AOSS 321, Winter 2009
Earth System Dynamics
Lecture 2
1/13/2009
Christiane Jablonowski
cjablono@umich.edu
734-763-6238
Eric Hetland
ehetland@umich.edu
734-615-3177
Characteristics of the Atmosphere
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Composition of the air, greenhouse gases
Temperature, atmospheric layers
Pressure, pressure systems
Wind
General circulation of the atmosphere, seasons,
ocean circulation, albedo, heat & heat transport
• Humidity, clouds, precipitation
• Sun, electromagnetic waves, Earth’s energy balance
Let’s form groups and discuss the elements, e.g.
• physical characteristics and units, how measured or
observed, graphical representation, characteristics of
the global circulation & distribution, typical values
1
Composition of the air
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Nitrogen (N2)
Oxygen (O2)
Argon (Ar)
CO2
Neon (Ne)
Helium (He)
Methane (CH4)
…
78.08%
20.95%
0.93%
383 ppm
18.2 ppm
5.2 ppm
1.7 ppm
• Water vapor (H2O) (1%-4%)
highly variable
Greenhouse gases: CO2
• CO2 measurements in Mauna Loa, Hawaii
• Well-mixed trace gas (greenhouse gas) in the
lower atmosphere
• Barren lava field of an active volcano19º32' N,
155º 35' W, 3397 m above mean sea level (MSL)
Greenhouse gases: CO2
• Carbon dioxide (CO2) concentrations in ppm
(parts per million) in Mauna Loa, Hawaii
380 ppm
‘Keeling’
curve
Year
May 2005
Greenhouse gases (GHG)
• Earth's most abundant greenhouse gases
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–
–
–
water vapor (H2O)
carbon dioxide (CO2)
methane (CH4)
nitrous oxide (NO2), commonly known as "laughing gas"
– ozone (O3)
– chlorofluorocarbons (CFCs)
• Ranked by their contribution to the greenhouse effect,
the most important ones are:
–
–
–
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water vapor, which contributes 36–70%
carbon dioxide, which contributes 9–26%
methane, which contributes 4–9%
ozone, which contributes 3–7%
• What are the atmospheric lifetimes of the GHGs?
460 ppm
CO2 2100
380 ppm
CO2 2005
Global CO2 and T Trends
350,000 years of
Surface
Temperature and
Carbon Dioxide
(CO2)
at Vostok, Antarctica
ice cores
• Carbon dioxide (CO2) is, because of our emissions,
much higher than ever experienced by human kind
• Temperature is expected to follow
• Do we need to be worried?
2
Atmospheric Layers
Temperature
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Weather maps: http://www.rap.ucar.edu/weather/
Station data, Boulder, CO
Lines of equal temperature are called isotherms
Typical plots:
– Time series at a fixed location (station)
– 1D vertical temperature profiles at a station
(radiosonde sounding, skew-T diagrams)
– 2D horizontal cross section near the surface or at a
specific pressure level, e.g. 500 hPa
– 2D meridional-vertical cross section (zonal-mean)
• Pay attention to the horizontal and vertical axis
labels
Temperature: Typical Vertical Profiles
• Weather:
takes place in
the troposphere,
the lower
10-15 km of the
atmosphere
Temperature:
Latitude-pressure cross section
• Annual mean, zonal-mean temperature T in oC,
linear pressure scale (vertical direction)
200
-55
10 km
0
-15
20
1000
SP
-60º
-30º
30º
60º
NP
Temperature:
Latitude-pressure cross section
• Temperature T (annual mean, zonal-mean) in K,
logarithmic pressure scale, includes the stratosphere
Pressure (hPa)
260
230
210
100
300
1000
NP
40ºN
Eq
40ºS
SP
3
Surface Pressure
• Surface pressure
map (at mean
sea level)
• Isobars: Contour
lines that connect
equal pressure
values
Weather maps: http://www.rap.ucar.edu/weather/
NCAR weather station:
http://www.eol.ucar.edu/cgi-bin/weather.cgi?site=fl
Mean sea level pressure (MSLP)
• Extrapolate the surface pressure to mean sea level
• Effects of surface pressure differences due to
mountains (orography) are eliminated
• Plot of annual-mean MSLP in hPa
Latitude
NP
Eq
SP
Longitude
Mean Sea Level Pressure
• Annual-mean zonal-mean MSLP in hPa
SP
Equator
NP
Pressure: Units
• Conversion of pressure units
29.92”Hg = 1.0 atm = 101.325 kPa = 1013.25 mb
101.325 kPa = 1013.25 hPa
• We use the unit: hPa (= 100 Pa)
4
Zonal (W-E) wind
Pressure (hPa)
• Zonal-mean, annual-mean zonal wind u in m/s
• Typical scale: 10 m/s, jet streams over 30 m/s
NP
Eq
SP
Jet stream
• Narrow band with high
wind speeds
• Wind direction is
mostly from west to
east (west wind)
• Approximate height:
10-12 km (just under
the tropopause around
200 hPa)
• Aircraft pay attention
to the position of the
jet stream, why?
