unit 3 presentation 1 july 10, 2015 solar radiation energy global radiation balance sun in local sky
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Unit 3 Presentation 1July 10, 2015
Solar radiationEnergyGlobal radiation balance Sun in local sky
SOLAR RADIATION
The Earth intercepts 1.5 quadrillion megawatt-hours per year or 28,000 times the power consumed by the entire population of the planet each year.
Strahler & Strahler 2003
i.e., low frequencies
i.e., high frequencies
Shortwave vs. longwave radiation
Ruddiman 2001
• Important principles regarding emission of electromagnetic radiation.
(1) There is an inverse relationship between wavelength l and the temperature T of the object emitting it (e.g. the Sun emits very short wavelength energy because it is so hot).
Planck’s Law: E = (2c2h /) e[hc/kT - 1]
where the speed of light c = 3x108 m/s, Boltzmann's constant k = 1.38 x 10-23 J/oK, and Planck's constant h = 6.64 x 10-34 J s.
Radiation and Temperature
NOTE: l is reported in length units: mm = 10-6 m (micrometers) nm = 10-9 m (nanometers) Å = 10-10 m (Ångstroms)
• Important principles regarding emission of electromagnetic radiation.
(2) Integrating E over all wavelengths gives the total energy flux emanating from the surface of the radiating body: Stefan’s Law: E = T4
where = emissivity of the radiator (= 1 for a blackbody radiator), s = the Stefan-Boltzmann constant (= 5.673 x 10-8 W/(m2 oK4)), and T is the absolute temperature of the radiator's surface in oK.
(3) The Wien Displacement Law explains wavelength shift of peak E with change in T:
max = b/T
With b=2.8977685×10−3m ºK (Wien displacement constant) and T in oK.
Radiation and Temperature
used in all Earth systemand climate models
Shortwave Radiation• Ultraviolet
– short wavelength (0.2 to 0.4 micrometers), high energy. Does not penetrate atmosphere easily.
• Visible light– shorter wavelengths are perceived as violet, longer wavelengths as
red (0.4 to 0.7 micrometers); penetrates atmosphere easily.• Near-infrared radiation
– in the wavelengths from 0.7 to 1.2 micrometers.• Shortwave infrared radiation
– wavelengths from 1.2 to 3 micrometers, invisible to the eye. Penetrates atmosphere easily.
THE SUN
~0.5 mm
E = 2c2h / e[hc/kT - 1]
withT=5800 oK
Longwave radiation
• Thermal infrared radiation– Wavelengths greater than 3 micrometers– This form of radiation is emitted by cooler bodies. – It is perceived as heat.
THE EARTH
~10 mm
E = 2c2h / e[hc/kT - 1]
withT=288oK
Wavelengths of Sun and Earth Radiation
• Earth Energy– Earth’s output peaks
in the thermal infrared
portion of the electro-
magnetic spectrum
Strahler & Strahler 2003
• Solar Energy – Sun’s output peaks
in the visible light
portion of the electro-
magnetic spectrum.
Effects of solar radiation on the ecosystem:
The Sun's surface has an energy flux density of 70 x 106 W/m2. (Note: W/m2 = J/s m2, where J = kg m2/s2). As this radiates out into space, the density diminishes as a function of distance2; by the time it reaches the Earth the density is only on the order of 1370 W/m2.
Path of Radiation from Sun to Earth (or other planet)
From Ruddiman 2001
At top-of-atmosphere (TOA),solar radiation is I=1368 W/m2.
I affects an area equivalent toa disk of dimension pr2.
Averaging I over a 24-hourperiod gives pr2I/4pr2=I/4=342 W/m2.
Incoming solar radiation on Earth
2
2
The Global Radiation Balance• The Sun transmits shortwave radiation into space where it is intercepted byEarth.
• The absorbed radiation is ultimately emitted by Earth as longwave radiation to space
TEarth = ( (1- ) 343 W m-2 / 5.67 x 10-8 W m-2 K-4)1/4
= 279 K (+6 ºC)
Earth blackbody radiator model
But we know Tearth = 288K!
TEarth = ( ( 0.7) 343 W m-2 / 5.67 x 10-8 W m-2 K-4)1/4
= 255 K (-18 ºC)
No albedo (=0):
With albedo ( =0.3):
Input = output
(1-)E=T4
T=[(1-) E/()]1/4 , where =1
TBR = +6ºC
TBR = -47ºC
TBR = +55ºC
The Seasonal Cycle
Summer
Sun in the local sky
EquatorC
elestial Equator
Baltimore
Olin Hall
Northern Summer Solstice
At 40ºN solar altitude A=73.5º
At 40ºN sunrise in NE
At 40ºN sunset in NW
Note:Zenith angle z =90º-A
Path of the Sun in the sky at selected
latitudes for summer, winter
solstices and equinox
From Strahler & Strahler 2003
Note: “8” = “ º ”
EARTH ALBEDO
Albedo (Latin for “white”) is the average reflection coefficient of an object (1=white; 0=black). “Bond albedo” is the total radiation reflected from an object compared to the total incident radiation from the Sun. “Geometric albedo” is the amount of radiation relative to that of a flat Lambertian surface which is an ideal reflector at all wavelengths.
De Pater & Lissauer 2010
Trees and grasses
Rocks and Soils
Water and Snow
http://www-surf.larc.nasa.gov/surf/pages/bbalb.html
Broadband Albedo (Oct. 1986)
http://snowdog.larc.nasa.gov/surf/pages/lat_lon.html
Baltimore MD Oct. 1986
END OF PRESENTATION
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