Download Air mass - Regional Climate Group

Weather
Chapter 8, 9
Ai M
Air
Masses, F
Fronts,
t and
d Middl
Middle-Latitude
L tit d C
Cyclones
l
September 10
10, 2009
1
O tli
Outline
•Air Masses
–cP
cP, cA
cA, mP
mP, mT,
mT cT
•Fronts
F t
–Stationary, cold, warm, occluded
•Middle-Latitude
Middle Latitude Cyclones
–Factors in their development
2
Surface
weather
chart
Surface
weather
chart
in in
today
yesterday
Frontt is
F
i an important
i
t t factor
f t which
hi h determines
d t
i
weather
th
Cyclones are usually in company with fronts
Front forms when two air masses of different
characteristics meet with each other
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Air Masses
• Ai
Air mass – an extremely
t
l llarge b
body
d off air
i
whose properties of temperature and
humidity are fairly similar in any horizontal
direction at any given latitude.
– Air masses may cover thousands of square
kilometers (huge in size)
• Part of weather forecasting is a matter of
determining air mass characteristics,
predicting how and why they change
change, and
in what direction the system will move.
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An example of cold air mass
Here,, a large,
g , extremelyy cold water air mass is dominating
g the weather over
much of the United States. At almost all cities, the air is cold and dry. Upper
number is air temperature (°F); bottom number is dew point (°F).
5
Source Regions of Air Masses
• S
Source Regions – are regions where air masses
originate. In order for a huge air mass to develop
uniform characteristics,
characteristics its source region should be
generally flat and of uniform composition with light
surface winds
winds.
– The longer air remains stagnant over its source
region, the more likely it will acquire properties of the
surface below.
– Best source regions are usually dominated by High
Pressure [e.g. ice and snow covered arctic plains
and subtropical oceans and desert regions.]
– Are the middle latitudes a good source region???
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Classification
Air masses are grouped
Ai
d iinto
t ;
• General categories according to their source
region
i
– Polar (P) are air masses that originate in polar latitudes
– Arctic (A) are air masses that originate in arctic regions
– Tropical (T) are air masses that form in warm tropical regions
• The nature of the surface in the source region
– Maritime (m) are air masses that originate over water (moist in
the lower layers)
– Continental (c) are air masses with a source region over land
(dry)
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Air Mass
Classification/characteristics
• cP – Continental Polar air
mass. Cold,, dryy and
stable.
• cT – Continental Tropical
air mass. Hot, dry, stable
air aloft, unstable air at
the surface
• *cA – Continental Artic.
Extremely cold cP air
mass
• mP – Maritime Polar air
mass. Cool,, moist,,
unstable.
• mT – Maritime Tropical
air mass. Warm, moist,
usually unstable.
• *mE – Maritime
Equatorial. Extremely hot
humid air mass
originating over
q
waters.
equatorial
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Air mass source regions and their paths.
There are typical routes for air masses
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cP
• cP
P and
d cA
A air
i masses b
bring
i bitt
bitterly
l cold
ld weather
th
to the midlatitude inland in the winter.
• Originate
O i i t over iice and
d snow covered
d regions
i
off
N. Canada, Alaska, and Siberia, where long
clear nights allow for strong radiational cooling
of the surface.
• Air becomes cold and stable.
stable Little moisture in
source region makes this air mass relatively dry.
• As air moves southward it is modified.
Temperatures moderate as it moves south
south.
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Temperature detected by satellite
Average upper-level wind flow (heavy arrows) and surface position of anticyclones
(H) associated with two extremely cold outbreaks of arctic air during December
December.
Numbers on the map represent minimum temperatures (°F) measured during each
cold snap.
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Visible
s b e sate
satellite
te image
age sshowing
o
g tthe
e modification
od cat o o
of ccP a
air as itt moves
o es
over the warmer Gulf of Mexico and the Atlantic Ocean. When cP
meets warm ocean, condensation occurs.
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Lake Effect Snows
cP
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2004 November 30
cP
Michigan, USA
Lake Effect Snow on Earth
Credit: SeaWiFS Project, NASA
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mP
• O
Originated
i i t d ffrom very cold
ld polar
l air
i b
butt contains
t i much
h
moisture compared to cP
• Ocean water modifies the cP air by adding warmth and
moisture creating mP air.
• mP air is cool and moist, but is modified as it moves from
ocean to the inside of continent.
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• After crossing several mountain ranges
ranges, cool
moist mP air from off the Pacific Ocean
descends the eastern side of the Rockies as
modified, relatively dry Pacific air.
