Download Lecture 20 Outline of Today`s Lecture What is an Air Mass?

Lecture 20
Outline of Today’s Lecture
• Air Masses
• Air Mass
Modification
• Fronts
1
What is an Air Mass?
Air Masses are large regions of air with similar
temperature and moisture content.
2
Air Mass
Air masses form over areas with
(1) Uniform underlying surface properties and
(2) High surface pressure, where divergence of surface
winds decreases contrasts in temperature and
humidity.
3
Air Mass Properties
• Air masses take on the properties of the underlying
surface.
• Air masses are classified according to their location
of “origin”.
• Geographical Characteristics
– Tropical, Polar, Arctic.
• Surface Properties
– maritime, continental.
• Source region characteristics are most predominant if
air mass remains over source region for a long time.
4
Air Mass Classification
Classified according to temperature and
moisture characteristics.
Continental (dry) - c
Maritime (moist) - m
Equatorial (extremely humid) - E
Tropical (warm) - T
Polar (cold) - P
Artic (extremely cold) - A
5
Air Mass Classifications
• A - Arctic
– Extremely cold cP air mass may be designated Arctic
– Cold, dry, stable
• cP - continental Polar
– Cold, dry, stable
• mP - maritime Polar
– Cool, moist, unstable
• mT - maritime Tropical
– Warm, moist, usually unstable
• cT - continental Tropical
– Hot, dry
– Stable air aloft, unstable surface air
• E - Equatorial
– Hot, very humid
– unstable through deep layer
6
Air Mass Source Regions
Mean sea-level pressure map, January: Air
Masses form where areas of high pressure prevail.
7
Air Mass Source Regions
8
Air Mass Source Regions
Mean sea-level pressure map, July: Air Masses
form where areas of high pressure prevail.
9
Air Mass
Modification
10
Air Masses Modify When they Move
cP air from Canada gradually warms as it is carried
across warmer ground on it way to the southern US.
11
Air Masses Modify with Seasons
cP air from Canada gradually warms as the sun becomes
stronger (more overhead) in spring and summer.
12
Air Mass Modification I
cP air from Canada is carried across the Great Lakes or
the Gulf Steam. Contact with the ocean warms and
moistens the air near the surface, transforming it to an
unstable mP air mass.
13
Air Mass Modification I
•
cP air from Canada is carried across the Great Lakes.
•
Contact with the lakes warms and moistens the air near the
surface, transforming it to an unstable mP air mass, and
resulting and lake-effect snow showers.
Annual average snowfall totals.
14
Air Mass Modification II
When mP air enters the West Coast and moves inland
it crosses several mountain ranges, removing
moisture as precipitation.
15
Air Mass Modification II
When mP air enters
the West Coast and
moves inland it
crosses several
mountain ranges,
removing moisture
as precipitation.
16
Air Mass Modification II
When mP air enters the West Coast and moves inland it
crosses several mountain ranges, removing moisture as
precipitation.
17
Fronts
Fronts
• Types of Fronts
• Identifying Fronts
• Formation of Fronts
18
Fronts
A Front - is the
boundary between
air masses.
Thus, a front is
characterized by a
zone of contrasting
temperature and
moisture.
19
Stationary Front
cP
cP
mT
Isobars
mT
Isotherms
Note the two air masses, cP and mT, that are
involved in the early formation of this front.
20
Air mass characteristics can differ
tremendously across a front
April 1976
temperature
contrasts
Jet stream axis is shown by shaded arrow
21
Four Types of Fronts
Warm Front
Cold Front
Stationary Front
Occluded Front
Frontal symbols are placed pointing in the direction of
movement of the front (except in the case of the
stationary front).
22
How do we determine
what kind of front it is?
From the vantage point of the ground:
• If warm air replaces colder air, the front is a
warm front.
• If cold air replaces warmer air, the front is a
cold front.
•
If the front does not move, it is a stationary
front
•
Occluded fronts are boundaries between cold
and cool air, with warm air pushed aloft.
