures at 850 millibars (about 1400 meters above the
surface) with frontal features overlaid. Notice the cold front is
a little further west and the warm front is a little further
north. The low is a little further northwest, as well.
The slope of the front
Temperatures at 700 millibars (about 2900 meters above the
surface) with frontal features overlaid. Notice, again, the cold
front is a little further west and the warm front is a little
further north than at 850. The low is even further northwest.