Understanding Fronts

1
Understanding Fronts

• A 3-D grasp on fronts and frontal movements and
  cyclones

2
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

3
Types of Air Masses

• Continental Polar, cold and dry
• Originates closer to the Poles over land-locked
  regions.
• 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

4
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.

5
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 ..
• Moisture gradient
• Temperature gradient
• Wind shift
• Pressure Trough
• Convergent boundary

6
Rules for finding fronts

• 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.)

7
Types of Fronts

• Cold
• Noted by cold air advancing and displacing warmer
  air that exists.
• Warm
• Noted by cold air retreating from an area.
• Stationary
• While differing air masses exist along a
  boundary, little movement is analyzed of the air
  masses.
• 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.)

8
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.

9
Cold Front

• Simple 3-D idea

http//www.physicalgeography.net/fundamentals/7r.h
tml
10
Cold Front
http//www.free-online-private-pilot-ground-school
.com/images/cold-front.gif
11
Cold Front

• An animation

12
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.

13
Warm Front

• Simple 3-D idea

http//www.physicalgeography.net/fundamentals/7r.h
tml
14
Warm Front
http//www.free-online-private-pilot-ground-school
.com/images/warm-front.gif
15
Warm Front

• An animation

16
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.

17
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.

18
Occluded Front

• Simple 3-D idea

http//www.physicalgeography.net/fundamentals/7r.h
tml
19
Occluded Front
http//www.free-online-private-pilot-ground-school
.com/images/occluded-front.gif
20
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.

21
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
• North-South
• East-West
• Up-Down
• Time

22
The cyclone
23
Cyclones Norwegian Model

• Step 1 A subtle boundary exists

Surface View
3-D View
Images source http//www.srh.noaa.gov/jetstream//
synoptic/cyclone.htm
24
Cyclones Norwegian Model

• Step 2 A wave forms on the boundary

Surface View
3-D View
Images source http//www.srh.noaa.gov/jetstream//
synoptic/cyclone.htm
25
Cyclones Norwegian Model

• Step 3 Cyclone becomes mature

Surface View
3-D View
Images source http//www.srh.noaa.gov/jetstream//
synoptic/cyclone.htm
26
Cyclones Norwegian Model

• Step 4 The occlusion process

Surface View
3-D View
Images source http//www.srh.noaa.gov/jetstream//
synoptic/cyclone.htm
27
Cyclones Norwegian Model

• Step 5 Cold air dominates, near the end.

Surface View
3-D View
Images source http//www.srh.noaa.gov/jetstream//
synoptic/cyclone.htm
28
Cyclone On satellite
29
Cyclone
30
Cyclone

• Radar Satellite view of a cyclone

Occlusion
Occlusion
Warm Front
Warm Front
Cold Front
Cold Front
31
A cyclone centered just south of Wichita, KS.
Fronts well developed
32
The cyclone six hours later, near Kansas City. A
mature cyclone at this point with well developed
fronts. Do you see them?
33
Another six hours, the cyclone (low) is beginning
to occlude. The surface low is being surrounded
by the colder air.
34
Another six hours, the cyclone (low) is now fully
occluded. The surface low is completely within
the colder air mass.
35
Another six hours, the cold front has surged well
east while the center of the cyclone has remained
over the Midwest.
36
Animation of a cyclone
37
Review Finding the fronts

• 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?

38
Review Finding the fronts

• 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?

39
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?

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.
40
Review Finding the fronts

• The last rule is to check cloud and weather
  patterns.

41
Review Finding the fronts

• Every cyclone presents unique characteristics.
• While the satellite and radar didnt 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? ?

42
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.

43
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.

44
Advanced Cross Section
Riverton North Platte
Omaha Davenport Gaylord
Rapid City Aberdeen
Chanhassen Lincoln White Lake

45
Advanced Using Profilers
46
Advanced Using forecast LI
47
Advanced Forecast Precip Thicks
48
The slope of the front
Temperatures at the surface with frontal features
overlaid.
49
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.
50
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.