Transcript end of weather.wind pptm

Local Wind Systems
Thermal Circulations: warm air
rises and cool air sinks
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Warm air leads to H in the
air (i,e, pushing isobar up);
Air moves from H to L;
increases surface pressure
(i.e., pushing near-surface
isobar up) over cool place;
Leads to circulation from
cool place to warm place
near surface
Pay attention to the change
of isobars with height
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Sea and Land Breezes
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sea breeze: from sea to land
land breeze: land to sea
sea breeze front: clouds
Florida sea breezes
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Sea and land breezes also
occur near the shores of
large lakes, such as the
Great Lakes
• Pay attention to the change
of isobars with height
Q: which is stronger in general?
a) sea breeze, b) land breeze,
c) the same
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Q: When do you expect to see the thunderstorm in summer
in Florida?
a) 10am,
b) noon, c) 3pm,
d) 6pm
Q: For the prevailing
northeasterly wind
over southern Florida,
where does the
strongest sea breeze
occur in general ? a)
Eastern coast of
southern Florida, b)
western coast, c)
central part
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Seasonally Changing Winds - the Monsoon
Monsoon wind system: change with season
 India and eastern Asian monsoon
 Global monsoons
Q: What are the differences and similarities between monsoon
and sea/land breeze?
A: Monsoon system is much greater in geographic area;
changes with season; sea/land breeze changes with diurnal
cycle; both due to horizontal temperature difference
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Mountain and Valley Breezes
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valley breeze: daytime; from valley to top
mountain breeze: nighttime; from top to valley
• The nighttime mountain breeze is sometimes called
gravity winds or drainage winds, because gravity
causes the cold air to ‘drain’ downhill.
Q: Which is stronger in general?
a) valley breeze, b) mountain breeze, c) the same
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Katabatic Winds
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Strong drainage wind from cold elevated plateau down steep
slope and/or narrow channel
• Katabatic winds are quite fierce in parts of Antarctica,
with hurricane-force wind speeds.
• Bora: a cold, gusty northeasterly wind along the Adriatic
coast in the former Yugoslavia
Q: why is the air parcel
temperature much
warmer at bottom of
the mountain in the
figure?
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Chinook (Foehn) Winds
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Chinook winds: one type of drainage wind; warm and
dry wind down the eastern slope of the Rocky Mountains
• It is called a Foehn along the leeward slopes of Alps.
Q: Which wind is weakest in general?
a) drainage wind, b) katabatic wind, c) Chinook wind
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Chinook wall cloud indicates that chinook is coming
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Santa Ana Winds
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Santa Ana wind: warm, dry wind from the elevated desert
plateau down to southern California
compressional heating
Could have very strong wind
wildfires
Q: which wind comes from elevated
desert plateau?
a) Chinook wind; b) Santa Ana wind;
c) Katabatic wind; d) mountain breeze
Q: The drainage wind over the lee
side of the Rocky Mountain is
a) Chinook wind; b) Santa Ana wind;
c) Katabatic wind; d) mountain breeze
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Desert Winds
dust and sand storms: occurs over
arid and semiarid regions
 dust devils – from surface;
usually with a diameter of a few
meters and a height of <100 m
Q: What is the difference between
tornado and dust devil? A: Tornado
is larger horizontally and deeper
vertically; from cloud base down
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Haboob: A spectacular example of duststorm caused by
thunderstorm downdraft; tens of kilometers horizontally and
hundreds of meters vertically
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General Circulation of the Atmosphere
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cause: unequal net heating of the earth’s surface and
atmosphere
effect: atmospheric circulation and ocean currents to transport
heat from the equator to the poles
Fig. 2.19 on p. 46
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Single-cell Model
basic assumptions: no rotation
 Originally proposed by George Hadley in England in the 18th
Century
 Hadley cell
Q: Why is the single-cell model wrong? A: Because single cell
does not exist due to earth’s rotation
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• UK’s Hadley Centre
for Climate
Research is named
after George Hadley.
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Three-cell Model
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model for a rotating earth; overall realistic for surface fields
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Three-cell Model
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realistic over the tropics in winter hemisphere:
Hadley cell; doldrums; subtropical highs
trade winds; intertropical convergence zone (ITCZ)
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Over mid- and high latitudes: Ferrel cell and polar cell do not
play major roles;
westerlies in the upper troposphere;
polar front;
polar near-surface easterlies
Q: if near-surface wind is southwesterly over NH midlatitudes,
what is the direction of upper troposphere wind?
a) westerly, b) easterly, c) southerly, d) northerly
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Average Surface Winds and
Pressure: The Real World
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semipermanent highs: Bermuda high & Pacific high
Pacific high moves north in summer;
Bermuda high moves west in summer
Semipermanent low: Icelandic low
it moves north in summer
Siberian high in winter due to very cold air
Aleutian low in winter due to storm track
ITCZ stays in the warm hemisphere (e.g., NH in July)
There are three semipermanent highs in SH
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The General Circulation and
Precipitation Patterns
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Most of the world’s
thunderstorms are found
along the ITCZ.
Low rainfall over the
subtropical regions
Fronts and precipitation
over the subpolar lows
Q: which is correct?
a) desert causes subtropical
high;
b) subtropical high causes
desert
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Q: Why is Los Angeles dry in
summer, while Atlanta is wet?
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Westerly Winds and the Jet
Stream
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jet streams
subtropical jet stream
polar front jet stream
Low-level jet stream over
the Central plains of the
U.S. (within 2 km above
surface), bringing moist
and warm air to form
nighttime thunderstorms
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(warm) Gulf Stream; (warm) Kuroshio Current;
(cold) California Current; (cold) Canary Current;
Equatorial Current and Counter Current in the Pacific
Global wind drives
ocean current
Q: These ocean
circulations are
consistent with
wind of
a) high pressure
system;
b) low pressure
system
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Fig. 7-29, p. 193
Winds and Upwelling
Upwelling is strongest when wind
is parallel to the coastline
Q: Why is ocean coldest in northern
California in the left figure?
A: wind is parallel to the coastline;
upwelling is strongest; cold deep
water is brought to surface
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El Niño and the Southern Oscillation
Q: what is the El Niño? A: warming, pressure decrease, and
weakened upwelling over the central and eastern Pacific; trade
wind weakened as well;
(cooling, pressure increase over western Pacific)
 La Niña: opposite
Q: What is the Southern Oscillation (SO)? A: oscillation of
surface pressure over tropical western and eastern Pacific
 ENSO: El Nino and SO are closely related
Q: What is teleconnections? A: local changes (e.g., in sea surface
temperature) affect weather in remote regions.
SST animation:
www.esrl.noaa.gov/psd/map/clim/sst.anom.anim.year.html
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Thermocline
is incorrect
in bottom
panel
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