Lecture # 04 January 27, 2010, Wednesday Energy & Radiation

Transcription

1 Lecture # 04 January 27, 2010, Wednesday Energy & Radiation Kinds of energy Energy transfer mechanisms Radiation: electromagnetic spectrum, properties & principles Solar constant Atmospheric influence on insolation & Fate of solar radiation absorption, forward & backward scattering, transmission Terrestrial radiation emitted by Earth s surface & atmosphere also termed longwave or thermal or infrared radiation longwave radiation budget, greenhouse effect Effects of clouds on solar and terrestrial radiation Radiation budgets of Earth s surface & of the atmosphere The Greenhouse effects

2 Solar radiation is transferred to the Earth s surface where it is absorbed. This radiation provides energy for atmospheric motion, weather processes, biological activity, conversions of state of material (e.g., water), and many other activities. Energy unit is J (joule) & Power unit is W (watt) = J s -1 First law of Thermodynamics Conservation of Energy Energy can neither be created nor destroyed. But it can change from one form to another.

3 (in use, motion) (in reserve, not used yet, position)

4 Kinetic Energy (motion)

5 Forms (Kinds) of Energy Potential energy (in reserve, position) Kinetic energy (in use, motion) Radiation (Solar energy) Heat energy (sensible) Nuclear energy Chemical energy

6 3 Mechanisms of Heat Transfer

7 Transfer energy through a medium by direct molecular contact, however, molecules do not move in the direction energy is transferred. Conduction Most effective in solid materials. Not effective in the atmosphere (poor conductor), EXCEPT in a very thin layer (mm) of air at the interface between the Earth s surface and the atmosphere: laminar airflow, no turbulent mixing.

8 Transfer by mixing fluids. Convection Very important in the atmosphere. Free convection: mixing by buoyancy generated turbulence. Forced convection: mixing by mechanical generated turbulence (wind shear).

9 Energy transfer without a physical medium. Radiation Propagates energy transfer through the vacuum of space. Emitted by substances > 0 K. Types differ by electrical and magnetic wave properties. Amplitude (quantity) & wavelength (quality, type). In any type of radiation, electrical and magnetic waves, although closely coupled, are perpendicular to each other. Travel at the speed of light. Shorter wavelength Longer wavelength

10 The Electromagnetic Spectrum Micrometer m Electromagnetic energy comes in an infinite number of wavelengths. Simplified by categorizing wavelengths into just a few individual bands along the electromagnetic spectrum.

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12 Stefan-Boltzmann Law: The radiation intensity ( I ) in unit of W m -2 from a body with an absolute temperature T (in unit of K) is proportional to the 4th power of T: Example: Radiation Properties & Principles I = ε σ T 4 σ = the Stefan-Boltzmann constant in W m -2 K -4 ε = emissivity Blackbodies (perfect emitters, ε = 1), Graybodies (ε < 1) Sun s temp = 6000 K Earth s temp = 300 K emits 73.5 x 10 6 W m -2 emits 460 W m -2 Hotter sun emits much more energy than cooler earth!

13 Wien s Law The wavelength ( max in μm) of maximum emitted radiation is inversely proportional to the temperature (T, in K) of the radiating body: Example: Radiation Properties & Principles max = 2900 m K / T max-sun = 2900 m K / 6000 K 0.5 m max-earth = 2900 m K / 288K 10 m Sun emits shorter wavelengths than the earth!

14 Radiation from Sun and Earth Solar Radiation In different scales! Terrestrial Radiation 4 m

15 Radiation Properties & Principles Kirchoff s Law: A molecule absorbs radiation of a particular wavelength is also capable of emitting radiation of the same wavelength. Good absorbers are good emitters at the same wavelength, poor absorbers are poor emitters at the same wavelength.

16 Solar Constant Insolation Intensity near the top of atmosphere Electromagnetic radiation does not lose its energy traveling in the vacuum of space. A reduction of radiation intensity is proportional to increasing distance only as energy is distributed over a larger area. Thus, radiation intensity decreases in proportion to the distance squared: I = Energy (W) / (4 r 2 ) Calculating this inverse square law for Earth s average distance from the Sun yields a solar constant of 1367 W m -2, compared to 445 W m -2 on Mars.

17 Atmospheric Influences on Isolation Direct Beam (~25%) Absorption (~25%) Back-scatter (reflection) (~25%) Stratosphere Troposphere (6%) (19%) Forward-scatter by gas molecule Forward-scatter by cloud Diffuse Radiation (25%) Earth Surface

18 Atmospheric Influences on Isolation Direct beam radiation (25%) Avoid gas molecules, particles and clouds Absorption (25%) Mainly by gas molecules Reflection (backscatter) (25%) By gas molecules & cloud droplets Diffuse radiation (forward scatter) (25%) By gas molecules, aerosols, clouds Transmission (50%) It is the % of insolation goes through the atmosphere & reaches the Earth s surface.