Meridional (S-N) wind
Height (km)
Pressure (hPa)
• Monthly-mean, zonal-mean meridional wind v
in m/s are relatively weak (a few m/s)
• Compare to instantaneous v (typical scale: 10 m/s)
Vertical velocity
• Vertical velocity (Cartesian system) in m/s
w = dz/dt
• Vertical pressure velocity in Pa/s
 = dp/dt
• Typical scale of w in midlatitudinal weather
systems is on the order of 1 cm/s
• Typical vertical velocity in thunderstorms: on
the order of 10 m/s (and more)
• Upward motion in low pressure systems
• Downward motion in high pressure systems
Vertical pressure velocity
Height (km)
Pressure (hPa)
• Monthly-mean, zonal-mean vertical pressure
velocity  = dp/dt in Pa/s
Wind direction, speed & wind shear
• Wind direction is the direction from which the
wind is blowing
– West wind: blows from west to east
– South wind: blows from south to north
– Eastward: means a west wind
– Easterly: means an east wind
• Wind speed: Magnitude of the wind vector
• Isotach: lines with equal wind speed
• Wind shear: difference between the wind
directions and speeds at two different heights
(subtract the two wind vectors)
Wind direction
• Cardinal directions (NESW) and azimuth degrees
0º
315º
Azimuth:
increases
45º
counterclockwise
90º
270º
225º
135º
180º
How to draw wind vectors
Length of a wind vector indicates the speed
Draw:
• Wind vector with speed 10 m/s and wind vector
with speed 20 m/s (identical direction)
• Westerly wind
• West wind
• Westward wind
• SSE wind
• Wind from 30°
• Wind shear between a NW wind at 10 km and
NE wind at 5 km. Both wind speeds are identical.
Wind symbols on weather maps
Zoom into some surface
measurements (station
reports)
Wind barbs
• International convention
• Indicate wind direction
and speed in knots
• 1 knot = 0.51 m/s
5
General Circulation of the Atmosphere
• Global wind and circulation systems
Seasons
• Due to tilt of the Earth's axis to its orbital plane
• Tilt is about 23.5 degrees
Seasons
• Solstices and equinoxes
Heat transport: Ocean & Atmosphere
equally important
total
atmosphere
ocean
Equator
Latitude
90° N
Ocean-Atmosphere Interaction:
Ocean surface currents are wind-driven
Surface Currents: Ekman transport
Ocean circulation:
Oceanic conveyor belt
The conveyor belt:
Heat engine for Europe
Albedo
• Defined as:
Ratio of diffusely
reflected to incident
electromagnetic
radiation
• Most often expressed as
a percentage
• Also depends on the
direction of incoming
radiation
• Highly variable
Earth-Ocean-Atmosphere Interactions
Gases
Particles
Heat and Heat transport
• Heat is a form of energy that can change the
temperature of an object
• Measured in energy units, e.g. Joule (J) which is
equivalent to Nm
• Mechanisms of heat transfer:
– Conduction: molecular transfer of energy
– Convection: transfer by fluid motion
– Radiation: transfer by electromagnetic waves even
through vacuum
• Check the difference between conduction and
convection:
http://www.atmos.washington.edu/~durrand/demos/convection_conduction.htm
6
Humidity, Clouds & Precipitation
• Annual mean zonal-mean specific humidity (g/kg):
ratio of water vapor (in g) to moist air (in kg)
Almost all moisture is in the troposphere
Water Cycle
Clouds
• Convective
clouds in
Intertropical
Convergence
Zone (ITCZ)
• Cloud bands
associated
with frontal
zones in
midlatitudes
Clouds
• What Are Clouds?
