1	Lecture 2: Global Energy Balance
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3	Planetary Energy Balance
4	Solar Flux and Flux Density Solar Luminosity (L) the constant flux of energy put out by the sun L = 3.9 x 10²⁶ W Solar Flux Density (S_d) the amount of solar energy per unit area on a sphere centered at the Sun with a distance d S_d= L / (4 p d²) W/m²
5	Solar Flux Density Reaching Earth Solar Constant (S) The solar energy density at the mean distance of Earth from the sun (1.5 x 10¹¹ m) S = L / (4 p d²) = (3.9 x 10²⁶ W) / [4 x 3.14 x (1.5 x 10¹¹ m)²] = 1370 W/m²
6	Solar Energy Absorbed by Earth
7	Solar Energy Incident On the Earth Solar energy incident on the Earth = total amount of solar energy can be absorbed by Earth = (Solar constant) x (Shadow Area) = S x p R²_Earth
8	Albedo = [Reflected] / [Incoming] Sunlight
9	What Happens After the Earth Absorbs Solar Energy? The Earth warms up and has to emit radiative energy back to the space to reach a equilibrium condition. The radiation emitted by the Earth is called “terrestrial radiation” which is assumed to be like blackbody radiation.
10	Blackbody Radiation Blackbody A blackbody is something that emits (or absorbs) electromagnetic radiation with 100% efficiency at all wavelength. Blackbody Radiation The amount of the radiation emitted by a blackbody depends on the absolute temperature of the blackbody.
11	Energy Emitted from Earth
12	Planetary Energy Balance
13	Greenhouse Effect
14	Two Key Reasons for the Greenhouse Effect Solar and terrestrial radiations are emitted at very different wavelengths. The greenhouse gases selectively absorb certain frequencies of radiation.
15	Spectrum of Radiation Radiation energy comes in an infinite number of wavelengths. We can divide these wavelengths into a few bands.
16	Wien’s Law Wien’s law relates an objective’s maximum emitted wavelength of radiation to the objective’s temperature. It states that the wavelength of the maximum emitted radiation by an object is inversely proportional to the objective’s absolute temperature.
17	Apply Wien’s Law To Sun and Earth Sun l_max = 2898 mm K / 6000K = 0.483 mm Earth l_max = 2898 mm K / 300K = 9.66 mm Sun radiates its maximum energy within the visible portion of the radiation spectrum, while Earth radiates its maximum energy in the infrared portion of the spectrum.
18	Wavelength and Temperature The hotter the objective, the shorter the wavelength of the peak radiation.
19	Selective Absorption and Emission The atmosphere is not a perfect blackbody, it absorbs some wavelength of radiation and is transparent to others (such as solar radiation). è Greenhouse effect. Objective that selectively absorbs radiation usually selectively emit radiation at the same wavelength. For example, water vapor and CO2 are strong absorbers of infrared radiation and poor absorbers of visible solar radiation.
20	Why Selective Absorption/Emission? Radiation energy is absorbed or emitted to change the energy levels of atoms or molecular. The energy levels of atoms and molecular are discrete but not continuous. Therefore, atoms and molecular can absorb or emit certain amounts of energy that correspond to the differences between the differences of their energy levels. è Absorb or emit at selective frequencies.
21	Different Forms of Energy Levels The energy of a molecule can be stored in (1) translational (the gross movement of molecules or atoms through space), (2) vibrational, (3) rotational, and (4) electronic (energy related to the orbit) forms.
22	Energy Required to Change the Levels The most energetic photons (with shortest wavelength) are at the top of the figure, toward the bottom, energy level decreases, and wavelengths increase.
23	Three Factors To Determine Planet Temperature Distance from the Sun Albedo Greenhouse effect
24	Earth, Mars, and Venus
25	Global Temperature
26	Greenhouse Effects On Venus è 510°K (very large!!) On Earth è 33°K On Mars è 6°K (very small)
27	Why Large Greenhouse Effect On Venus? Venus is too close to the Sun Venus temperature is very high Very difficult for Venus’s atmosphere to get saturated in water vapor Evaporation keep on bringing water vapor into Venus’s atmosphere Greenhouse effect is very large A “run away” greenhouse happened on Venus Water vapor is dissociated into hydrogen and oxygen Hydrogen then escaped to space and oxygen reacted with carbon to form carbon dioxide No water left on Venus (and no more chemical weathering)
28	Why Small Greenhouse Effect on Mars? Mars is too small in size Mars had no large internal heat Mars lost all the internal heat quickly No tectonic activity on Mars Carbon can not be injected back to the atmosphere Little greenhouse effect A very cold Mars!!
29	Vertical View of the Energy Balance
30	Vertical Distribution of Energy
31	Where Is Earth’s Radiation Emitted From?
32	Cloud Type Based On Properties
33	Cloud Types Based On Height
34	Important Roles of Clouds In Global Climate
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36	Zenith Angle and Insolation The larger the solar zenith angle, the weaker the insolation, because the same amount of sunlight has to be spread over a larger area.
37	Latitudinal Variations of Net Energy Polarward heat flux is needed to transport radiation energy from the tropics to higher latitudes.
38	Polarward Energy Transport
39	How Do Atmosphere and Ocean Transport Heat?
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