Transcript Document

Air Masses & Fronts
This chapter discusses:
1. Classification of 4 North American air masses based
on cold or warm land mass and ocean origin
2. Air mass delineation and movement along stationary,
cold, warm, and occluded fronts
Regional Weather Patterns
•Surface maps of
US temperature,
dew point, and
pressure reflect
synoptic (largescale) trends.
•In this image,
nearly every
station around the
high pressure
anticyclone
reports cold, dry
air, suggesting the
air mass formed in
a common source
region.
Air Mass Classification and
Characteristics
** cA (continental Artic) - very cold, dry and very stable. Very shallow air mass.
Source Regions & Classification
•
Air masses of similar temperature and humidity form above flat,
uniform regions with light surface winds.
•
Air masses are associated with surface high pressure.
•
Typical U.S. air masses:
maritime Polar (mP)
- moist and cold
continental Polar (cP)
- dry and cold
maritime Tropical (mT)
- moist and warm
continental Tropical (cT) (warm season only)
- dry and hot
continental Artic (cA) (cold season only)
- very dry and very cold
cP Wind Flow
•Western
mountains, such
as the Rocky and
Sierra Nevada
ranges, normally
protect the Pacific
Northwest from
cP air.
•Strong
anticyclone highs,
however, can
create northeast
winds that cause
cold outbreaks
along the western
coast.
Lake-effect snows and the associating air masses
• Lake-effect snows form as a result of colder air traversing over
“relatively” warm lake water.
• 25F (air temperature) vs. 40F (lake surface temperature)
• The warmth of the lake provides an instability which promotes
cloud growth.
• Lake effect snow events can last between several hours to a few
days.
Artic outbreak associated
with continental artic air
mass.
Top image: 12 UTC 24
December 1983
Bottom image: 00 UTC
February 18 2006
Summer & Winter cP Air
•Summer cP air over
the US brings welcome
relief from heat, but
also triggers steeper
environmental lapse
rates and cumulus
cloud development.
•Cold surfaces during
the winter create
temperature
inversions.
Modification of cP Air
As the cP air mass moves over the warmer Gulf of Mexico and Gulf Stream, ocean
surface warmed the cooler air. The lower atmosphere becomes unstable and forms
extensive rows of cumulus cloud streets.
Origin of mP Air
Cold Asian &
polar air passing
over the ocean
south of the
Aleutian low will
pick up warmth
and moisture, and
reaches the
Pacific Coast as
cool, moist, and
unstable, bringing
rain and snow.
Modification of mP Air
•Orographic precipitation lowers the moisture content of mP air,
called Pacific air east of the Rockies, during its westward flow.
•Leeward of the Rockies, the air is dry and may develop chinook
winds.
East Coast mP Air
•A strong anticyclone in eastern
Canada creates northeasterly
winds that may bring cold,
unstable Atlantic mP air and
storms into New England and
the middle Atlantic States.
•This creates a damming effect
of the cold air due to the
Appalachian mountain range.
This cold mP air mass adjacent
to the warm Gulf Stream air
produces a temperature
difference zone where
developments of extratropical
cyclones are favorable.
•These storms are known as
northeasters.
Tropical Pacific mT Air
Regular STJ
El Nino STJ
La Nina STJ
•Warm and moist maritime air from the tropical Pacific may reach the West Coast as a
series of unstable and powerful thunderstorms.
•This stream of moist air is sometime referred to as the “pineapple express.”
•The subtropical jet (STJ) is associated with bringing in moist, warm, mT air into southern
California during the El-Nino-influenced winter causing landslides.
Gulf & Caribbean mT Air
Thunderstorms
Threat
•Gulf of Mexico and Caribbean Sea warmth and moisture flows into the East Coast by a
strong anticyclone.
•When the moist, hot mT air rises above dense cP air, heavy and widespread precipitation
can result.
•Thunderstorms can occur in the moist mT air mass just ahead of the cold front.
Mexican Origin of cT Air
•Dry, hot air from the
Mexican desert can cause lowlevel instability in the U.S.
interior during summer, and
may trigger dust devils.
•An upper level ridge of high
pressure may add
compressionally heated air to
the region, enhancing the dry,
hot conditions.
•This summer upper-level
high pressure is a common
feature in the central US
which causes the Polar Front
Jet to migrate even further
north.
The Results of Different Air Masses Collision
•Two air masses entering a region, such as the U.S. middle latitudes, have a front, or
transition zone, between the strong temperature, humidity, wind direction differences.
•Fronts can also be defined as troughs of lower pressure from analyzing isobars.
•Four different fronts are used on weather maps: cold, warm, stationary, and occluded
fronts.
•An extratropical cyclone (mid-latitude cyclone) is made up of 2 or more different air
masses.
Fronts
“the transition zone between two distinct air masses”
• Cold – a transition zone where a cold air mass
advances and replaces a warm air mass.
• Warm – a front that moves in such a way that warm
air replaces cold air.
• Occluded (occlusion-“closed off”) – a complex frontal
system that ideally forms when a cold front overtakes
a warm front.