23
Identifying Fronts
Across the front - look for one or more of the
following:
1. Change of Temperature
2. Change of Moisture characteristic (RH, Td)
3. Change of Wind Direction
4. Change in pressure readings (falling vs rising
pressure
5. Characteristic Precipitation Patterns
6. Characteristic Cloud Patterns
24
Wind Shift Across Cold Front
25
Typical Cold Front Structure
Cold air replaces warm; leading edge is steep in fastmoving front shown below due to friction at the ground
– Strong vertical motion and unstable air forms cumuliform clouds
– Upper level winds blow ice crystals downwind creating cirrus and
cirrostratus
Slower moving fronts have less steep boundaries and less
vertically developed clouds may form if warm air is stable
26
Cold Front Passage
27
Cold Front on Weather Map
Note that the front is located at the leading edge of the
colder air.
28
Cold Font Striking Europe
29
Cold Font Striking West Coast
30
Typical Warm Front Structure
• In an advancing warm front, warm air rides up over colder
air at the surface; slope is not usually very steep.
• Lifting of the warm air produces clouds and precipitation
well in advance of boundary.
• At different points along the warm/cold air interface, the
precipitation will experience different temperature histories
as it falls to the ground (snow, sleet, fr.rain,& rain).
31
Warm Front on Weather Map
The warm front is also located on the warm air side of
the colder air.
32
Stationary Front can bring Flooding
• Warm, moist mT air moves into California on Jan. 1, 1997
• Heavy flooding caused 100,000 people to flee their homes
• Yosemite NP experienced nearly $200 million in damages
and was closed for two months
mP
mT
The Pineapple Express
brings heavy rain to CA.
33
Frontogenesis: Formation of Fronts
• Fronts form when air of differing origins converges
(e.g., tropical vs polar).
• Convergence of air happens in areas of low pressure.
• Frontogenesis is the hallmark of midlatitude cyclones.
34
Map of Midlatitude Cyclone
Temperature dashed lines
Pressure - solid
lines
1016
1 0 2 1 1 7 1023
2 0 1022
30
24 1 0 2 1
2 4 1019
29
1016
1022
25
3 3 1011
33
1006
35
2 4 1013
49
3 5 1012
4 9 1005
17
34
1019
4 1 1014
4 8 1016
4 2 1025
1016
3 8 1020
1005
4 2 1009
4 5 1025
1022
5 3 1022
4 5 1016 4 8
1004
4 1 1002
2 3 1022
28
3 9 1021
3 7 1024
38 1 0 2 1
33
1014
2 2 1020
16 1 0 2 1
1020
2 9 1023
1020
3 8 1023
3 5 1024
3 5 1026
3 0 1021
1021
25
31
2 9 1023
1023
1 0 1023
18
13
32 1 0 2 1
3 2 1024
2 2 1022
1 4 1019
1 2 1024
1 9 1025
2 7 1023
3 2 1023
22 1 0 2 4 2 1 1 0 2 6
1 9 1023
Fonts - heavy
lines with barbs
25
19 1 0 2 4
1 8 1025
2 0 1023
21
1 4 1020
2 5 1023
1 0 1024
49
6 3 1013
7 2 1011
7 6 1008
7 2 1005
6 4 1020
7 0 1017
64
74
5 5 1013
75
66
5 5 1013
1010
7 7 1017
35
Location of Hazards in a
Midlatitude Cyclone
36
Frontogenesis: Formation of Fronts
Fronts are an fundamental part of winter storms
37
Lifecycle of Midlatitude Cyclone
The lifecycle
takes several
days to a week,
and can move
1000’s of km
during this time.
Incipient stage
Mature Stage
Incipient stage
Occluded stage
Stationary front
Mature stage
Dissipated stage
38
Lifecycle of Midlatitude Cyclone
Polar front separates cold easterlies and
westerlies.
39
Incipient Stage
A kink forms on the front and cold air starts to move
southward. Warm air starts to move northward.
40
Mature Stage
Cold air continues to move south, and warm air
north. Fronts strengthen and low pressure develops
in the center.
41
Occluded Stage
Cyclone matures, precipitation and winds become
more intense.
42
Dissipating Stage
Cyclone continues to occlude (end of life cycle)
and cyclone dissipation starts
43