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 =σT 4 Wien displacement law: λ m T = 2.90×10 3 m·K

Perfect black body The most perfect black body radiation ever measured: The cosmic microwave background radiation at 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/m 2 )

Source: = nanometer = meter

Earth’s incoming and outgoing radiation Characteristic 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 -2 μm -1 μmμm Sun - Incoming Earth - Outgoing UV absorbed by Ozone

Ozone Depletion in Stratosphere revisited - Chemical Reactions Production mechanism: UV + O 2  2O O + O 2  O 3 Loss mechanism: UV + CFC  Cl Cl + O 3  ClO + O 2 ClO + O  Cl + O 2 Ozone absorbs UV: O 3 + O  2O 2 UV + O 3  O + O 2 Will recover ~2070 Chlorofluorocarbons (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 Direct or diffused shortwave solar radiation received in atmosphere or at surface Central Australia = 5.89 kWh/m 2 /day - High Helsinki, Finland = 2.41 kWh/m 2 /day - Low Insolation: Incident Solar Radiation – Energy received per unit are per unit time θ d d / cos θ e.g. cos 30 o = 0.5, half as much insolation Hence, poles colder than equator But heat also transmitted in atmosphere (convection…)

Daily variation due to rotation of Earth on axis Absorbed radiation dependent on time of day Daytime has net surplus energy input Night time has net loss of Infrared radiation Temperature of atmosphere lags behind absorbed radiation due to heat capacity of atmosphere sun

Variation in insolation due to time of year Greater tilt  more extreme Seasons Smaller Tilt  Polar regions colder

April January 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 –-130 o C in winter in south –CO2 dry-ice caps Year nearly twice as long as Earth –Orbital period 1.88 x Earth

Albedo Defined as the fraction of incident radiation which is reflected ObjectAlbedo Global cloud0.23(different types have different albedo) Forests0.15(depends on type of tree) Water0.10(highly dependent on incident angle) Snow0.8 Sand0.3 Grass0.2 Planet Earth0.31 Extremes: Water efficiently absorbs Snow/Ice effficiently reflects

Water – absorbs Ice – reflects Desert reflect > grass Variation of albedo

Source: Nasa ERBE BB 5800 KBB 300 K

Incoming budget Output budget Normalised to 100 Greenhouse Effect 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 CO 2 cycle Solar radiation 30% higher in past

Earth – rescue from ice age Deep ice ages – ‘snowball Earth’ –Ice reflects, water absorbs  more cooling –CO 2 buildup  greenhouse effect  heating

Venus – high temperatures Increased temperature – water evaporates, CO 2 released –Water vapour also greenhouse gas Venus lost its water early, prob. never forming oceans –CO 2 dominated atmosphere

Temperature Profiles Revisited: Venus, Earth, Mars Temperature difference due to greenhouse effect –Mars: +6 o C – lost CO 2 in atmoshere –Earth: +31 o C – moderate CO 2, stored in rocks due to water cycle –Venus: +500 o C – CO 2 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) Troposphere Stratosphere Mesosphere Thermosphere