Chapter 7: Atmospheric Circulations

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Chapter 7 Atmospheric Circulations

• Scales of atmospheric motions
• Eddies - big and small
• Local wind systems
• Global winds
• Global wind patterns
• and the oceans

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Scales of Atmospheric Motions

• Microscale meters - kilometers
• Mesoscale km a few hundred km
• synoptic scale a few hundred km a few thousand
  km
• planetary scale a few thousand km and larger
• Q what is the scale of atmospheric boundary
  layer turbulence?
• a) microscale, b) mesoscale, c) synoptic
  scale
• Q what is the scale of weather fronts?
• a) microscale, b) mesoscale, c) synoptic
  scale
• Q what is the scale of lake breeze over the
  Great Lakes
• a) microscale, b) mesoscale, c) synoptic
  scale

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Eddies - Big and Small

• eddy or turbulent eddy caused by convection
  (heating or cooling), wind shear (or near surface
  wind), or waves
• Rotor caused by mountain waves
• wind shear change of
• wind speed or direction
• with height

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• Clear-air turbulence caused by wind shear
  important for aviation

Billow clouds caused by mountain waves in a wind
shear zone
Q what could cause bumpy aircraft flight in the
upper troposphere? a) clear-air turbulence,
b) rotor, c) billow clouds
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Local Wind Systems

• Thermal Circulations warm air rises and cool air
  sinks
• 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

• sea breeze from sea to land
• land breeze land to sea
• sea breeze front clouds
• Florida sea breezes

• 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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height
Q7 given the isobars as left, what is the
near-surface wind direction? a) from A to B b)
from B to A Q8 During the day, if you stand on
beach, what would be the wind direction due to
sea breeze? a) from sea to beach b) from land to
sea
A B
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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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North American monsoon Q Coriolis force is
important for monsoon circulation. Is it
as important for sea breeze as for monsoon? a)
yes b) no Q Still, is Coriolis force important
for sea breeze? a) yes b) no
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Mountain and Valley Breezes

• valley breeze daytime from valley to top
• mountain breeze nighttime from top to valley

• The nighttime mountain breeze is sometimes
  calledgravity winds or drainage winds, because
  gravitycauses 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

• 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

• 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

• 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 lt100 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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Q give a phenomenon and its typical
temporal/spatial scales for each scale (micro,
meso, synoptic, global) Q What is the
direction of sea breeze? What is the formation
mechanism? Q what wind would produce clouds? a)
valley breeze, b) mountain breeze Q Indian
summer monsoon brings rainfall to India. The
winter monsoon is a) wet, b) dry Q
Suppose you are fishing in a mountain stream in
the early morning. The Wind is more likely
blowing a) upstream, b) downstream
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General Circulation of the Atmosphere

• cause unequal net heating of the earths 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 earths
  rotation

• UKs Hadley Centre for Climate
• Research is named after George Hadley.

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Three-cell Model

• model for a rotating earth overall realistic for
  surface fields

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Three-cell Model

• realistic over the tropics in winter hemisphere
• Hadley cell doldrums subtropical
  highs
• trade winds intertropical convergence
  zone (ITCZ)
• 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

• 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

• Most of the worlds 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

• 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
Fig. 7-29, p. 193
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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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• ENSO is an example of a global-scale weather
  phenomenon.
• Q What is the El Nino effect on winter weather
  in the U.S.? A Northwestern U.S. usually
  has a warmer winter, Southeast usualy has a
  wetter winter, and often Arizona has a wetter
  winter

El Nino effect
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Other Atmosphere-Ocean Interactions

• North Atlantic Oscillation (NAO) based on
  pressure difference between Bermuda and Iceland
• Arctic Oscillation (AO) pressure difference
  between about 45oN and Arctic describes the same
  phenomenon in a similar way as NAO.
• Positive (negative) phase stronger (weaker)
  zonal wind

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Pacific Decadal Oscillation Pacific surface
temperature pattern changes every 20-30
years Over the tropical Pacific, PDO pattern is
not very different from ENSO Over midlatitude
Pacific, PDO pattern is different from
ENSO Positive (negative) phase warmer (cooler)
surface water along the west coast of North
America Amplitude is smaller than ENSO
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Q Most U.S. is located in a) westerly wind
belt, b) easterly wind belt, c) southerly wind
belt, d) northerly wind belt Q Why do
summers along the west coast of U.S. tend to be
dry? Q What coastal wind along the west coast
of North America would produce strong upwelling?
a) northerly, b) southerly, c) easterly, d)
westerly Q What is a major El Nino? Q for the
positive phase of NAO, a) Greenland is colder and
northern Europe is milder, b) Greenland is milder
and northern Europe is colder, c) Greenland and
northern Europe are both colder, d) Greenland
and northern Europe are both milder