Download Chapter 9: Weather Patterns

Fronts:
Chapter 9: Weather Patterns
Mid Latitude Cyclones: extratropical cyclones, Nor’easters
-boundary surfaces that separate air masses of different densities
temperature
Region southern FL to Alaska
moisture
Lifecycle and associated weather
-15-200km wide
Regional Influence
-line on the weather map
Polar Front Theory:
Fronts:
-Norwegian Cyclone Model
-surface slope is gradual
-WWI
-describes birth, growth and decay of cyclones
Generates cyclone
at the surface
-warm air overlies cold air
-air masses move at different speeds
-one air mass will advance
-clashing produces weather
Midlatitude Cyclone:
Fronts:
-primary weather producers
-warm air is always forced aloft
-low pressure systems, 1000 km dia.
-overrunning: warm air gliding on top of cold air
-counterclockwise circulation toward center
-warm and cold fronts
-upward flow initiates precipitation
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5 Types of Fronts:
Warm Front:
-warm
-precipitation and temperature profile
-Cold
-Stationary
-Occluded
-Dryline
Warm Front:
Cold Front:
-warm air mass is advancing, displaces colder air at the surface
-cold air mass is advancing, displaces warmer air at the surface
-red line with half circles
-cold air more dense forces warm air aloft
-gradual slope (1:200)
-blue line with triangles
-speed = 25-35 km/hr
-steeper frontal boundary (1:100)
-speed = 35-50 km/hr
1
1
200
100
Warm Front:
Cold Front:
-adiabatic cooling
-precipitation follows the passage of the front
-cloud sequence
-steeper frontal boundary & speed: more violent weather
-gradual slope & slow advance: widespread, light precip. long duration
-altocumulus and cumulonimbus
-precipitation precedes front
-E to SW wind shift
1
1
200
100
2
Cold Front:
Occluded Front:
-same lifting of warm air, only quicker over a shorter distance
-cold type occluded front
-heavy downpours, short duration, narrow band of precipitation
advancing air is colder than air mass it is overtaking
-passage of front temperature drops, polar air, clear skies
common east of the Rockies (cP overtakes mP)
-SW to NW wind shift
1
100
Stationary Front:
Occluded Front:
-little to no horizontal movement across the front by either air mass
-warm type occluded front
-lateral motion
advancing air is warmer than air mass it is overtaking
-overrunning, light precipitation
Pacific coast (mP overtakes cP)
Occluded Front:
Drylines:
-rapid moving cold front overtakes a warm front
-fronts based on moisture content
-warm air driven aloft
-not necessarily a difference in temperature
-precipitation from wedging
-dry air forces moist air aloft
-strong temperature gradients
-cT (southwest US) displaces mT (Gulf) spring and summer
-intense weather
-severe T-storms from Texas to Nebraska
3
Life of a Midlatitiude Cyclone (2-10 days)
6 basic stages
Front develops
Wave develops
Cyclonic circulation established
Cyclonic circulation established
Warm air invades north (warm
front)
Cold air advances south (cold
front)
Low pressure at the crest
Occlusion begins
Occluded front developed
Cyclone dissipates
Cyclogenesis = cyclone formation
Life of a Midlatitiude Cyclone
Front develops
Occlusion: beginning of the end
Stationary front
Cold front advances past the
warm front
cP on the North (easterlies)
mT on the South (westerlies)
Strong temperature gradients,
storm intensifies
Life of a Midlatitiude Cyclone
Life of a Midlatitiude Cyclone
Wave develops
Occluded front developed
wavelength (O 100 km)
blizzards, strong winds
wave steepens
energy is being exhausted
within a few days warm front
driven aloft
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Life of a Midlatitiude Cyclone
Idealized Weather of a Midlatitiude Cyclone
Cyclone dissipates
A) Cirrus clouds
cold air mass surrounds the low
at the surface
horizontal temperature gradient
eliminated
Front ~1200 km away
Warm front advances,
cloud base lowers
(cirrostratus, altostratus,
stratus)
Idealized Weather of a Midlatitiude Cyclone
B) Nibostratus clouds
Light precipitation, gets
heavier as front advances
Temperatures increase
Winds shift from an
easterly direction to a
southerly direction
Idealized Weather of a Midlatitiude Cyclone
Idealized Weather of a Midlatitiude Cyclone
Cyclone generally move from west to east
Steered by the general westerly circulation
Right side of the storm passes first
C) mT Air mass
Warm, moist
Clear skies
Southerly winds
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Idealized Weather of a Midlatitiude Cyclone
Veering: winds rotate/shift in a clockwise direction
South of the storm
D) Cumulonimbus Clouds
Skies will clear as you move into the mT region or cP region
Heavy rains
Violent weather as cold
front approaches
Idealized Weather of a Midlatitiude Cyclone
Backing: winds rotate/shift in a counterclockwise direction
North of the storm, pass through the occlusion
E) Temperatures Drop
Cold with precipitation
cP Air mass
Descending air
Clear skies
Low precipitation
Wind shifts from southerly
to westerly.
