Planetary Atmospheres, the Environment and Life (ExCos2Y)

Topic 4: Solar Radiation

Chris Parkes

Rm 455 Kelvin Building

3. Structure of Planetary Atmospheres

• 4 distinctive layers with boundaries – Troposphere, Stratosphere,

Mesosphere, Thermosphere

• Temperature profile– Greenhouse gases

– Ozone in stratsophere

• Comparison of atmosphere’s of Earth, Venus & Mars

Revision

Radiation from the sun,

Radiation Balance & the Greenhouse Effect

image from SOHO satellite

Electromagnetic Radiation

• All bodies emit EM radiation

• Distribution of intensity as a function of wavelength depends on temperature of the body

• The peak of the distribution from the sun is in the visible region

• The Earth’s radiation distribution peaks in the infra-red region

Peak wavelength 1/temperature

Stefan-Boltzmann Law: I =σT4

Wien displacement law: λmT = 2.90×103 m·K

Perfect black body

The most perfect black body radiation ever measured:The cosmic microwave background radiation at 2.725 Kelvin

= 1 / wavelength

Absorption of radiation by atmospheric gases

Solar radiation Earth radiation

Visible light gets through

UV absorbed by Ozone

Outgoing Longwave Radiation

During 2003 heat wave (range up to 350W/m2)

Source: http://en.wikipedia.org/wiki/Solar_radiation

= nanometer = 10-9 meter

Earth’s incoming and outgoing radiationCharacteristic Blackbody

radiation shapes

Actual solar spectrum at sea level shows gaps where absorption occurs

Likewise earth radiation reaching upper atmosphere show gaps

There are short and long wavelength “windows”

Atmospheric Ozone responsible for absorbing UV radiation

Wm

-2μ

m-1

μm

Sun - Incoming

Earth - Outgoing

UV absorbed by Ozone

Ozone Depletion in Stratosphere revisited - Chemical Reactions

Production mechanism:

UV + O2 2O

O + O2 O3

Loss mechanism:

UV + CFC Cl

Cl + O3 ClO + O2

ClO + O Cl + O2

Ozone absorbs UV:

O3 + O 2O2

UV + O3 O + O2

Will recover ~2070Chlorofluorocarbons (CFCs): Cl, F, C

Exposure to UV radiation

TOMS (Total Ozone Mapping Spectrometer)

Ozone and clouds both

absorb UV radiation

Measure UV on ground

level need to combine: incoming UV

reflected UV

cloud cover

Variation of “Insolation” with latitude

At an angle less energy density on surface

Solar energy

Solar energy

Direct or diffused shortwave solar radiation received in atmosphere or at surface

Central Australia = 5.89 kWh/m2/day - HighHelsinki, Finland = 2.41 kWh/m2/day - Low

Insolation: Incident Solar Radiation – Energy received per unit are per unit time

θ

d

d / cos θ

e.g. cos 30o = 0.5, half as much insolationHence, poles colder than equator

But heat also transmitted in atmosphere (convection…)

Daily variation due to rotation of Earth on axis

Absorbed radiationdependent on time ofday

Daytime has net surplusenergy input

Night time has net loss of Infrared radiation

Temperature ofatmosphere lags behindabsorbed radiation dueto heat capacity ofatmosphere

sun

Variation in insolation due to time of year

Greater tilt more extremeSeasons

Smaller Tilt Polar regionscolder

April 1984-1993

January 1984-1993

Variation in insolation due to time of year

Seasons on Mars

• Elliptical Orbit – closer to sun in

southern hemisphere summer,

– further southern hemisphere winter

• Extreme seasons in south– -130oC in winter in

south – CO2 dry-ice caps

• Year nearly twice as long as Earth– Orbital period 1.88 x

Earth

AlbedoDefined as the fraction of incident radiation which is reflected

Object AlbedoGlobal cloud 0.23 (different types have different

albedo)

Forests 0.15 (depends on type of tree)Water 0.10 (highly dependent on incident

angle)Snow 0.8Sand 0.3Grass 0.2

Planet Earth 0.31

Extremes:•Water efficiently absorbs•Snow/Ice effficiently reflects

• Water – absorbs• Ice – reflects• Desert reflect > grass

Variation of albedo

Source: Nasa ERBE

BB 5800 K

BB 300 K

Incoming budget

Output budget

Normalised to 100

GreenhouseEffect

balance

Radiation reflected from Earth

Top: shortwave

Bottom: longwave

Globally annual average of in & out is balanced

but there are seasonal & regional variations

CERES (Clouds & the Earth’s Radiant Energy System, NASA)

The greenhouse effect• The earth takes energy from solar radiation • Earth is in a “steady state” (constant surface temperature) • re-emit in “blackbody” radiation (longwave)

in order to keep energy balance• Greenhouse gases absorb longwave radiation from

earth’s surface and re-emit part of this back to surface• To maintain energy balance surface temperature must be

increased to increase the output radiation energy– The higher the surface temperature the more energy is being emitted

Without greenhouse effect

temperature down by ~30ºC

Greenhouse Gas – self-regulation

• Negative feedback mechanism– Interacts with CO2 cycle

• Solar radiation 30% higher in past

Earth – rescue from ice age

• Deep ice ages – ‘snowball Earth’– Ice reflects, water absorbs more cooling

– CO2 buildup greenhouse effect heating

Venus – high temperatures

• Increased temperature – water evaporates, CO2 released– Water vapour also greenhouse gas

• Venus lost its water early, prob. never forming oceans– CO2 dominated atmosphere

Temperature Profiles Revisited: Venus, Earth, Mars

• Temperature difference due to greenhouse effect– Mars: +6oC – lost CO2 in atmoshere– Earth: +31oC – moderate CO2, stored in rocks due to

water cycle– Venus: +500oC – CO2 dominated atmosphere

Summary: Radiation and Climate

Example exam questions

Q1. Explain why on average the surface temperature along the equator is higher than that of the poles?

Q2. Sketch the daily variation of earth’s input and output radiation. Explain how this relate to the temperature variation.

Q3. Draw a diagram explaining the radiation budget of the earth?

Next lecture – convection in the atmosphere

Radiation transfer mechanisms

Global radiation budget & energy transfer

Height (km)

Temperature (K)

00 300 600

50

100

150

Troposphere

Stratosphere

Mesosphere

Thermosphere