– When the air behind the front is colder than the air ahead
of it, the front is called a cold occlusion. Cold  Cool
– When the air behind the front is milder than the air ahead
of it, the front is called a warm occlusion. Cool  Cold
• Stationary – a front that is nearly stationary with
winds blowing parallel and from the opposite
directions on each side of the front at the surface.
Front Identification
•Locating a front on a weather
map involves finding sharp
changes in:
a) temperature
b) dew point
c) wind direction
d) pressure and
e) cloud/precipitation patterns.
•Pressure tendency values are
different along a front.
Cold Front Passage
Click Here for Animation
•West of the front the surface
stations are reporting rising
pressure trend while stations
ahead of the front are
experiencing falling pressure
tendency.
•(/) rising
•(\) falling
•(/\) rise than fall
•(\/) fall than rise
Cold Front Transition
cold, dry air
warm, moist air
•Important cloud, wind, and temperature changes are revealed in this cross-section view of a typical
cold front.
• The front rises steeply (1km rise across a 50km distance) and high clouds protrude ahead.
• The slope is also dependent on the speed of the front as well, therefore the fast the front, the steeper
the slope which leads to greater upward vertical motion along the cold front.
•Slow moving cold fronts are associated with broader cloud and precipitation coverage behind the
front.
• Fast moving cold fronts may send a fast moving squall line of showers ahead of the front.
Strengthening Front
•Frontogenesis is the process in which a front is strengthening or developing;
tightening of the thermal gradient; strong vertical motion is associated with this
process.
•Frontolysis is the process where a front is weakening or dissipating; thermal
gradient becomes diffuse; vertical motion is weak.
•Satellite imagery shows the temporal transition between a weak front and its
frontogenesis, or strengthening, as it moves offshore over warmer water.
Cold Front Weather
Most cold fronts move toward the south, southeast, or east.
Back Door Cold Front
•Eastern Canadian high pressure can
generate cold fronts from the
northeast, which mix with the warm,
moist Gulf air over the northeastern
US.
•Cold air damming describes how the
Appalachian Mountains confine the
front's westward movement.
•Surface winds shifts from westerly to
easterly or northeasterly, and
temperature falls.
•Denver area does experience back
door cold front.
6:00 CDT
28 Sept 2005
Frontal Weather Trends
•Observed wind, temperature,
pressure, humidity, clouds, and
rain weather experiences typical
patterns before, during, and after
a warm front passage as well.
•Note the cyclonic rotation of
winds and change in temperature
along this warm front.
Warm Front Passage
Click Here for Animation
Warm Front Transition
warm, moist air
cold, moist
air
•Unique clouds and precipitation patterns are associated with warm fronts, with a broader
range of showers than in a cold front.
•The cross-sectional view shows the gentle slope of “overrunning” warm air, a typical
temperature inversion, and the shifting winds.
•Overrunning is where a warmer air mass is “gliding” over the top of a colder air mass.
•A warm front is associated with a gentle slope (2km rise across a 600km distance).
•The upward vertical motion associated with a warm front is not as intense as a cold front.
Warm Front Weather
Cold Occluded Fronts
•Fast moving cold fronts
may overtake the slower
moving warm front,
particularly when they are
influenced by cyclonic winds
eventually leading to an
occlusion.
•The colder, heavier air
behind a cold front is able to
lift the less cold, lighter air
ahead of the warm front.
Click Here for Animation
•Cold occlusion describes
where very cold air is behind
the cold front and cool/cold
air ahead of the warm front.
Cold Occlusion
Cross-Sectional
Views
Warm Occluded Fronts
•The air ahead of the warm
front is colder than the air
behind the cold front.
•Warm occlusion describes the
case where the cold front
catches up to and overtakes the
warm front but the milder,
lighter air is unable to
“undercut” the colder, heavier
air ahead of it.
•The cold air associated with
the cold front rides along the
slope of the warm front –
“piggy-backing.”
Occluded Front Weather
Mid-Latitude Cyclone
•Occluded fronts are common
along mid-latitude cyclones, or
deep low pressures centers about
which the cold and warm fronts
pivot.
•Mid-latitude cyclones are also
known as extratropical cyclone.
•These storms appear frequently
in the mid-latitudes.
•These storms are formed as a
result of colliding air masses
(polar front theory).
Upper Air Front
A division between cold and warm air masses in the tropopause is
described as an upper-air front, which forms when polar jet rides
near the tropopause through tightly packed isotherms.
Dryline
• Not a true front but rather a moistures boundary.
• Separates warm humid air to the east (Gulf of Mexico)
and hot, dry air to the west (high Mexican plateau).
• Moisture and wind shift differences are used to depict
the dryline.
• Acts as a focus mechanism for severe weather.
• Climatologically found in the south-central US during
Spring and Fall but can be observed over the CO high
plains.
• There are two types of dryline:
– Synoptically-driven (active)
– Quiescent (passive)
Active Synoptic Pattern Dryline: 1800 UTC 4 May 2003
Passive Synoptic Pattern Dryline: 1800 UTC 6 May 2002
High Plains Dryline: 1800 UTC 18 Aug 2005
High Plains Dryline: 1800 UTC 18 Aug 2005