Transcript Geography 120 Earth Systems II: The Atmospheric Environment

Midterm I Review
The most common atmospheric
circulation structure
H
L
Cooling
or No
Heating
Heating
H
L
Imbalance of heating
 Imbalance of temperature
 Imbalance of pressure
 Wind
Topics we have discussed
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Overviews I: Extreme weather and climate
Overview II: Success and failure of weather and climate prediction
Overview III: Why is it so difficult to predict weather and climate?
Evolution of the atmosphere
The incoming solar energy
What cause the four seasons?
What is the greenhouse effect?
Vertical structure of the atmosphere
What set the atmosphere in motion?
How does air move around the globe?
The mission of the atmospheric
sciences is to understand and predict
weather, climate, and related
disasters
Overview I: Extreme weather and climate
• Atmosphere: A mixture of gas molecules, microscopically
small particles of solid and liquid, and falling
precipitation
• Meteorology: The study of the atmosphere and the
processes that form weather
• Weather: The state of the atmosphere at a given time and
place
• Climate: The statistical properties of the atmosphere. (i.e.
averages and variability)
• Weather- and climate-related disasters: tropical cyclones,
tornados, floods, droughts, winter storms, extreme heat,
extreme cold, lightning, El Nino, global warming
• Impacts of weather/climate on agriculture, business,
international relationships, history, science, philosophy,
public health, psychology, social work, education, …
Weather- and Climate-related Disasters
Weather/climate and Psychology, Public Health,
Economy, Transportation, Education, …
Overview II: Success and failure of
weather and climate prediction
• The modern climatology (meteorology) was born in the
1940s (a very young science!), but has been growing
very fast! Now we have a global observational network
with many satellites, ships, radars and surface
stations, as well as very comprehensive prediction
models running on the world’s fastest supercomputers.
• The current status of weather and climate predictions:
(1) weather prediction good to 10 days, (2) tropical
cyclone prediction good in track but not in intensity,
(3) climate prediction good to two seasons, (4) climate
change projections have a 3-fold difference in
magnitude.
Observing the atmosphere
World’s major climate modeling centers
Overview III: Why is it so difficult to predict
weather and climate?
• The main reasons of the difficulties: (1) Teleconnection
problem, (2) Feedback problem, and (3) Subgrid-scale
problem, (4) Limitation of concept/theory/model.
Problem I: Different parts of the world are
strongly connected to each other
(The “Teleconnection Problem”)
Global atmospheric flow
Factors affecting US weather and
climate
Arctic
N. Atlantic
Atlantic/
Sahel
Madden-Julian
Oscillation
El Nino
Amazon
Any location is affected by all the other
locations, and in turn is affecting all the
other locations
Problem II: Different components of the earth
system (atmosphere, land, ocean, ice, clouds, etc)
are strongly interacting with each other
(The “Feedback Problem”)
Problem III: The global climate models divide the earth
into many small pixels (called grids), but the earth system
composes of both very big objects (such as the whole Pacific
Ocean) and very small objects (such as the cloud droplets),
making it very difficult to draw them on the same page
(The “Subgrid-Scale Problem”)
Evolution of the atmosphere
– The standard units of measurements (SI)
– Earth’s three atmospheres:
1st: 4.6 billion years ago, H, He
Transition: formation of magnetic field, volcano activities
2nd: 4 billion years ago, CO2, H2O, N2
Transition: emergence of life, formation of ocean
3rd: 400 million years ago, O2
Important event: formation of seven continents
– What is the residence time? What is the difference between
the permanent and variable gases? Name 3 of each. What are
the most and second most abundant gases?
– Given that variable gases are so rare, why are they considered
at all? How are CO2 and O3 changing?
– Earth’s climate history: ice ages (at least 5 have occurred so far.
We’re in an ice age!), 100,000-year cycle, little ice age (1350-1850AD)
Standard units of measurement
SI (System International)
Quantity
Length
Mass
Time
Temperature
Density
Speed
Force
Pressure
Energy
Power
Name
meter
kilogram
second
Kelvin
kilogram
per cubic meter
meter per
second
newton
pascal
joule
watt
Units
m
kg
s
K
kg/m3
Symbol
m
kg
s
K
kg/m3
m/s
m/s
m.kg/s2
N/m2
N.m
J/s
N
Pa
J
W
Evolution of the Sun and the Earth
The Earth was born 4.6 billion years ago.
Permanent gases and variable gases
• Residence time: The amount of
time a gas is in the atmosphere
• The permanent gases: gases
having long residence times
(N2=42,000,000 y, O2=5,000 y),
99.999% of total atmosphere
mass
• The variable gases: Gases
generally having shorter
residence times (H2O=10 days,
CO2=150 y).