Idealized Weather of a Midlatitiude Cyclone
F-G)
Occluded front region
Temperature remains cool
Precipitation beneath the
front
Type of precipitation
depends on the lower
temperature profile
OF moves slower than the
warm or cold fronts
System rotates
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Surface cyclone: centered below the jet stream
downwind of a upper level trough
Polar Front Theory developed from surface observations
Cyclonic & Anticyclonic Circulation
Wave develops
Cyclones and anticyclones are typically found together
1. Topographic irregularities
(Mnts)
Surface divergence under an anticyclones feeds surface convergence
under the cyclone
2. Temperature contrasts (land/sea)
Divergence aloft must be greater than convergence at the surface
under a cyclone
3. Ocean current influence
(hurricanes)
Regions of Cyclogenesis
Conditions aloft
- Topographic irregularities (Mnts)
Surface cyclones are preceded by intensification of airflow aloft
- Temperature contrasts (land/sea)
Zonal airflow (W-E) little cyclonic activity
- Ocean current influence (hurricanes)
Longitudinal airflow (N-S) increase cyclonic activity
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Storm Tracks: Patterns of Movement
Warm Conveyor Belt:
In general east to northeast track
mT air moves toward the middle of cylone, north over mP or cP air
Ascends to middle troposphere (JS) joins the general westerly flow
Primary producer of precipitation
Troposphere
Most of the north Pacific storms that influence the west coast do
not make it over the Rockies in tact (redevelop)
Storm Tracks: Patterns of Movement
12km = avg. thickness
16km = tropics
09km = poles
Cold Conveyor Belt:
Originates at surface ahead of the warm front
Flows westerly around the center and ascends, precipitation
Air joins the general westerly circulation aloft
Nor’easter, mP air is entrained from the North Atlantic
Modern View: The Conveyor Belt Model
Dry Conveyor Belt:
3 intersecting air streams (belts)
Originates at the uppermost troposphere
2 belts originate at the surface and ascend
Cold and dry
1 belt originates aloft and descends
Splits and descends behind the cold front
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Nor’easters
10/21/04 – 10/27/04
Hs = 3.8 to 4 m
Waves
T = 14 sec.
Max setup = ~ 65 cm (~2ft)
November 1950 La Guardia Airport
FDR Drive December 1992
Ref: Bloomfield, J., M. Smith and N. Thompson, 1999. Hot Nights in the City. Environmental Defense Fund, NY.
Tides
10/21/04
10/23/04
10/25/04
10/27/04
The Perfect Storm (12 UTC October 31,1991)
Regions of Cyclogenesis
Level 4
Alberta
East Coast
Nevada
Colorado
FL/Bahamas
Gulf
Davis and FitzGerald, 2004
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General Observations by Month
Nor’easter Intensity Scale
1347 Hindcast Storms Off North Carolina (1942-1984)
Number of Storms:
April (135)
March (128)
September (125)
Level 3 Storms (194):
March (32)
January (27)
December (25)
Level 4 Storms (30):
March, October, December (5)
January (4)
Relative Power = (Hosig(m))2 x Storm Duration (hr)
Storm Class
Hosig (m)
Duration (hrs)
Range (m2hr)
1 Weak
2.0
8
Power <= 71
2 Moderate
2.5
10
71 < Power <= 163
3 Significant
3.3
34
163 < Power <= 929
4 Severe
5.0
63
929 < Power <= 2322
5 Extreme
7.0
96
Power > 2322
Dolan & Davis, 1992, Journal of Coastal Research.
General Observations by Winter Season (Oct-Apr)
1991 - 2005
Number of Storms:
92-93 (77)
97-98 (71)
04-05 (68)
Level 3 Storms (194):
04-05 (18)
02-03 (16)
97-98 (14)
Level 4 Storms (30):
02-03 (4)
93-94, 94-95, 97-98 (3)
Severe Winters:
97-98 (14,3,1)
02-03 (16,4)
04-05 (18,2)
Nor’easter Intensity Scale
900
800
Number of Storms
700
600
1189
Level
%
Hosig (m)
853
1 (Weak)
72
1.8
hrs
4
110
2 (Moderate)
9
2.5
20
194
3 (Significant)
16
3.2
39
500
30
4 (Extreme)
2-3
4.0
94
400
2
5 (Severe)
<1
4.4
146
300
200
100
0
1
2
3
Level
4
5
10