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mT
• Wi
Wintertime
t ti
source region
i ffor Pacific
P ifi M
Maritime
iti
Tropical air mass is the subtropical east Pacific
Ocean.
• Must travel many many miles over the ocean
before reaching
g the California coast.
• Warm, moist air masses that produce heavy
precipitation. Warm rains can cause rapid snow
melt leading to disastrous mud slides
slides.
• mT air that influences the weather east of the
Rockies originates over the Gulf of Mexico and
Caribbean Sea.
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• An infrared satellite image
g that shows subtropical
p
((mT))
air (heavy red arrow) moving into northern California on
January 1, 1997. The warm, humid airflow (sometimes
called "the pineapple express") produced heavy rain and
extensive flooding in northern and central California.
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A typical weather map showing surface-pressure systems, air masses, fronts,
and isobars (in millibars) as solid gray lines
lines. Large arrows in color show air
flow. (Green-shaded area represents precipitation.)
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Criteria for locating a front
• Sharp temperature changes over a
y short distance
relatively
• Changes in the air’s moisture content
(changes in dew point)
• Shifts in wind direction
• Pressure and pressure changes
• Clouds
Clo ds and precipitation patterns.
patterns
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Cold Front
• Represents a zone where cold
cold, dry stable
polar air is replacing warm moist unstable
tropical air
air.
• Drawn as solid blue line with the triangles
along the front showing its direction of
movement.
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Slope of a cold front
• The leading edge of the front is steep
• Air aloft pushes forward blunting the frontal surface
• Distance from leading edge of front to cold air =50 km.
But the front aloft is about 1 km over our head
head. Thus it
is said to have a slope of 1:50
• This is a fast moving front. Slow moving fronts have
l
less
slope
l
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Typical weather with a cold front
• Winds shift from S or SW to W or NW
• Temperature – warm before drops at front and
keeps dropping
• Pressure – Falls steadily before, minimum at
frontal line
line, rises after
• Precipitation – Showers before; heavy precip at
front TSTMS
front,
TSTMS, snow; precip decreases then
clearing
• Visibility – Hazy before; poor at front; improving
after
• Dewpoint – High before; sharp drop at FROPA;
lowering after
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Warm Fronts
•
•
•
•
•
A warm front
f t is
i a front
f t that
th t moves in
i such
h a way that
th t warm air
i
replaces cold air
Depicted by solid red line with half circles pointing into the cold air
Average speed of movement = 25 ~ 35 km/h (c.f., 35~50 km/h for
cold front)
Overrunning
g – rising
g of warm air over cold;; produces
p
clouds and
precipitation well in advance of the front’s surface boundary
Average slope is 1:300 (c.f. 1:50 for cold front)
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29
• Vertical view of the temperature and winds across the
warm front in Fig
Fig. 8
8.15
15 along the line P
P-P
P'.
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Typical weather with a warm front
• Winds before (S or SE); variable at front; S or SW after
front line
• Temperature
p
– cool to cold before; rising
g at FROPA;
Warmer then steady after
• Pressure – Usually falling before; steady at FROPA;
slight rise then falling after
• Clouds Ci, Cs, As, Ns, St, Fog, occasional CB before;
Stratus with the front; clearing with scattered SC after
• Precipitation
P i i i – light
li h to mod
dR
Rain,
i snow, sleet,
l
or d
drizzle
i l
(with showers in summer) before; Drizzle at FROPA;
little to no p
precip
p after FROPA
• Dew point – steady rise before FROPA; Steady with
FROPA; Rise then steady after FROPA
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Detecting the approaches of
cold and warm front
From vertical structure of wind
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Veering
L
X
Here wind direction will show
clockwise change with increasing
altitude, this is called veering, warm
air (front) is coming
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Backing
L
X
Here wind direction will show counterclockwise change with increasing
altitude, this is called backing, cold air
(front) is coming
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Occluded Fronts
• Wh
When a cold
ld ffrontt catches
t h up tto and
d overtakes
t k a warm
front, the frontal boundary created between the two air
masses is called an occluded front or simply
p y an
occlusion
• Represented as a solid purple line with alternating cold
f t type
front
t
triangles
ti
l and
d warm front
f t half
h lf circles
i l
• Two types: Warm and cold occlusions; cold occlusions
most prevalent in the Pacific coastal states; warm
occlusions occur when the milder, lighter air behind a
cold front is unable to lift the colder heavier air off the
ground
d and
d iinstead
t d rides
id up along
l
th
the sloping
l i warm ffrontt
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The formation of a cold-occluded front.
