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Energy Balance and Temperature

Energy Balance and Temperature. Leila M. V. Carvalho. A Few questions that you might have wondered in a sunny day …. Why is the sky blue? Why is the sky red near the horizon during sunrises and sunsets? Why is the sky often hazy near the horizon above the ocean?

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Energy Balance and Temperature

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  1. Energy Balance and Temperature Leila M. V. Carvalho

  2. A Few questions that you might have wondered in a sunny day … • Why is the sky blue? • Why is the sky red near the horizon during sunrises and sunsets? • Why is the sky often hazy near the horizon above the ocean? • Why in polluted regions the sunsets are sometimes exceptionally red? • What is the fate of solar radiation?

  3. Why is the sky blue? • Solar radiation is attenuated by a variety of processes as it pass through the atmosphere. • A portion of the solar radiation (basically UV) is absorbed and is transformed to heat (mainly by the ozone in the stratosphere). • Visible radiation, in contrast is not absorbed by the atmosphere. If that was not true, what color would appear the sky? • Near infrared (approximately 0.7-5 μm), which represents ~ 50% of Sun’s radiation is absorbed by water vapor and to a lesser extent by CO2. • This is why direct sunlight in the desert feels so hot and shade is so welcome, where in humid regions it does not make much difference VERY DARK

  4. But…Why is the sky blue??? Because of scattering… Some particles and molecules found in the atmosphere have the ability to scatter solar radiation in all directions. The particles/molecules which scatter light are called scatterers and can also include particulates made by human industry Selective scattering (or Rayleigh scattering) occurs when certain particles are more effective at scattering a particular wavelength of light Blue skies are produced as shorter wavelengths of the incoming visible light (violet and blue) are selectively scattered by small molecules of oxygen and nitrogen -which are much smaller than the wavelength of the light. The violet and blue light has been scattered over and over by the molecules all throughout the atmosphere, so our eyes register it as blue light coming from all directions, giving the sky its blue appearance.

  5. On the moon, which has no atmosphere, there is no scattering and this explains why astronauts saw the moon sky black even during the daylight The bluish of the Earth is also the effect of backscattering by the atmosphere

  6. Let’s see if I understand… Income Visible radiation violet and blue are selectively scattered by small molecules of oxygen and nitrogen -which are much smaller than the wavelength of the light Scattering occurs in all directions and multiple times

  7. The intensity of the ‘blue’ depends on the presence of bigger particles such as aerosols or droplets Can you guess why the sky is not as blue near the ocean as it is up in the mountains?

  8. Now I have another question: what is reflection and what is the difference? • Reflection of energy is a process whereby radiation making contact with some material is simply redirected away from the surface without being absorbed. • This is why we see the objects around us: By Gregoire (UCSB)blog

  9. Green because it is REFLECTING GREEN LIGHT White because it is REFLECTING ALL VISIBLE LIGHT • Yellow because it is REFLECTING • Yellow LIGHT Black because it not REFLECTING ANY LIGHT

  10. Two types of reflection: Specular Reflection Light reflects back as a bean of equal intensity (with the same angle with respect to the vertical line) θ θ

  11. Diffuse reflection or scattering • When a beam is reflected from an object as a larger number of weaker rays traveling in many different directions, it is called diffuse reflection or scattering. • You could not see your image reflected in the shirt of the guy, although it is reflecting most of the light it received from the sun!

  12. The percentage of visible light reflected by an object or substance is called its ALBEDO. Which surface should have the highest and lowest albedo and why?

  13. Albedo varies widely for different surfaces • Deep ocean - 7% • Varies with sun angle • Moist, dark soil - 10% • Forest – 3-10% • Grassland - 20% • Desert soil - 30% • Dry, light sand - 35% • Fresh snow – 75-95% • Clouds (depend on the cloud) (Cirrus 20-40%, Stratus 40-65%, Cumulus 75%, Cumuloninbus 90%)

  14. Application of the concept of albedo (or reflectance of VIS light) observed from satellites (wavelength ~ 0.6 μm) in meteorology (forecast) Questions that should be answered about this picture: What do the different gray levels represent? Where should be the thickest clouds and why?

  15. Try now with these images to identify interesting features

  16. SUN GLITTER If the sea surface is calm, a single mirror-like reflection of the sun can be seen at the horizontal specular point. However, there are generally many thousands of small sun glints observed, where each glint is a reflection of the sunlight from one of many waves, each with a different slope. As an ensemble this is called sun glitter Question: Based on the position of the sun glitter, is it summer or winter in the NH and why??

