Transcript Part 2

The Atmosphere, Part 2:
Winds & Storms
v 3.1
Thomas V.
Dagenhart, M.S.
Storm over the Atlantic near the Sargasso Sea; Dagenhart, 2003
A Convection-Driven Circulation Cell
• Similar to circulation on a non-rotating Earth.
Equator
Poles
Garrison, 2005
Wind Circulation on a Non-Rotating Earth
•
•
•
•
•
Driven by
unequal
heating.
Warm air
rises at
equator.
Cool air falls
at poles.
Surface air
flows directly
to equator.
Upper air
flows directly
to poles.
Gross &
Gross, 1996
The Coriolis Effect
• Acts on winds, ocean
Gross &
Gross, 1996
currents, and anything
in motion.
• Acts to right in Northern
•
•
Hemisphere and to left
in Southern Hemisphere.
Zero at equator and
strongest at poles.
Earth actually rotates out
from beneath moving
object.
North
Pole
Reason for the
Coriolis Effect
•
•
Buffalo &
Quito rotate
through the
same number
of degrees
each day.
But Quito
covers many
more
kilometers,
i.e. Quito has
a greater
rotational
speed.
Garrison,
2005
Reason for
the Coriolis
Effect
(continued)
• Has
Garrison,
2005
The Coriolis
Effect
• Objects veer
to the right in
the northern
hemisphere.
Garrison,
2005
Coriolis Effect on Moving Objects
• Causes curved paths except at equator.
• Effect increases with velocity, mass &
•
•
distance.
Ball moves horizontally on slope when
Coriolis effect balances gravity (requires
high velocities and ignores friction)
Leads to geostrophic (“earth turned”)
winds and currents
Gross &
Gross, 1996
Global Wind Patterns
Horse Latitudes
Horse Latitudes
Garrison,
2005
Global Wind Patterns
Gross &
Gross, 1996
Intertropical Convergence Zone (ITCZ)
•
•
•
•
•
Meteorological
equator, ~5o north
of geographical
equator.
Marked by cloud
bands.
Moves N-S with
seasons.
Controls location
of rain forests.
Location of weak
intermittent
surface winds
known as
Doldrums.
Gross &
Gross, 1996
ITCZ Seasonal Variation
• Average position deflected northward by:
– greater land mass in Northern Hemisphere; land heats
•
faster than water.
– greater reflectance by Antarctic snows than by Arctic
Ocean ice cover; limits solar heat gain.
Surface salinity diluted by heavy rain bands under ITCZ.
Rainfall
1979-2001
Gross &
Gross, 1996
Generalized Surface Winds over
the Ocean in February
• Local winds vary as storms pass through.
• Wind bands migrate N-S with seasons.
• Shifted S with Southern Hemisphere summer.
Gross &
Gross, 1996
Generalized Surface Winds Over
Ocean in August
• Note Northward shift of all belts compared to February.
Gross &
Gross, 1996
• Lines of equal temperature.
• Lines are close together where the temperature
Isotherms
changes rapidly, a steep temperature gradient.
X
X
X
Wikipedia,
2009
Low and High Pressure Systems
• Vertical
cross
section
•
Map
view
for N.
hemisphere
Gross &
Gross, 1996
Formation of An Extratropical Cyclone, A & B
• Front forms along boundary between two air masses, one
•
•
•
•
warm & one cold, along Polar Front.
Then a wave develops along the front (not well understood).
Extratropical = “outside of tropics”, ~ 30 to 60o from Equator.
Synonyms = low pressure system, low, mid-latitude storm,
nor’easter.
May generate huge ocean waves.
Gross &
Gross, 1996
Stationary
Front
Formation of An Extratropical Cyclone
•
•
•
•
•
Parts C & D
Cyclonic circulation flows counterclockwise in N. hemisphere.
Cyclonic circulation flows clockwise in S. hemisphere.
Occlusion develops as the cold front catches up to the warm
front.
Lows usually move from W to E with Prevailing Westerlies.
Steered by jet streams, cold core, more common in winter.
Cold
Front
Warm
Front
Gross &
Gross, 1996
Formation of An Extratropical Cyclone
•
•
Parts E & F
Note well developed comma shape of clouds in fully
occluded cyclone.
As cyclone dissipates warm air has moved aloft, cold is
found below.
Secondary low
may develop here.
Occluded
front
Gross &
Gross, 1996
Another Look at
Extratropical Cyclone
Formation, A & B
• Form along Polar Front
especially during winter.
Garrison,
2005
Another Look at
Extratropical Cyclone
Formation, C
•
Note the difference in
width of cloud bands for
cold vs. warm fronts.
Vilhelm Bjerknes, a Norwegian,
first theorized about air
masses, fronts & cyclones.
Garrison, 2005
Warm Fronts
• Broad cloud & precipitation band, 100’s km wide.
• Classic progression of clouds from high altitude cirrus along
leading edge to low altitude nimbostratus as front passes.
Gross &
Gross, 1996
Cold Fronts
• Narrow cloud & precipitation band, 10’s km wide.
• Mostly cumulonimbus clouds.
• Cloud band may be well ahead of actual front.
• Sudden wind shift as front passes.