Importance of the Variable Gases
• CO2 and water vapor are the major greenhouse gases
• Water can exist in all three states on Earth. Global
water cycle is the process of water being cycled from the
planet to the atmosphere and back again.
• O3 protects us against harmful ultraviolet radiation
IPCC (2001)
Change of CO2
Change of O3 Montreal Protocol in
1987 to ban freons
The incoming solar energy
–
What is energy? 3 methods of energy transfer
The names of the 6 wavelength categories in
the electromagnetic radiation spectrum
Intensity of radiation (Stefan-Boltzman law):
–
Wavelength of radiation (Wein’s law):
–
The wavelength range of Sun (shortwave) and
Earth (longwave) radition
The 11-year solar cycle
–
–
–
I=T4
max = b/T
Methods of
Energy Transfer
• Conduction
– Molecule to molecule transfer
– Heat flow: warm to cold
– e.g. leather seats in a car
• Convection
– transferred by vertical movement
– physical mixing
– e.g. boiling water
• Radiation
– propagated without medium (i.e. vacuum)
– solar radiation provides nearly all energy
– The rest of this chapter deals with radiation
The Electromagnetic Spectrum
Sun =
“shortwave”
(0.4-0.7 μm)
Peak 0.5 μm
(green)
The limitations of the
human eye!
Earth =
“longwave”
(4-100 μm)
Peak 10 μm
(infrared)
What cause the four seasons?
–
–
–
The two basic motions of the Earth
What causes the four seasons: the Earth’s
tilt and the 3 ways it affects the solar
insolation (change of length of the day, beam
spreading, beam depletion)
Change of the Earth’s orbit at longer time
scales (Milankovitch cycles): eccentricity,
axial tilt, and precession
The Earth’s two basic motions:
1.
revolution with a period of 1 year,
and rotation with a period of 1 day.
The change of seasons is caused by
the Earth’s 23.5o tilt from the line 2.
perpendicular to its orbit plane
(toward the sun during summer),
which affects the receipt of solar
insolation in three ways:
3.
Length of Daylight period
Angle at which sunlight hits the
surface (“Beam Spreading”)
Thickness of atmosphere through
which sunlight must travel
(“Beam Depletion)
What is the greenhouse effect?
• Earth’s energy balance at the top of the atmosphere and at
the surface. What percentage of solar energy is absorbed by
the surface?
• Atmospheric influences on radiation (3 ways)
• The three types of atmospheric scattering. What causes the
blue sky? Why causes the reddish-orange sunsets?
• What cause the greenhouse effect? What are the major
greenhouse gases? Why is methane important?
• Sensible heat flux (dry flux from warm to cold regions) and
latent heat flux (wet flux from wet to dry regions)
Earth’s energy budget (averaged over the whole
globe and over a long time
Yellow:
shortwave
Red:
longwave
Sensible
heat 7%
•
Net Longwave 21%
Latent heat
23%
At the top of the atmosphere (3-way balance):
Incoming shortwave = Reflected Shortwave + Emitted longwave
•
At the surface (5-way balance):
Incoming shortwave = Reflected shortwave + Net emitted longwave (emitted - incoming)
+ Latent heat flux + sensible heat flux
Atmospheric absorption - The Greenhouse Effect
Transparent
to solar
(shortwave)
radiation
Opaque to
earth’s
(longwave)
radiation
Major GH gases:
CO2, H20(v), CH4
The greenhouse effect helps to keep the earth surface at a comfortable
temperature. But when it’s too strong, the temperature becomes too warm.
The importance of methane (CH4)
• 23 times more powerful as a greenhouse gas than CO2
• The livestock sector is a major player, which accounts
for 35-40% global anthropogenic emissions of methane
(their burps!)
• The livestock sector is responsible for 18% of total
greenhouse gas emissions, which is higher than
transportation (cars, airplanes, etc)
• Therefore, consuming less meat is more efficient in
reducing global warming than not driving cars.
Vertical structure of the atmosphere
• Thickness of the atmosphere: less than 2% of Earth’s
thickness
• Definition of temperature. 3 units.
• Definition of pressure and its unit.
• Definition of pressure gradient. Pressure gradient sets
the air in motion.
• Equation of state (P=ρTR)
• Vertical Pressure Distribution. How does pressure
change with height? What is the hydrostatic
equilibrium?
Temperature, pressure, winds
• Temperature – measure of average kinetic energy
(motion) of individual molecules in matter. 3 units:
Kelvin (K), Celsius (C), Fahrenheit (F)
• Pressure – force exerted/unit area (weight above you).
units - Pascals (Pa) or millibars (mb) (1 mb = 100 Pa)
• Pressure gradient – pressure difference between two
locations divided by the distance between those two
locations
• Winds
– Zonal winds (east-west): Eastward is called
westerly
– Meridional winds (north-south): Northward is
called southerly
Temperature Layers
The names of the 4 layers
What separate them?