  17. Why is the sky red near the horizon during sunrises and sunsets? • The Rayleigh scattering is also responsible for the redness of the sunrises and sunsets • When the sun is near the horizon, sunlight must travel a greater distance through the atmosphere than it does during the middle of the day • That increases the amount of Rayleigh scattering • As a result the short wavelengths are depleted (green and blue) so long wavelenghts (red) constitute an increasing percentage of the direct sunlight

  18. Why in polluted regions the sunsets are exceptionally red? Well, it can be dark … Why is the sky hazy near the horizon above the ocean? Sunset during Gap-fire – Goleta July 2008 Red sunsets in the Netherlands 7 days after the explosive eruption of the Kasatochi Volcano in Alaska's Aleutian Islands Credits: http://www.fotothing.com/marijke06/photo/a43ad13b6b63ba292e7f172b17f25922/

  19. The answer is in the Mie “mee” scattering • We saw before that the atmosphere always contains suspended aerosols • Aerosols are bigger than molecules • Unlike Rayleigh scattering, MIE scattering is predominantly forward, diverting relatively little energy backward to space

  20. In the case of fog and mist • Mie scattering is not strongly wavelength dependent and produces the almost white glare around the sun when a lot of particulate material is present in the air. It also gives us the white light from mist and fog. Ocean waves eject many aerosols (salt) in the atmosphere every day. Mie scattering is responsible for the ‘mist’ we observe next to the ocean surface

  21. Mie scattering also causes the sunsets and sunrises to be redder, in the presence of particles, such as those produced during eruptions Sunset observed in the USA after the eruption of the Pinatubo Volcano, 1991 http://www.spc.noaa.gov/publications/corfidi/sunset/

  22. However… • If, however, it were strictly true that tropospheric dust and haze were responsible for brilliant sunsets, cities such as New York, Los Angeles, London, and Mexico City would be celebrated for their twilight hues. The truth is that tropospheric aerosols when present in abundance as they often are over continental areas do not enhance sky colors they subdue them. Credit for this picture: Phillip Schneider, Ph.D.

  23. Non-selective scattering MIE scattering is also responsible for the white appearance of clouds. Cloud droplets with a diameter of 20 micrometers or so are large enough to scatter all visible wavelengths more or less equally. This means that almost all of the light which enters clouds will be scattered. Because all wavelengths are scattered, clouds appear to be white.

  24. The new solar cycle 2012 http://www.nasa.gov/topics/earth/features/2012-superFlares.html

  25. What is the fate of the solar radiation? • Let’s consider that there is a constant supply of radiation at the top of the atmosphere. Let’s assume that it corresponds to 100 unities =Earth Albedo 20% + 6% +4%= 30% 20% UV by Ozone (7%) and Near IR by water vapor CO2 (12%) 4% 19% Radiation available to heat the surface of the planet (direct and diffused) 51%

  26. January 18, 2011 VIS pictures. Based on these images, what features seem to play a major role for the earth-planetary albedo and why?

  27. NOAA Radiation Budget monthly mean (December 2010) (W/m2). Work with your partner to explain the observed features in these images. Questions to think: Why does SW radiation vary with the latitude and not with the longitude? Explain the maximum observed. Explain the patterns of absorbed solar radiation http://www.nesdis.noaa.gov/SatProducts.html Absorbed solar radiation is the difference between the incoming solar radiation at the top of the atmosphere and the outgoing reflected flux at the top of the atmosphere

  28. Based on the lecture on Radiation, explain the patterns of Outgoing Longwave Radiation observed here. Compare with the image of absorbed solar radiation – check your explanations for the previous question

  29. Energy transfer between the surface and the Atmosphere • Earth surface and the atmosphere radiate energy almost completely in the long wave (thermal infrared – peak in 10μm) • Earth surface heats and transfer this energy to the atmosphere in distinct forms: radiation, sensible heat and latent heat • Latent heat: • Water in the oceans, soil, rivers, etc, absorb heat from the surface to change state (from liquid to vapor) • This heat is further released in the atmosphere as this vapor is transported to high levels and become cloud droplets • This heat is used to heat the atmosphere.

