Atmospheric Circulation

1
Atmospheric Circulation
Lecture 12
OEAS-306
March 3, 2009

• Outline
• Review from Last Lecture
• The Geostrophic Balance
• Surface Wind Patterns
• Global Heat Budget
• Atmospheric Circulation
• Storms (Hurricanes and Noreasters)

2
Conservation of Mass The Continuity Equation
This states that a divergence or convergence of
the flow in one direction must be compensated for
by a convergence or divergence in one or both of
the other directions so that the combined effects
sum to zero.
3
Newtons 2nd Law (F ma) The Momentum Equation
Horizontal Momentum Equation
x-momentum
acceleration
friction
Coriolis
Pressure Gradient
z
Velocity in x-direction u Velocity in
y-directon v Velocity in z-direction w
y
x
y-momentum
4
Unbalanced pressure gradient will cause an
acceleration.
?x
B
A

• Is the pressure higher at location A or location
  B?
• So, what is the direction of the pressure
  gradient?
• What direction will this tend to accelerate the
  flow?

5
What about Rotation?
http//www.youtube.com/watch?v_36MiCUS1ro
Low Pressure
?z
y
u
v
x
High Pressure
Rotation appears to deflect flow to the right
(northern hemisphere.)
6
In the absence of friction, Coriolis force can
balanced by Pressure Gradient.
This is called the Geostropic Balance
In the absence of friction, wind (or current) is
parallel to pressure gradient.
7
Composition of Atmosphere
Nitrogen (N2) 78
Oxygen (O2) 21
Argon (Ar) Carbon dioxide (CO2) Neon, Helium,
Methane, and other elements and compounds

• The lower atmosphere is a fairly homogeneous
  mixture of gases.
• Water vapor occupies up to 4 of the volume of
  the atmosphere
• The density of air is influenced by temperature
  and water content.
• Average density is 1.2 kg/m3 _at_ 20C and sea-level
• Humid air is less dense than dry air.

8

• Ascending air cools (adiabatically) as it
  expands. Cooler air can hold less water, so water
  vapor condenses into tiny droplets clouds (or
  rain).
• Descending air warms (adiabatically) as it
  compresses the droplets (clouds) evaporate.

9
Cyclonic and Anti-cyclonic Circulation

• Flow is convergent at center of Low Pressure and
  divergent at center of High Pressure (due to
  slight influence of friction).
• So, air rises at Low Pressure center a descends
  at High Pressure Center.
• Rising air cools (adiabatically) forming clouds
  and rain. Low Pressure Bad Weather
• Falling air warms (adiabatically) so moisture
  evaporates. High Pressure Good Weather

10
What was up with the weather yesterday?
11
The Atmosphere Moves in Response to Uneven Solar
Heating and Earths Rotation

• Atmospheric circulation is powered by sunlight.
  Since Earth is in thermal equilibrium, what
  assumption can be made about the input and output
  of heat on Earth?

(above) An estimate of the heat budget for Earth.
On an average day, about half of the solar energy
arriving at the upper atmosphere is absorbed at
Earths surface. Light (short-wave) energy
absorbed at the surface is converted into heat.
Heat leaves Earth as infrared (long-wave)
radiation. Since input equals output over long
periods of time, the heat budget is balanced.
12
The Solar Heating of Earth Varies with Latitude
How solar energy input varies with
latitude. Equal amounts of sunlight are spread
over a greater surface area near the poles than
in the tropics. Ice near the poles reflects much
of the energy that reaches the surface there.
13
The Solar Heating of Earth Varies with Latitude
Earth as a whole is in thermal equilibrium, but
different latitudes are not. (top left) The
average annual incoming solar radiation (red
line) absorbed by Earth is shown along with the
average annual infrared radiation (blue line)
emitted by Earth. Note that polar latitudes lose
more heat to space than they gain, and tropical
latitudes gain more heat than they lose. Only at
about 38 N and 38 S latitudes does the amount
of radiation received equal the amount lost.
Since the area of heat gained (orange area)
equals the area of heat lost (blue areas),
Earths total heat budget is balanced.

• What factors govern the global circulation of
  air?
• Uneven solar heating
• The Coriolis effect
• (bottom left) The ocean does not boil away near
  the equator or freeze solid near the poles
  because heat is transferred by winds and ocean
  currents from equatorial to polar regions.

