Transcript Ch. 6 lecture - U

Chapter 6—Weather Systems
General Introduction
• What is a weather system?
– An atmospheric circulation pattern and the
weather associated with it
• Spatial scale: ~ 1km  ~ 2000km
• Temporal scale: hour(s)  weeks
• Tornado/tornadic thunderstorms are at
smallest spatial and temporal scales, while
cyclones, anticyclones, hurricanes are at
largest spatial and temporal scales
Scales of Weather/Climate
Phenomena
Air Masses
• Definition: a large body of air that has fairly uniform
weather attributes, mainly with respect to temperature
and moisture
• Horizontal and vertical extents vary from one air mass
to another
• Vertical temperature profile also differ between air
masses
• Location, season, and upper-air flow play roles in
determining type of air masses affecting a region
(think of San Diego vs. Tucson, or just Tucson in May
vs. Tucson in July and August)
Air Masses (cont’d)
• Names depend on temperature and moisture
characteristics, which are determined by the source
region (area where air mass develops)
– Moisture: maritime (m) vs. continental (c)
– Temperature: Arctic (A), Antarctic (AA), Polar (P), Tropical
(T), Equatorial (E)
• Combine moisture and temp letters to get various types
• Most common air mass types to affect U.S.: mP, mT, cP,
cT, and cA
Air Masses Affecting North
America During Winter
Air Masses Affecting North
America During Summer
Movement of Air Masses
• Upper-level wind patterns move surface
weather systems and air masses
• Air masses modify from original state as
change location due to:
– Distance from source region
– Characteristics of new location
– Advection of other temp and moisture qualities
** Stability is affected by movement **
Fronts
• Front: leading edge of an air mass or the
boundary between different air masses
• There are four main types of fronts in terms
of temperature and and moisture
characteristics:
1. Cold
2. Warm
3. Occluded
4. Stationary
• Other types of fronts exist, but usually only
occur on small scales (gust front) or in
specific regions (dryline)
Fronts and Air Masses
Cold Front
• Leading edge of cool/
cold air
• Designated as blue
line with triangles on
side of front in
direction of movement
• Cooler, denser air
forces warm air up
• Generally associated
with showers and tstorms
Interaction of Cold and Warm Air
cold air mass
warm air mass
Cold Front
Warm Front
• Leading edge of warmer
air
• Designated as red line
with half circles on side
of front in direction of
movement
• Warmer air rides up and
over cooler air
• Generally associated
with steady
precipitation, but
depends on stability
Warm Front
Occluded Front
• Occur when cold front
catches warm front
• Designated as purple
line with alternating
triangles and half circles
that indicate direction of
movement
• Different types
• Can see steady or intense
precip, depending on
stability
Occluded Fronts
In an occluded front, a warm front is overtaken by a cold front.
