1. Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 4: Solar Radiation Chris Parkes Rm 455 Kelvin Building

2. 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

3. Radiation from the sun, Radiation Balance & the Greenhouse Effect image from SOHO satellite

5. 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

6. Perfect black body The most perfect black body radiation ever measured: The cosmic microwave background radiation at 2.725 Kelvin = 1 / wavelength

7. Absorption of radiation by atmospheric gases Solar radiation Earth radiation Visible light gets through UV absorbed by Ozone

8. Outgoing Longwave Radiation During 2003 heat wave (range up to 350W/m 2 )

9. Source: http://en.wikipedia.org/wiki/Solar_radiation = nanometer = 10 -9 meter

10. 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

11. 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

12. 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

13. 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…)

14. 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

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

16. April 1984-1993 January 1984-1993 Variation in insolation due to time of year

17. 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

18. 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

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

20. Source: Nasa ERBE BB 5800 KBB 300 K

21. Incoming budget Output budget Normalised to 100 Greenhouse Effect balance

22. 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)

23. 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

24. Without greenhouse effect temperature down by ~30ºC

25. Greenhouse Gas – self-regulation Negative feedback mechanism –Interacts with CO 2 cycle Solar radiation 30% higher in past

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

27. 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

28. 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

29. Summary: Radiation and Climate

30. 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

32. Radiation transfer mechanisms

33. Global radiation budget & energy transfer

34. Height (km) Temperature (K) 0 0300600 50 100 150 Troposphere Stratosphere Mesosphere Thermosphere