14
Earths Uneven Solar Heating Results in
Large-Scale Atmospheric Circulation
A convection current forms in a room when air
flows from a hot radiator to a cold window and
back. Air warms, expands, becomes less dense,
and rises over the radiator. Air cools,
contracts, becomes more dense, and falls near the
cold glass window.
15
The Coriolis Effect Influences the Movement of
Air in Atmospheric Circulation Cells
Global air circulation as described in the
six-cell circulation model. Air rises at the
equator and falls at the poles, but instead of
one great circuit in each hemisphere from equator
to pole, there are three in each hemisphere. Note
the influence of the Coriolis effect on wind
direction. The circulation show here is idea
that is, a long-term average of wind flow.
16
The Coriolis Effect Influences the Movement of
Air in Atmospheric Circulation Cells

• A large circuit of air is called an atmospheric
  circulation cell.
• Three cells exist in each hemisphere.
• Hadley cells are tropical cells found on each
  side of the equator.
• Ferrel cells are found at the mid-latitudes.
• Polar cells are found near the poles.
• What are some of the wind patterns found between
  and within cells?
• Doldrums are calm equatorial areas where two
  Hadley cells converge
• Horse latitudes are areas between Hadley and
  Ferrel cells.
• Trade winds are surface winds of Hadley cells.
• Westerlies are surface winds of Ferrel cells.

17
Monsoons Are Wind Patterns That Change with the
Seasons
Monsoons are patterns of wind circulation that
change with the season. Areas with monsoons
generally have dry winters and wet summers. Sea
breeze is cool air from over the water moving
toward land. Sea breezes occur after
sunrise. Land breezes occur after sunset when
air warmed by the land blows toward the water.
18
Monsoons Are Wind Patterns That Change with the
Seasons
Locations where monsoons occur typically have wet
summers and dry winters.
19
Sea Breezes and Land Breezes Arise from Uneven
Surface Heating

• Land experience much larger variations in
  temperature than the ocean because of the higher
  specific heat of water. As a result, under
  stable weather conditions there is a daily sea
  breeze in many locations.
• (a) In the afternoon, the land is warmer than the
  ocean surface, and the warm air rising from the
  land is replaced by an onshore sea breeze.
• (b) At night, as the land cools, the air over the
  ocean is now warmer than the air over the land.
  The ocean air rises. Air flows offshore to
  replace it, generating an offshore flow (a land
  breezes).

20
Storms Are Variations in Large-Scale Atmospheric
Circulation

• Storms are regional atmospheric disturbances.
  Storms have high winds and most have
  precipitation.
• Extratropical cyclones occur in Ferrel cells, and
  are winter weather disturbances. These storms can
  also cause extensive damage.
• Tropical cyclones occur in tropical regions.
  These storms can cause millions of dollars worth
  of damage and endanger life.
• Both types of storms are cyclones, or rotating
  masses of low-pressure air.
• Cyclones rotate counter-clockwise in northern
  hemisphere and clockwise in southern hemisphere.

21
Extratropical Cyclones Form between Two Air Masses
Polar Cell
Polar Front
Ferrel Cell
22
A disturbance at this boundary leads to the
intensification of Low Pressure
This is what happened Sunday and Monday!
23
Saturday, February 28, 2009
Warm Front
Cold front
24
Yesterday
25
Tropical Cyclones
A tropical cyclone is a warm-core, low pressure
system with an organized circulation that forms
over tropical or subtropical waters. In the
Atlantic/East Pacific Oceans, it is called a
hurricane. In the West Pacific Ocean, it is a
typhoon. In the Indian Ocean, it is known as a
cyclone
Hurricanes are a study in the transfer of heat
Release of latent heat of vaporization (540
cal/gram)
Air expands and cools (adiabatically)
Remember, moist air is less dense than dry air.
Unlike extra-tropical storm, tropical storms get
their energy from warm ocean waters (need ocean
temps 80F)
26
Because of Coriolis effect, winds spin around the
low pressure center of a Hurricane, continually
feeding it warm moist air.
27
Hurricane Katrina August 2005
Cyclone Monica April 2006
Australia
28
The strong winds and storm surge associated with
hurricanes can cause extensive damage to coastal
communities.
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• Summary
• In the absence of friction, many flows in both
  the atmosphere and ocean are in geostrophic
  balance (Pressure Gradient Coriolis
  Acceration).
• As a result, winds generally blow parallel to
  lines of equal pressure (isobars).
• Surface flow around low pressure centers is
  convergent, leading to rising air. This air
  cools adiabatically causing water vapor to
  condense leading to clouds, rain and generally
  bad weather.
• Surface flow around high pressure centers is
  divergent, leading to falling air. This air
  warms adiabatically causing water vapor to
  evaporate leading few clouds and generally good
  weather.
• The earth as a whole is in thermal equilibrium.
• The atmosphere responds to uneven solar heating
  by flowing in three great circulating cells over
  each hemisphere. The flow of air within these
  cells is influenced by Earths rotation (Coriolis
  effect).
• Daily variations in surface heating can lead to
  the development of a sea breeze.
• Seasonal variations in surface heating lead the
  development of a monsoon, or wind patterns that
  change with the seasons.
• Large storms are spinning areas of unstable air
  that develop between or within air masses.
  Extratropical cyclones originate at the boundary
  between air masses.
• Tropical cyclones, the most powerful of Earth's
  atmospheric storms, occur within a single humid
  air mass.