The approximate height of
tropopause, stratopause
and mesopause
Vertical pressure distribution: Hydrostatic equilibrium
 Pressure decreases with height
Because downward gravity force is
balanced by vertical pressure gradient
force (called hydrostatic equilibrium)
Δp/Δz = ρg
 Pressure decreases non-linearly w/
height (Because air is compressible, so
denser near the surface)
Δp/Δz
ρg
What set the atmosphere in motion?
• Know 3 Forces that affect wind speed /direction
• Especially work on Coriolis force, as this is the hardest to
understand. Which direction is air deflected to by Coriolis
force?
• What is the geostrophic balance? At which level is it
valid? Difference between upper level and surface winds
• Troughs, ridges, cyclones and anticyclones. Do they
correspond to high or low surface pressure? Is the air
moving clockwise or counter-clockwise around them?
Forces affecting the horizontal winds
•
Horizontal pressure gradients responsible for wind generation
•
Three forces affecting horizontal winds:
1. Pressure Gradient Force (PGF)
2. Coriolis Effect (CE)
3. Friction Force (FF)
DV
= PGF + CE + FF
Dt
CE:
• The Earth’s rotation deflects any moving object
to the right of its moving direction in NH (left in
SH). Like walking in a turning bus.
• CE increases poleward (greatest at the poles, 0
at the equator), and increases with the speed of
moving object
Geostrophic Balance (Geostrophic flow)
PGF = - CE
• When the effects of friction can be neglected (such as in the upper air
away from surface roughness), the wind speed/direction is simply a
balance between the PGF and CE.
• Air motion is deflected by the Coriolis force to be perpendicular to PGF
PGF
Cyclones, Anticyclones, Troughs and Ridges
Upper air: isobars usually not closed
off
• Troughs (low pressure areas)
• Ridges (high pressure areas)
Near surface: isobars usually closed
off due to surface friction
• Cyclones (Low pressure areas)
• Anticyclones (High pressure
areas)
How does air move around the globe?
• Three precipitation (heating) belts. Primary high and
lows
• Three-cell model. Mechanism for each cell
• Two characteristics of zonal mean temperature
structure
• Two characteristics of zonal mean wind structure.
Why does westerly winds prevail in the extratropical
troposphere? What cause the jet streams?
• Semipermanent pressure cells. Low pressure is
associated with clouds and precipitation. High
pressure is associated with warm surface
temperature, drought, and desert.
• What drives the ocean surface currents? In the case of
Ekman spiral, what is the direction of surface current
relative to surface wind?
Vertical structure and mechanisms
Polar Cell (thermal):
Driven by heating at
50 degree latitude and
cooling at the poles
Ferrel Cell (dynamical):
Dynamical response to
Hadley and polar cells
Polar
Hadley
Hadley Cell (thermal):
Heating in tropics  forms
surface low and upper level
high  air converges
equatorward at surface, rises,
and diverges poleward aloft
 descends in the subtropics
Vertical structure of temperature
Two characteristics:
•
Horizontally uniform
in the tropics
•
Steep gradient in
the extratropics
Vertical structure of zonal wind
Two characteristics:
•
Westerly winds in
the extratropical
troposphere
(caused by the
Coriolis force)
•
Jet streams: local
maximum of winds
(caused by sharp
pressure gradient
across the boundary
between warm
tropical air and cold
polar air)
General circulation of the oceans
• Ocean surface currents – horizontal water motions
• Transfer energy and influence overlying atmosphere
• Surface currents result from frictional drag caused by wind - Ekman
Spiral
• Water moves at a 45o angle (right)
in N.H. to prevailing wind direction
• Due to influence of Coriolis effect
• Greater angle at depth
About the midterm
• There will be ~50 multiple-choice
questions
• Please pay special attention to the key
points highlighted in red on the review
slides (some easy grades)
• Sample questions
1)
In the SI system, the standard unit of length is:
A) yard, B) meter, C) gram, D) pound.
2) Which of the following is NOT a variable gas?
A) water vapor. B) nitrogen. C) carbon dioxide. D) ozone.
3)
In the northern hemisphere, when the surface wind blows toward the east, the
underlying ocean current flows toward:
A) the west. B) the north. C) the southeast. D) the northwest.
4) Anticyclones:
A) are associated with low-pressure systems in the northern hemisphere.
B) experience Coriolis effects that deflect air to the left in the Northern
Hemisphere.
C) are associated with supersonic winds.
D) are associated with counter-clockwise flow in the southern hemisphere.