Gross &
Gross, 1996
Nor’easter
Damage
• A fierce extratropical
•
•
cyclone.
Ash Wednesday
storm, Mar. 7, 1962.
Wreaked havoc from
NC to New England.
Nor’easter Damage
Outer Banks, NC
Nov. 2009
Fire Island, NY
Garrison, 2005
Fully-Developed Extratropical Cyclone
• Following Prevailing Westerlies and approaching western N.
•
America on October 27, 2000.
Visible-light photo by GOES-10 satellite, note commas shape.
Cold Front
Garrison,
2005
Warm Front
Satellite Photo of Extratropical Cyclone
• Which
direction
does the
storm
circulate?
• In which
•
hemisphere
is this storm
located?
Appears
hurricanelike, but no
eye & larger
diameter.
Gross &
Gross, 1996
Hurricanes
• AKA typhoons (W.
•
•
•
•
•
•
Pacific, tropical cyclones
(Indian O.), baguios
(Philippines)
CCW circulation in N.
Hemisphere
CW circulation in S.
hemisphere
Well developed eye.
Formed in tropics inside
warm mass, warm core.
Derive energy from
warm ocean waters.
Rapidly dissipate over
land.
Hurricane Andrew
approaches Gulf Coast
of U.S., Aug., 1992
N.O.
Worst Path!
Storm Surge!
Gross &
Gross, 1996
Hurricane Formation Areas
• Mostly in tropics 23.5o S to 23.5o N.
• Formation zones (in orange) never within 5o of equator since
•
•
Coriolis effect equals zero there.
Note typical paths indicated by red arrows.
Almost never in S. Atlantic or SE Pacific; water too cold.
Garrison,
2005
Ocean Temperatures Limit Areas
of Hurricane Formation
Sea Temp.
1987-1999
• Reds & oranges indicate highest humidity & air
temperatures, places favorable for hurricane formation.
• This moist air lies above ocean waters warmer than 26oC.
October
1992
Garrison,
2005
Track of Hurricane Georges, Sept. 1998
• Hurricane’s eye varies
in definition.
Garrison,
2005
Hurricane Alberto Viewed from Space
• In this oblique view, note how thin the atmosphere appears
relative to Earth’s size.
Garrison,
2005
•
•
Hurricane Damage in Galveston, TX
Sept. 8, 1900, little warning.
Deadliest U.S. natural disaster (for now), ~8,000 dead.
Garrison,
2005
Anatomy of a Hurricane
• High level clouds exhausted from hurricane’s top & rotate CW.
Garrison,
2005
• Most hurricane winds
& clouds rotate CCW.
Hurricane Near
Hawaii
• Directions of arrows
•
indicate directions of
winds.
Lengths of arrows
indicate speeds of
winds.
Gross &
Gross, 1996
Seasonal Wind Patterns
• Caused by tilt of Earth’s rotational axis.
• Zones of most intense heating & cooling shift N-S with seasons.
Gross &
Gross, 1996
The Summer • Warm air rises over Asia to be replaced by
moist Indian Ocean air.
Monsoon
• Causes intense rains over India & SE Asia.
Note shift
in wind
circulation
as picture
changes.
Ocean
currents
change as
a result.
Gross &
Gross, 1996
dry, dense air flows Southward off Asia
The Winter • Cold,
onto the Indian Ocean, dry season on land.
Monsoon • Monsoons unique to Indian Ocean, no N half
Note shift
in wind
circulation
as picture
changes.
Ocean
currents
change as
a result.
Gross &
Gross, 1996
The Sea Breeze
• Forms in daytime as land heats faster than water.
• Cloud bands form over land & parallel to shore.
• In afternoon, helps fishermen sail home after a morning’s work.
Gross &
Gross, 1996
The Land Breeze
• Forms at night as sea cools more slowly than land.
• Cloud bands form over sea & parallel to shore.
• In early morning, helps fishermen sail out to sea for another
day of work.
Gross &
Gross, 1996
Cumulus Clouds Building over Land as Sea
Breeze Reaches Maximum Development
• Late afternoon facing the mainland.
• Sea breeze hits photographers back.
Emerald Isle, NC; Dagenhart, 2003
Island Effect
• Heavy precipitation occurs on island’s windward (upwind) side.
• A rain shadow forms on leeward (downwind) side of island.
• Mainly on islands with significant elevation, not atolls.
Gross &
Gross, 1996
Island Effect:
Dry Side of Island
• Semi-arid conditions.
• Intermittent streams.
• Like the more extensive
rain shadows found on
leeward side of mountain
ranges.
Dry side of
Kauai, Hawaii
Gross &
Gross, 1996
Island Effect: Wet Side of Island
• Lush rain forest vegetation, perennial streams.
Gross &
Gross, 1996
wet side of
Kauai, Hawaii
References
• Environment Canada
•
•
(2002) Hurricane Terms.
http://www.ns.ec.gc.ca/weather/hurricane/hurricanes9.html
Garrison, T. (2005) Oceanography: An Invitation to Marine
Science, 5th ed. Brooks/Cole Thomson Learning, Stamford, CT,
522 p.
Gross, M.G. and E. Gross (1996) Oceanography: A View of the
Earth, 7th ed. Prentice Hall, Upper Saddle River, NJ, 472 pp.