19 Fate of Solar Radiation Reaching surface 50% Atmospheric absorption: 25% (atmospheric absorptivity = 0.25) Atmospheric reflection: 25% = 19% (clouds) + 6% (gases) (atmospheric reflectivity or albedo = 0.25) 50% (25% direct, 25% diffuse) reaches Earth s surface: atmospheric transmissivity = 0.5, and of this 50%, 5% reflected back to space: Earth-surface s reflectivity = % absorbed: Earth-surface s absorptivity = 0.9 Combine atmosphere and Earth s surface as a whole: (Reflectivity or albedo = 25% + 5% = 0.3, absorptivity = 25% + 45% = 0.7)

20 Albedo reflectivity of a surface = reflected solar radiation incoming solar radiation affected by color, surface texture, angle of incidence

21 Atmospheric Influences on Isolation Characteristics of scattering depend upon the size of the scattering agents

22 Atmospheric Influences on Isolation Rayleigh Scattering: smaller gas molecules < ~ 1/10 radiation wavelength. both forward & backward scattering. scatter more shorter wavelength (blue end of the visible end) thus blue sky in clear day. but, when solar angle is small (sunrise, sunset), longer optical path, depletion of shorter wavelengths in incoming beam reddish Sun. Mie Scattering: larger aerosols (air pollution, wild fire, volcano) mainly forward (enhances redder Sun at sunrises & sunsets during high atmospheric aerosol concentration). scatter cross wavelengths in visible band (grayish sky in a hazy day).

23 Why blue Sky & reddish sunrise - sunsets? Longer optical path length at sunset, so blue wavelengths are greatly scattered and diffused, leaving only the red part of the spectrum (longer wavelengths) to reach the earth s surface Mie scattering enhances it especially with air pollutions Shorter wavelengths of the visible band are scattered in all directions in the sky by gas molecules causing blue sky

24 Atmospheric Influences on Isolation Nonselective Scattering: even larger water droplets in clouds. water droplets larger than radiation wavelengths in the visible band. water droplets acts like lenses which have different refractions on colors and thus creates rainbow. clouds (many droplets of different sizes) equally scatter across visible band which leads to white or gray sky. more backscatter (reflection), less forward scatter (diffuse radiation) than gas molecules (19% by clouds vs. 6%by gases).

25 Atmospheric Influences on Isolation Terrestrial radiation Absorption Near IR Atmospheric Window (visible band) Generally transparent (poor absorption) in visible ( m) band (large energy) Selective absorptions O 3 & O 2 in the UV band ( m) H 2 O & CO 2 in the near-ir (infrared) bands ( m)

26 Terrestrial Radiation Atmospheric Window (longwave band) Terrestrial radiation A window (relatively poor absorption) at m, except a narrow band around 9.5 m due to O 3 & O 2 absorption. This window is at peak terrestrial radiation allowing cooler atmosphere Outside this window, H 2 O & CO 2 absorb most thermal IR longwave radiation greenhouse gases, without which, Earth would be 33 o C cooler

27 Terrestrial Radiation Terrestrial radiation Atmospheric Window (longwave band) Clouds absorb virtually all longwave radiation (including those in the Atmospheric Window) cloudy nights not as cold as clear nights Outside this window, longwave radiation by Earth s surface is largely absorbed by atmosphere its temperature increases and radiates more longwave radiation Kirchoff s law, a good absorber is also a good emitter of radiation at the same wavelength Longwave radiation is transferred in all directions, including downward to the Earth s surface, which causes additional surface heating, and the cycle repeats.

28 Clouds Impacts on Solar & Terrestrial Radiation Cooler top Warmer bottom Diffuse solar radiation Cooling effect Warming effect

29 Longwave Radiation Budgets of the Surface & Atmosphere Net longwave (LW) Radiation of the Earth s Surface or Atmosphere = absorbed LW (gain) emitted LW (loss) Both the Earth s surface and atmosphere have net LW radiation losses

30 Radiation Balance (Equilibrium) Radiation Budget (Loss) Radiation Budget (Gain) Combine both shortwave (SW) and longwave (LW) radiation to evaluate radiation budget Net all-wave radiation or simply net radiation = absorbed (SW and LW) emitted (LW) Different processes for shortwave & longwave radiation

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33 Atmospheric Window in the visible band, as glass is transparent to visible light allow most solar radiation pass through the atmosphere or glass to heat the Earth s surface or objects inside a greenhouse. Greenhouse gases in the atmosphere (H 2 O & CO 2, etc.) are good absorbers of (thermal IR) longwave radiation as does the glass raise their own temperature emit more IR radiation downward transfer to heat the atmosphere below. Terrestrial Radiation Greenhouse Effect Longwave Shortwave

34 Terrestrial Radiation Greenhouse Effect Like the atmosphere, glass is also good at absorbing UV and IR