1. Chapter 7: Atmospheric Circulations
Scales of atmospheric motions
Eddies - big and small
Local wind systems
Global winds
Global wind patterns
and the oceans

2. 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?
Q: what is the scale of lake breeze over the Great Lakes
Fig 7.1

3. Figure 7.2: The scales of atmospheric motion with the phenomena’s average size and life span. (Because the actual size of certain features may vary, some of the features fall into more than one category.)

4. 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

5. Clear-air turbulence: caused by wind shear; important for aviation
Billow clouds: caused by mountain waves in a wind shear zone
Figure 1: The formation of clear air turbulence (CAT) along a boundary of increasing wind speed shear. The view of wind is from the side, with a top layer of air moving over a layer below.
Q: what could cause bumpy aircraft flight in the upper troposphere?
a) clear-air turbulence, b) rotor, c) billow clouds

6. 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

7. 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

8. 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
Figure 7.6: Typically, during the summer over Florida, converging sea breezes in the afternoon produce uplift that enhances thunderstorm development and rainfall. However, when westerly surface winds dominate and a ridge of high pressure forms over the area, thunderstorm activity diminishes, and dry conditions prevail.

9. height
Q: given the isobars as left, what is the near-surface wind direction?
a) from A to B
b) from B to A
Q: 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

10. 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

13. 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?
Figure 7.10: Enhanced infrared satellite image with heavy arrow showing strong monsoonal circulation. Moist, southernly winds are causing showers and thunderstorms (yellow and red areas) to form over the southwestern section of the United States during July, 2001.

14. Mountain and Valley Breezes
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

15. 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?

16. 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

17. Chinook wall cloud indicates that chinook is coming
Figure 7.14: A chinook wind can be enhanced when clouds form on the mountain’s windward side. Heat added and moisture lost on the upwind side produce warmer and drier air on the downwind side.

18. 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

19. 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

20. Haboob: A spectacular example of duststorm caused by thunderstorm downdraft; tens of kilometers horizontally and hundreds of meters vertically

21. 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

22. General Circulation of the Atmosphere
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 on p. 46

23. 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
UK’s Hadley Centre for Climate
Research is named after George Hadley.

24. Three-cell Model
model for a rotating earth; overall realistic for surface fields

25. 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

26. 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

27. Figure 7.22: Average sea-level pressure distribution and surface wind-flow patterns for January (a) and for July (b). The heavy dashed line represents the position of the ITCZ.

29. The General Circulation and Precipitation Patterns
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

30. Q: Why is Los Angeles dry in summer, while Atlanta is wet?

31. 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

33. (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
Figure 7.29: Average position and extent of the major surface ocean currents. Cold currents are shown in blue; warm currents are shown in red.
Fig. 7-29, p. 193

34. 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

35. 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:

36. Thermocline is incorrect in bottom panel
Figure 7.32: In diagram (a), under ordinary conditions higher pressure over the southeastern Pacific and lower pressure near Indonesia produce easterly trade winds along the equator. These winds promote upwelling and cooler ocean water in the eastern Pacific, while warmer water prevails in the western Pacific. The trades are part of a circulation (called the Walker circulation) that typically finds rising air and heavy rain over the western Pacific and sinking air and generally dry weather over the eastern Pacific. When the trades are exceptionally strong, water along the equator in the eastern Pacific becomes quite cool. This cool event is called La Niña. During El Niño conditions—diagram (b)—atmospheric pressure decreases over the eastern Pacific and rises over the western Pacific. This change in pressure causes the trades to weaken or reverse direction. This situation enhances the countercurrent that carries warm water from the west over a vast region of the eastern tropical Pacific. The thermocline, which separates the warm water of the upper ocean from the cold water below, changes as the ocean conditions change from non-El Niño to El Niño.

38. 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

39. 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

40. Over midlatitude Pacific, PDO pattern is different from ENSO;
Pacific Decadal Oscillation: Pacific surface temperature pattern changes every 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
Figure 7.36: Typical winter sea surface temperature departure from normal in °C during the Pacific Decadal Oscillation’s warm phase (a) and cool phase (b).
(Source: JISAO, University of Washington, obtained via Used with permission of N. Mantua.)

41. 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