1	Lecture 2: Global Energy Cycle
2	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
3	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
4	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
5	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.
6	Solar Energy Absorbed by Earth
7	Albedo = [Reflected] / [Incoming] Sunlight
8	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.
9	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.
10	Stefan-Boltzmann Law The single factor that determine how much energy is emitted by a blackbody is its temperature. The intensity of energy radiated by a blackbody increases according to the fourth power of its absolute temperature. This relationship is called the Stefan-Boltzmann Law.
11	Apply Stefan-Boltzmann Law To Sun and Earth Sun E_s = (5.67 x 10^-8 W/m² K⁴) * (6000K)⁴ = 73,483,200 W/m² Earth E_e = (5.67 x 10^-8 W/m² K⁴) * (300K)⁴ = 459 W/m² Sun emits about 160,000 times more radiation per unit area than the Earth because Sun’s temperature is about 20 times higher than Earth’s temperature. è 20⁴ = 160,000
12	Energy Emitted from Earth
13	Planetary Energy Balance
14	Greenhouse Effect
15	Greenhouse Gases
16	Factors Determine Planet Temperature Distance from the Sun Albedo Greenhouse effect
17	Mars, Earth, and Venus
18	Global Temperature
19	Greenhouse Effects On Venus è 510°K (very large!!) On Earth è 33°K On Mars è 6°K (very small)
20	Why Large Greenhouse Effect On Venus? Venus is very 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 liquid water left on Venus
21	Saturation Vapor Pressure Saturation vapor pressure describes how much water vapor is needed to make the air saturated at any given temperature. Saturation vapor pressure depends primarily on the air temperature in the following way: è Saturation pressure increases exponentially with air temperature.
22	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!!
23	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.
24	Stefan-Boltzmann Law The single factor that determine how much energy is emitted by a blackbody is its temperature. The intensity of energy radiated by a blackbody increases according to the fourth power of its absolute temperature. This relationship is called the Stefan-Boltzmann Law.
25	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.
26	Micrometer (mm)
27	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.
28	Spectrum of Radiation Radiation energy comes in an infinite number of wavelengths. We can divide these wavelengths into a few bands.
29	Solar and Terrestrial Radiation All objectives radiate energy, not merely at one single wavelength but over a wide range of different wavelengths. The sun radiates more energy than the Earth. The greatest intensity of solar energy is radiated at a wavelength much shorter than that of the greatest energy emitted by the Earth.
30	Shortwave and Longwave Radiations Solar radiation is often referred to as “shortwave radiation”. Terrestrial radiation is referred to as “longwave radiation”.
31	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.
32	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.
33	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.
34	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.
35	Vertical Distribution of Energy
36	Vertical Distribution of Energy
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39	Greenhouse Effect and Diurnal Cycle The very strong downward emission of terrestrial radiation from the atmosphere is crucial to main the relatively small diurnal variation of surface temperature. If this large downward radiation is not larger than solar heating of the surface, the surface temperature would warm rapidly during the day and cool rapidly at the night. è a large diurnal variation of surface temperature. The greenhouse effect not only keeps Earth’s surface warm but also limit the amplitude of the diurnal temperature variation at the surface.
40	Important Roles of Clouds In Global Climate
41	Atmospheric Influences on Insolation Absorption - convert insolation to heat the atmosphere Reflection / Scattering - change the direction and intensity of insolation Transmission - no change on the direction and intensity of insolation
42	Reflection and Scattering Reflection: light bounces back from an objective at the same angle at which it encounters a surface and with the same intensity. Scattering: light is split into a larger number of rays, traveling in different directions. Although scattering disperses light both forward and backward (backscattering), more energy is dispersed in the forward direction.
43	Scattering
44	Rayleigh Scattering (Gas Molecules) Involves gases, or other scattering agents that are smaller than the energy wavelengths. Scatter energy forward and backward. Violet and blue are scattered the most, up to 16 times more than red light. Responsible for (1) blue sky in clear days, (2) blue tint of the atmosphere when viewed from space, (3) why sunsets/sunrises are often yellow, orange, and red.
45	Scattering and Colors Short wavelengths (blue and violet) of visible light are scattered more effectively than longer wavelengths (red, orange). Therefore, when the Sun is overhead, an observer can look in any direction and see predominantly blue light that was selectively scattered by the gases in the atmosphere. At sunset, the path of light must take through the atmosphere is much longer. Most of the blue light is scattered before it reaches an observer. Thus the Sun appears reddish in color.
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47	Mie Scattering (Aerosols) Larger scattering agents, such as suspended aerosols, scatter energy only in a forward manner. Larger particles interact with wavelengths across the visible spectrum. Produces hazy or grayish skies. Enhances longer wavelengths during sunrises and sunsets, indicative of a rather aerosol laden atmosphere.
48	Nonselective Scattering (Clouds) Water droplets in clouds, typically larger than energy wavelengths, equally scatter wavelengths along the visible portion of the spectrum. Produces a white or gray appearance. No wavelength is especially affected.
49	Lecture 3: Temperature
50	Seasonal and Latitudinal Variations The amount of energy absorbed and emitted by Earth changes geographically and seasonally. Seasonal variations: the angle of inclination is responsible for the seasonal variation in the amount of solar energy distributed at the top of the atmosphere. Latitudinal variations: the variations of solar energy in latitude is caused by changes in: (a) the angle the sun hits Earth’s surface = solar zenith angle (b) the number of day light hours (c) albedo
51	Angle of Inclination = the Tilt
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53	Length of Day
54	Solar Zenith Angle Solar zenith angle is the angle at which the sunlight strikes a particular location on Earth. This angle is 0° when the sun is directly overhead and increase as sun sets and reaches 90 ° when the sun is on the horizon.
55	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.
56	Solar Zenith Angle Affects Albedo The larger the solar zenith angle, the larger the albedo. When the zenith angle is large, sunlight has to pass through a thicker layer of the atmosphere before it reaches the surface. The thinker the atmospheric layer, more sunlight can be reflected or scattered back to the space.
57	What Determine Zenith Angle?
58	Sun in the Sky
59	Length of Day
60	Insolation at Top of Atmosphere
61	Insolation in Summer Solstice
62	Albedo = [Reflected] / [Incoming] Sunlight
63	Surface Types Affect Albedo
64	Global Distribution of Albedo
65	Latitudinal Variations of Net Energy Polarward heat flux is needed to transport radiation energy from the tropics to higher latitudes.
66	Polarward Energy Transport
67	How Do Atmosphere and Ocean Transport Heat?
68	Global Atmospheric Circulation Model
69	Ocean Circulation
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71	Diurnal Temperature Variations The difference between the daily maximum and minimum temperature is called the daily (or diurnal) range of temperature.
72	Diurnal Cycle Changes with Altitude The diurnal cycle (the daily range of temperature) is greatest next to the ground and becomes progressively small as we move away from the surface. The diurnal cycle is also much larger on clear day than on cloudy ones.
73	Daytime Warming Sunlight warms the ground and the air in contact with the ground by conduction. Air is a poor heat conductor, so this heating is limited to a layer near the surface. Air temperatures above this layer are cooler. Wind stirring can reduce this vertical difference in air temperatures.
74	Nighttime Cooling Both the ground and air above cool by radiating infrared energy, a process called radiational cooling. The ground, being a much better radiator than air, is able to cool more quickly. Shortly after sunset, the earth’s surface is cooler than the air directly above.
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76	How to Measure Temperature The thermometer has to be mounted 1.52m (5 ft) above the ground. The door of the instrument shelter has to face north in Northern Hemisphere.