the warm air is pushed aloft and it is not longer in contact with the
ground
Warm air
Cold air
Cool air
Stationary Front
• Occur when boundary
between air masses is
not moving
• Designated as blue
triangles pointing into
warmer air and half
circles “pointing” into
cooler air
• If any precip is falling, it
tends to be light and
steady
Cyclones and Anticyclones
Review from yesterday
• Cyclone
– Associated with low pressure
– In N.H.  counterclockwise motion
– Convergence at surface: air spirals toward center and
“piles up” (like pouring water into clogged funnel)
– In general, can expect cloudy weather and precipitation
• Anticyclone
– Associated with high pressure
– In N.H.  clockwise motion
– Divergence at surface: air flows away from center (like
pouring water on an up-side down bowl)
– In general, can expect fair weather
Types of Cyclones
• Wave cyclone or mid-latitude cyclone
– Large spiral of air that repeatedly forms,
intensifies, and dissolves along polar front
• Tropical cyclone
– Smaller than wave cyclone, forms between 815°N and between 8-15°S
• Tornado
– Small area of intensely spiraling air
Wave or Mid-latitude Cyclone
•
•
Large spiral of air that repeatedly forms, intensifies,
and dissolves along polar front
Life cycle:
1. Early: wave forms along boundary between warm and
cold air (polar front); cold air moving southward, warm
air moving northward
2. Open: wave “deepens,” cold front and warm fronts
develop; precip strongly developed
3. Occluded: cold front has caught warm front; warm air
forced aloft at center of L; precip intensifies
4. Dissolving: polar front distinguishable; still warm air
aloft; precip slowly ends; clouds go bye-bye
Early Stage
• Wave forms along
boundary between
warm and cold air
(polar front)
• Cold air moving
southward, warm air
moving northward
Open Stage
• Wave “deepens,”
cold front and warm
fronts develop
• Precip strongly
developed
Occluded Stage
• Cold front has caught
warm front
• Warm air forced aloft
at center of L
• Precip intensifies
Dissolving Stage
• Polar front
distinguishable again
• Still warm air aloft
• Precip slowly ends
• Clouds go away
Variation of Weather Within a
Wave/Mid-latitude Cyclone
•
If you stop a wave cyclone and walk/drive
through it from behind the cold front to past the
warm front, you would experience:
1.
Behind cold front: NW winds, cold temps, low
humidity
2. At cold front: stronger winds, precip (usually intense)
3. Between cold and warm fronts: SW or S wind,
warmer temps, higher humidity
4. As approach warm front: still warm temps and high
humidity, sometimes precip (usually light)
5. Past warm front: SE or E winds, cooler temps, lower
humidity (but higher than behind cold front), precip
As cyclone passes over a region you would experience
these in reverse order
Tornado
• Small area of intensely spiraling/rotating air
• May only last short time (20-30 minutes),
but are very destructive
• Can occur just about anywhere when
conditions are appropriate (topography does
not necessarily preclude development)
• All states in US have tornadoes
Tornadoes
• Small vortex with very intense winds that
extends from base of thunderstorm to the
Earth’s surface
• Visible due to dust, debris, and condensed
moisture
• Diameter: 100s of meters to > 2 km at base
• Usually short-lived (20-30 minutes)
• Most common in US, especially during
spring and summer, due to appropriate
ingredients
• Tornado Alley – region from west-central
Texas to western Great Lakes
Fujita Scale (or F-scale)
Rating
F0
F1
F2
F3
F4
F5
F6*
* hypothetical
Category
Weak
Strong
Violent
Catastrophic
Estimated wind
mph
km/hr
40-73
65-118
74-112
119-181
113-157
182-253
158-206
254-332
207-260
333-419
261-318
420-513
> 319
> 513
Tornado Alley
A Few Examples
• Tri-State Outbreak (March
18, 1925)
– Missouri, Illinois, and Indiana
– 7 tornadoes over ~ 440 mi
(700km), 695 deaths
• Palm Sunday Outbreak
(1965)
– Midwest US
– 30 tornadoes in 5 states, 256
deaths
• Topeka, KS (June 8, 1966)
– 16 deaths, hundreds of houses
destroyed or damaged,
Washburn University badly
damaged
A Few More Examples
• Lubbock, TX (May 11, 1970)
– 26 deaths, > $100 million in damage
– Weather observer records