The faster-moving cold front
35~50 km/h
...
25~35 km/h
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...catches up to the slower-moving warm front...
39
...and
and forces it to rise off the
ground. (Green-shaded area
represents precipitation.)
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The formation of a warm-type occluded front.
The faster-moving cold front in this figure..
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...overtakes the slower-moving warm front in this figure.
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The lighter air behind the cold front rises up and over the denser
air ahead of the warm front. Here is a surface map of the situation.
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Typical weather with
an occluded front
• Temperature (Cold type) – Cold or cool before; dropping
with FROPA;; Colder after
• Temperature (Warm type) – Cold before; Rising with
FROPA; Milder after.
• Precipitation – All intensities before; during and after
followed by clearing
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Stationary Front
• A ffrontt with
ith essentially
ti ll no movementt
• Drawn as alternating
g red and blue line.
Semicircles face toward colder air on the
g
p
point toward warmer
red line and triangles
air on the blue line.
• Winds tend to blow parallel to a stationary
front.
• If either a cold or warm
arm front stops mo
moving,
ing
it becomes a stationary front
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• East Asian summer monsoon results from
massive precipitation caused by quasiquasi
stationary front.
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Polar Front Theory
• The first model to explain the development and
intensification of a midlatitude cyclone
• Describes the g
genesis, g
growth, and lysis
y
((decaying)
y g) of
mid-latitude cyclone
• Developed by Norwegian scientists (Bjerknes
(Bjerknes, Solberg,
Solberg
Bergeron)
• Published shortly after WW I
• Today the work has been modified to serve as a
convenient way to describe the structure and weather
associated with migratory storm systems.
• “Polar front” is a (imaginary) front which separates cold
polar air from warm subtropical air
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Step One
A segment of the polar front as a stationary front. (Trough of low pressure with
higher pressure on both sides. Cold air to the North, warm air to the south.
Parallel flow along the front.
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Step Two
Under the right conditions a wavelike kink forms on the front.
The circulation of winds about the cyclone tends to produce a wavelike
deformation on the front
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Step Three
Steered by the winds aloft, the system typically moves east or northeastward and
gradually becomes a fully developed open wave in 12 to 24 hours. Open wave –
the stage of development of a wave cyclone where a cold front and a warm front
exist, but no occluded front. The center of lowest pressure in the wave is located
at the junction of two fronts.
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Step Four
Central pressure is now lower, several isobars encircle the wave. The more tightly
packed isobars create a stronger cyclonic flow
flow, winds swirl counterclock-wise
counterclock wise and
inward toward the low’s center. The cold front advances on the warm front…
52
Step Five
As the open wave
moves eastward,,
central pressures
continue to decrease,
and the winds blow
more vigorously. The
faster-moving cold
front constantly inches
closer to the warm
front, squeezing the
warm sector into a
smaller
ll area.
Eventually the cold
front overtakes the
warm front and the
system becomes
occluded. The storm
is usually most intense
at this time, with
clouds and precip
covering a large area
area.
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Step Six
•
•
The intense
Th
i t
storm
t
from
f
step
t
five gradually dissipates,
because cold air now lies on
both sides of the cyclone.
cyclone
Without the supply of energy
provided by the rising warm,
moist air,
air the old storm system
dies out and gradually
disappears. Occasionally,
however a new wave will form
on the westward end of the
trailing cold front.
The entire life cycle of a
wave cyclone can last from a
few days to over a week.
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What are results?
• Meridional temperature gradient is disappeared
• Warm air moves aloft, cold air resides below
• During the processes, strong wind and
precipitation have been generated,
• Namely, potential energy (temperature gradient)
is converted to kinetic energy
energy.
• Condensation supplies additional energy.
• This is one of important engines to drive the
Earth’s atmospheric motions
55
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Conversion of potential energy
to
W
C
W
C
Available
potential energy
Kinetic energy
Latent heat
supports
57
Upper level support
• Divergence aloft is important to cyclogenesis.
IF there is no divergence aloft, surface low and rising motion
will be easily dissipated.
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Developing Mid-Latitude
Cyclones and Anticyclones
• Suppose an upper level low is directly
above the surface low
low. Significant
convergence does not occur in a low aloft
as it does at the surface (no friction aloft)
aloft).
A low that is not supported by some
divergence aloft will dissipate
• Same is true for a high pressure system;
di
divergence
att th
the surface
f
needs
d some
support from convergence aloft to survive
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Summary of clouds, weather, and vertical
motions associated with a developing wave
cyclone.
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Thanks!
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