AOSS 321, Winter 2009 Earth System Dynamics

Lecture 21/13/2009

Christiane Jablonowski Eric Hetland

cjablono@umich.edu ehetland@umich.edu

734-763-6238 734-615-3177

Characteristics of the Atmosphere• 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

Composition of the air• Nitrogen (N2) 78.08%

• Oxygen (O2) 20.95%

• Argon (Ar) 0.93%

• CO2 383 ppm

• Neon (Ne) 18.2 ppm• Helium (He) 5.2 ppm

• Methane (CH4) 1.7 ppm

• …

• Water vapor (H2O) (1%-4%)highly variable

1

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

May 2005Year

‘Keeling’curve

380 ppm

• Carbon dioxide (CO2) concentrations in ppm (parts per million) in Mauna Loa, Hawaii

Greenhouse gases (GHG)• Earth's most abundant greenhouse gases

– 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:– 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?

Global CO2 and T Trends

• 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?

350,000 years of Surface

Temperature and Carbon Dioxide

(CO2) at Vostok,

Antarctica ice cores

CO2 2005

CO2 2100

380 ppm

460 ppm

Atmospheric Layers2

Temperature• 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

• Annual mean, zonal-mean temperature T in oC,linear pressure scale (vertical direction)

10 km

Temperature:Latitude-pressure cross section

SP NP-30º 30º 60º-60º

200 -55

0

20-15

1000

• Temperature T (annual mean, zonal-mean) in K, logarithmic pressure scale, includes the stratosphere

Temperature:Latitude-pressure cross section

Pre

ssur

e (h

Pa)

SPNP Eq

210

230

260

30040ºN 40ºS

1000

100

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

3

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

Latit

ude

NP

SP

Eq

Longitude

Mean Sea Level Pressure

• Annual-mean zonal-mean MSLP in hPa

NPSP Equator

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)

• Zonal-mean, annual-mean zonal wind u in m/s• Typical scale: 10 m/s, jet streams over 30 m/s

Zonal (W-E) windP

ress

ure

(hP

a)

NP SPEq

4

• 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?

Jet stream

Meridional (S-N) wind• 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)

Hei

ght

(km

)

Pre

ssur

e (h

Pa)

Vertical velocity

• Vertical velocity (Cartesian system) in m/sw = 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• Monthly-mean, zonal-mean vertical pressure

velocity = dp/dt in Pa/s

Hei

ght

(km

)

Pre

ssur

e (h

Pa)

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

90º

180º

270º

45º

135º225º

315º

Azimuth:increasescounterclockwise

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 surfacemeasurements (stationreports)

Wind barbs• International convention• Indicate wind direction and speed in knots• 1 knot = 0.51 m/s

General Circulation of the Atmosphere• Global wind and circulation systems

5

Seasons• Due to tilt of the Earth's axis to its orbital plane• Tilt is about 23.5 degrees

Seasons• Solstices and equinoxes

total

atmosphere

ocean

Latitude

Nor

thw

ard

heat

tran

spor

t (10

15 W

)

90° NEquator

Heat transport: Ocean & Atmosphere equally important

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

GasesParticles

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

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

6

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 re-emit 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

Latit

ude

NP

SP

Eq

Longitude

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

7

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 at the top

of the atmosphere,

displayed for each

month and latitude

• Global mean

annual-mean value

342 W/m2

• Based on ERBE

satellite measurements

Incoming solar radiation

January December

Equator

NP

SP

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 where there are high, cold clouds.

Clouds and the Earth’s Radiant Energy System (CERES) instrumentJanuary 2002.

• Earth’s radiation balance (annual mean)

Energy: Surplus and deficit regions

NP SP

But the Earth’s surface temperature is observed to be, on average, about 15 C (~59 F).

The Greenhouse Effect

SUN

Earth

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).

This greenhouseeffect in not controversial.

Glossary

• 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 : 2a*cos(), at the equator (=0): 2a

• Total surface area of the Earth: 4a2

• Volume of the Earth: 4/3 a3

Units• 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:– h (hecto) 102

– k (kilo) 103

– c (centi) 10-2

– m (milli) 10-3