  30. The other efficient way of transferring heating is by convection • There is free convection (heated surface has less density and more buoyancy) • Forced convection: mechanical turbulence, caused for instance by the wind • Eddies can efficiently transport heat upward

  31. Convection can be also efficient to for the release of Latent Heat • Water absorb Latent heat from surface to evaporate (change the state from water to vapor • Latent Heat can be released in the atmosphere due to convection as the air parcels move upward. Water vapor will convert into water – forming clouds Surface Heating

  32. Sensible heat • When energy is added to a substance, an increase in temperature can occur that we physically sense (then the name sensible). This is the amount of energy necessary for a given substance to increase its temperature • Sensible heat travels by conduction through the laminar boundary layer and is then dispersed upward by convection

  33. Longwave Radiation • Wavelengths longer than 4 microns • Emitted by Earth's surface and atmosphere • Atmosphere absorbs strongly in longwave • Atmosphere absorbs strongly at wavelengths emitted from surface, except 8-11 micron window (by the way the peak of the Earth Black body emission) • This radiation is absorbed by clouds • Water vapor, CO2 and other greenhouse gases also strong absorbers • Clouds absorb nearly all longwave emitted from surface

  34. Absorbed directly from the Sun Radiation from Earth that is lost to space without interaction with the atmosphere Lost to space – from what was absorbed Atmosphere Emission IR, Latent H, Sens H Returns to Earth Sensible Heat (Conduction Convection) Latent Heat (evaporation) Emission IR Radiation surface

  35. Absorbed directly from the Sun Radiation from Earth that is lost to space without interaction with the atmosphere Lost to space – from what was absorbed in the atmosphere = -64 -6 Greenhouse gases absorb, heat the atmosphere and emit up and down +19 (19+111+23+7=160) Returns to Earth -96 Emission IR, Latent H, Sens H 117-6=+111 Sensible Heat (Conduction Convection) Latent Heat (evaporation) Emission IR Radiation +96 -7 -117 -23 Surface heats with solar radiance and transfer energy to the atmosphere =+96-117-23- 7 =-51 = net income solar radiation

  36. DID YOU KNOW… • Atmospheric Greenhouse Effect Atmosphere transmits most visible wavelengths, absorbs most infrared • Most longwave escaping to space is emitted by the atmosphere • Surface would be 255K without greenhouse effect • Actual surface temperature is about 33K warmer due to greenhouse effect • Surface heated by downward flux of longwave from atmosphere, as well as incoming shortwave • Water vapor and CO2 are the most important greenhouse gases (CO2 has a long residence time in the atmosphere whereas H2O vapor doesn’t) • Increase in CO2 and other greenhouse gases appear to be intensifying the greenhouse effect

  37. 160 unities of energy from Earth are absorbed by the atmosphere by greenhouse gases :Water vapor most important, CO2 and other greenhouse gases also strong absorbers Atmosphere absorbs strongly at wavelengths emitted from surface, except 8-11 micron window Red curve: Blackbody radiation (theoretical)

  38. Absortion %

  39. Clouds absorb nearly all longwave emitted from surface T

  40. Did you know… Part of solar radiation is reflected Cirrus clouds are thin clouds that let incoming solar radiation to pass through them but absorb outgoing long-wave radiation and emit it back to earth and space Because of that Cirrus clouds increase earth temperature

  41. Satellite infrared images: sensors measure emitted radiation ~ 10μmIR band – longwave radiation Principle: High cloud tops are colder than low cloud tops or surfaces on earth The colder a feature is the less IR radiation ~ 10μm it emits. The intensity of IR radiation can be used to infer temperature Example – B~ 190 T= 418-B T=228 K or -45.15 C or -49.27 F In this example the following conversion is used : For B ( Brightness) > 176 T = 418 - B or when B <=176 T = 330 - (B/2) Note that the resulting temperatures are in Kelvin.

  42. Long-term trend shows more than 70 ppm (22%) increase since 1958 Rate of growth has increased from less than 1 ppm/year to 2 ppm/year Likely related to big increase in surface temperatures over 20th Century 1990s warmest decade of 20th century by far 2005 warmest on record, 2007 fifth warmest 2007 land temperatures warmest in record 7 of 10 warmest years since 2001, all since 1995 Global temperatures increasing at rate of 0.25oF per decade

  43. http://www.ncdc.noaa.gov/sotc/global/

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