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