Fronts

1
Fronts

• EAS 211
• Spring 2005
• 02/14/05

2
Front

• A narrow transition zone between two different
  air masses.
• Signify differences in Temperature, moisture, or
  density.
• Can be 10-100s of km wide and 1000s of km long.
• With vertical extent often reaching to near 500
  mb.
• The two air masses (usually one cold and one
  warm) do not readily mix because the colder (more
  dense) air mass sinks while the warmer (less
  dense) air rises.
• When a front passes through a region, there is
  typically a change in Temp and Pressure, a
  wind-shift, changes in cloud cover, and sometimes
  precip.

3
How do we identify a front?

• What parameters do we need to look at?
• Changes in wind direction.
• Temperature changes.
• Pressure changes.
• Clouds and Precip.
• Moisture changes.

4

• Frontogenesis
• A strengthening of the temperature contrast
  across the front, causing the front to
  strengthen.
• Frontolysis
• When the temperature contrast across the front
  becomes less distinct, causing front to weaken
  (e.g. Strong solar heating behind a cold front
  will weaken the cold front and clouds could cause
  it to die out.)

5
Types of Fronts

• Cold Front
• Warm Front
• Stationary Front
• Dryline
• Occluded Front
• Cold Occlusion
• Warm Occlusion

6
Cold Front

• cP air mass replacing an mT air mass
• Average speed is approximately 35 km/h, but
  ranges from slow to 50 km/h.
• Cu develop into Cbs asSE winds ahead and NW
  behind create CON in unstable conditions to
  promote UVM and storms of short duration.
• CAA occurs behind the front (stronger temperature
  gradient behind the front)
• Clouds and precip ahead of the frontusually dry
  behind
• Slope of the front approximately 1100
• Vertical extent reaches approximately 500 mb.

7
Warm Front

• A warm mT air mass replaces cooler mP or even cP
  air mass
• Warm air (less dense) runs over the colder air
  mass (overrunning) this leads to stable
  conditions and stratus type clouds.
• The slope of the front is less steep slope is
  approximately 1200
• Less intense precip (light to omod.) usually
  steady, and located ahead of the front
• Advances more slowly (20 km/h)
• WAA ahead of the front (Temp gradient ahead of
  the front)
• Can result in sleet or freezing rain in winter,
  and often produces precip fog.

8
Stationary Front

• Similar to the warm front except neither air mass
  undergoes substantial movement
• Sometimes the result of a cold front that has
  stalled out
• Often, there is little or no precip if there is
  precip, it usually occurs on the cool side of the
  front
• Sometimes, in the case of overrunning, warm,
  moist air rises up above the cold air to cause
  widespread cloudiness with extended periods of
  rain
• This is commonly associated with east-west
  oriented quasi-stationary fronts, and located on
  the cool side of the front.
• The temp gradient is located on the cool side
• The precip (if occurring) is also located on the
  cool side.

9
Dryline

• A low-level mesoscale boundary or transition zone
  hundreds of kilometers in length and up to tens
  of kilometers in width separating dry air from
  moist air.
• The length of the dryline is related to
  large-scale terrain or large-scale weather system
  features, whereas its width is related to
  mesoscale processes.
• In its quiescent state, the dryline may be
  considered the intersection of the top of a
  low-level moist layer with large-scale features
  of sloping terrain. In this state the shallow
  layer of moisture near the higher terrain is
  eroded by turbulent mixing with daytime heating.
  Moisture gradients are additionally strengthened
  by horizontal convergence resulting from downward
  transport of horizontal momentum in the dry air.
• In a more dynamically active state the dryline
  often advances away from the higher terrain as an
  integral component of an ETC. The dryline is
  found all over the world.
• In the United States the dryline, which marks the
  boundary between moist air from the Gulf of
  Mexico and dry continental air from the west, is
  found in the Plains region. It is most often
  present during the spring, where it is often the
  site of thunderstorm development. Typically the
  dryline in the United States advances eastward
  during the day and retreats westward at night.

10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
Occluded Front

• Form when a cold front catches up with, and
  merges with a warm front (recall that cold fronts
  move faster than warm fronts)
• Usually (though not always) a sign that the front
  is dissipating
• There are two types
• Cold occlusion
• Warm occlusion

14
(No Transcript)
15
Cold Occlusion

• Air behind cold front is colder than the air
  ahead of the warm front
• Cold front lifts both air masses adjoining the
  warm front so the coldest air is at the surface.
• There is a trough behind the cold occlusion under
  the axis of the deepest warm air.

16
(No Transcript)
17
Warm Occlusion

• Air behind the cold front is warmer than the air
  ahead of the warm front.
• Air ahead of the warm front (coldest) remains at
  the surface while the warmer air masses get
  lifted.
• The trough ahead of the warm occlusion is located
  along the axis of warm air.
• Rare e.g. Washington/British Columbia coast when
  a polar air mass spills into the Pacific and is
  driven northward along a warm front.

18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
(No Transcript)
22
(No Transcript)