• April 3-4 Outbreak (1974)
– 13 states (central AL to s. MI, central IL to central VA)
– 16 hrs, 148 tornadoes, 307 deaths, > 6000 injuries,
approximately $600 million in damage
• Central Oklahoma Outbreak (May 3, 1999)
– ~ 74 tornadoes through Oklahoma
– In OKC, estimated$1.2 billion in damage, 41 deaths,
675+ injuries (winds estimated at 300 mph)
Cyclone Tracks
• Wave cyclones
tend to form in
certain areas
and travel
common paths
• Mid-latitude
wave cyclones
tend to travel
eastward
• Tropical
cyclones tend
to move
westward
Tropical Cyclones
• Development involves 3 stages
– Tropical Disturbance (Easterly Wave)
– Tropical Depression
– Hurricane
• Actually have different names in different
region during stage 3
– Hurricane: Atlantic and eastern Pacific Oceans
– Typhoon: western Pacific
– Cyclone: Indian Ocean
Storm Life Cycle
• Tropical Cyclones or Tropical Disturbance
(Easterly waves) are troughs of low pressure
moving eastward with a mass of thunderstorms
and counterclockwise flow
• Tropical Depression: winds up to 38 mph (33
knots)
– Referred to as TD and Number
• Tropical Storm: wind range from 39-73 mph (3463 knots), isobars “packed,”
– Referred to with Name
• Hurricane: wind exceeds 74 mps (64 knots) then
ranked based on central pressure, storm surge
height, wind speed using Saffir-Simpson scale
Rare Satellite Image
Hurricanes
•the most powerful
and destructive
tropical cyclone in
the western
hemisphere
•Hurricane=Tropical
Cyclone
Hurricanes
• Intense tropical cyclone containing strong winds
and excessive precipitation
• Energy from latent heat release, loses energy over
land or cooler water
• Diameter—Average approximately 300 miles
wide
• Hurricane force winds extend 25 miles (small) to
150 miles (large) and tropical storm force winds
(300 miles)
• Associated problems include: strong winds,
tornadoes, heavy rain, storm surge (most
devastating aspect), death, water and food supply
Hurricanes
characteristic central “eye” (clear skies and calm winds)
air descends from high altitudes, warming wind speeds are
highest at the “eye wall” winds spiral outward creating high
wind speeds
Ingredients for a Hurricane
• 1. Pre-existing disturbance with
thunderstorms (Easterly wave)
• 2. Warm (80ºF) ocean temperatures (150 ft)
• 3. Light upper level winds (which provide
little wind shear)
• 4. Convergence of moisture and heat
Hurricanes (cont’d)
• Different names throughout globe
– Hurricane in Atlantic and eastern Pacific
– Typhoon in western Pacific and Philippines
– Cyclone in India and Australia
• Official hurricane seasons
– N. Atlantic Ocean: June 1 – November 30 (peak
occurrence from mid-August to late October)
– N. Pacific Ocean: May 15 – November 30 (peak
occurrence from July through October)
Simpson-Saffir Scale of
Tropical Cyclone Intensity
Categories 1 to 5 (5 is the most intense and
devastating)
categorized by central pressure, storm surge
wave height and mean wind speed
Simpson-Saffir Scale of Tropical
Cyclone Intensity
Category
1 (weak)
2 (moderate)
3 (strong)
4 (very strong)
5 (devastating)
Central
Pressure
(mb)
> 980
965-979
945-964
920-944
< 920
Storm
Surge
(ft)
4-5
6-8
9-12
13-18
>18
Mean
Wind
(mph)
74-95
96-110
111-130
131-155
> 155
Naming Hurricanes
• Before 1953: used latitude and longitude (or location of
damage)
• 1953 – 1977: used only female names (in alphabetical
order)
• 1978: alternate between male and female names for
eastern Pacific Ocean
• 1979: alternate between male and female names for
Atlantic Ocean
• Also:
–
–
–
–
In Atlantic, begin at start of alphabet each year
In Pacific, continue where left off previous year
Names retired if causes much devastation (life, property)
See http://www.aoml.noaa.gov/hrd/tcfaq/B2.html for recent
and upcoming names
MH Katrina- Hit as Cat. 4 (145 mph winds)
August 29th 2005
Eye is 30 miles across
Hurricane winds extend 125 miles from center
New Orleans and Biloxi both in Eye Wall
Biloxi
New Orleans
Atlantic Sea Surface Temperature Departures
1870
1900
1930
1970
1995
Strong multi-decadal fluctuations in Atlantic temperatures
dating back to the 1870’s.
Florida Coastal Population
*87M Atlantic/Gulf Residents
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