Clouds are made up of billions of tiny droplets of ice
or water. Each cloud droplet is so small and light that
it is held up by air currents.
• How Are Clouds Made?
Clouds are condensation, formed when warm air
rises and is cooled to below a certain temperature,
called the 'dew point'.
• Why Study Clouds?
Clouds provide one of the keys to understanding the
weather. Modeling clouds and their feedbacks is a
key challenge when assessing climate change.
Clouds & Precipitation
• Cloud classifications according to their vertical
position:
– Low
– Mid-level
– High clouds (ice clouds: cirrus)
• Cloud photos:
http://australiasevereweather.com/photography/
• UCAR’s digital image library:
http://www.fin.ucar.edu/ucardil/
• Fun facts: Earth’s annual mean globally-averaged
precipitation:
Rainiest spot on Earth:
Clouds
• Clouds influence the planetary albedo
• Cloud classification depends primarily on the cloud
altitude
Vertical distribution of clouds
ice clouds
water clouds
Role of low clouds
• Stratus clouds, which are
mostly composed of liquid
water droplets, reflect most
of the incoming shortwave
radiation (thin lines), but reemit large amounts of
outgoing longwave radiation
(thick lines).
• Their overall effect is to
cool the Earth.
Role of ice clouds
• Cirrus clouds, which are
mostly composed of ice
crystals, transmit most of
the incoming shortwave
radiation (thin lines), but trap
some of the outgoing
longwave radiation (thick
lines).
• Their overall effect is to
warm the Earth.
Precipitation
• Annual-mean precipitation in mm/day
Latitude
NP
Eq
SP
Longitude
7
The Sun
• Wikipedia:
http://en.wikipedia.org/wiki/Solar_constant
• Solar constant: amount of incoming solar
radiation per unit area, measured on the outer
surface of Earth’s atmosphere, in a plane
perpendicular to the rays.
• The solar constant includes all types of solar
radiation, not just the visible light. It is
measured by satellite to be roughly
S0 = 1366 W/m2
• Annually averaged radiant flux received on
Earth per unit area: S0/4 = 342 W/m2
Radiation Spectrum
visible UV (ultraviolet)
Radiated Energy: Black-Body
• Planck curves (radiative flux)
Earth’s energy balance
Balance requires: Incoming energy = Outgoing energy
Incoming solar radiation
• Incoming solar
radiation at the top
of the atmosphere,
displayed for each
month and latitude
NP
Equator
• Global mean
annual-mean value
342 W/m2
• Based on ERBE
satellite measurements
January
SP
December
Shortwave (solar) and longwave (infrared)
radiation & Energy budget
Source IPC AR4 (2007)
Energy balance
• Earth’s temperature:
Balance between the
amount of energy
received from the sun
and the amount of energy
radiated outward.
• Top: Solar radiation
reflected from the Earth
by clouds, ice, and bright
surfaces like desert.
• Bottom: Heat radiated
from the Earth. More
energy is emitted by
warmer surfaces, except
Clouds and the Earth’s Radiant Energy
where there are high,
System (CERES) instrument
cold clouds.
January 2002.
Energy: Surplus and deficit regions
• Earth’s radiation balance (annual mean)
NP
SP
The Greenhouse Effect
SUN
Based on conservation of energy: If
the Earth did NOT have an
atmosphere, then, the temperature at
the surface of the Earth would be
about -18 C ( ~ 0 F).
Earth
But the Earth’s surface
temperature is observed to
be, on average, about 15 C
(~59 F).
This greenhouse
effect in not
controversial.
Glossary
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Cyclone: low pressure system
Anticyclone: high pressure system
Isotherms
Isobars
Isotach
Lapse rate
Physical Constants and Parameters
• g: gravitational acceleration of the Earth,
approximately constant, g = 9.81 m/s2
• Rd: gas constant for dry air, Rd = 287 J/(kg K)
• Radius of the Earth: a = 6371 km
• E-W Circumference of the Earth at latitude  :
2a*cos(), at the equator (=0): 2a
• Total surface area of the Earth: 4a2
• Volume of the Earth: 4/3 a3
Units
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SI units: kg, s, K, m, J
Energy in Joule: J = N m
Force in Newton: N = kg m / s2
Pressure in Pascal: Pa = N / m2
• Some prefixes and factors:
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h (hecto)
k (kilo)
c (centi)
m (milli)
102
103
10-2
10-3