1 GEOS 110 Winter 2011 Earth’s Surface Energy Balance 1.Energy Balance and Temperature a.Atmospheric influences on insolation: absorption, reflection,

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GEOS 110 Winter 2011Earth’s Surface Energy Balance

1. Energy Balance and Temperaturea. Atmospheric influences on insolation:

absorption, reflection, and scatteringb. What happens to incoming solar

radiation? (global scale; local scale later)

c. Surface-atmosphere energy transferd. Greenhouse effecte. Temp. distributions

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Radiation is the transfer of electromagnetic (EM) energy via an electrical wave and a magnetic wave. When this energy is absorbed by an object there is an increase in molecular motion and hence in temperature.

Radiation

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Sun: T~6000K

E~7.4 x107 W/m2 ,

max0.44m

Earth: T~300K

E~ 460 W/m2 ,

max9.66m

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Radiation Laws

1. All objects, at whatever temperature, emit radiant energy.

2. Hotter objects radiate more total energy per unit area than colder objects.

 E = T4 (Stefan-Boltzman Law) =5.67e-8 Wm2K-4

3. The hotter the body the shorter wavelength of maximum radiation

max= c / T(K) (Wein’s Law) c=2897 mK 4. Objects that are good absorbers of radiation are also good

emitters. A perfect absorber/emitter is called a blackbody.

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InsolationWhat happens to incoming solar radiation (=insolation)? It is absorbed, reflected, and scattered.

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b. What happens to Solar Insolation?

The global energy budget = a balance between incoming solar radiation (+) and outgoing terrestrial radiation (-)

http://geography.uoregon.edu/envchange/clim_animations/gifs/three_rads_web.gif

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Absorption: Reduces energy reaching Earth surface –

different gases absorb different wavelengths of radiation

Scattering: Radiation is redirected

Scattering• Gas molecules in the atmosphere scatter incoming solar

radiation in all directions, not just back into space

• The smaller molecules scatter shorter wavelength, blue light.Gases and aerosols are more effective scattering different

wavelengths:

Gas molecules are most effective scattering shorter wavelengths of visual light (i.e. blue and violet), Aerosols scatter all wavelengths

Moonrise Earthrise

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19%

6%

25%

45%

5%19%

6%

25%

45%

5%

b. What happens to Solar Insolation?

On Average

50% does not reach surface: 25% absorbed by atmosphere

(7% via ozone) 19% reflected via clouds 6% back scattered via

atmosphere

50% that reaches the surface: 45% absorbed by Earth surface 5% reflected by ground

If we assume a constant supply of incoming solar radiation:

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b. The Fate of Solar Insolation

19%

6%

25%

45%

5%

planetary albedo = 30%(Average reflectivity)

Earth and Atmosphere absorb 45% + 25% = 70% of solar insolation

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Earth’s Energy Balance

1. Energy Balance and Temperaturea. Atmospheric influences on insolation:

absorption, reflection, and scattering

b. Fate of incoming solar radiation

c. Surface-atmosphere energy transfer

d. Greenhouse effect

e. Temp. distributions

C. Surface – Atmosphere Energy Transfer

Radiation Exchange: Earth emits radiation (longwave), almost like a

blackbody

Most of this radiation

(96%) is absorbed by

the atmosphere

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(Radiation emitted by Earth)

Radiation absorbed by atm.

C. Surface – Atmosphere Energy Transfer

Radiation Exchange:Selective absorption

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(Radiation emitted by Earth)

Radiation absorbed by atm.

Atmospheric “window”

C. Surface – Atmosphere Energy Transfer

• Radiation Exchange:

• Net loss of radiation

C. Surface – Atmosphere Energy Transfer

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Net Radiation = absorption of insolation

+ net longwave radiation

C. Surface – Atmosphere Energy Transfer

Atmosphere = net radiation deficitSurface = net radiation surplus

Energy must transfer between the surface and the atmosphere

Conduction: transfers radiant energy into Earth, and warms the laminar boundary layer ( = thin layer of air above surface)

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Heat TransferMechanisms of Heat Transfer

C. Surface – Atmosphere Energy Transfer

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http://nepalmountaintrek.com/images/paragliding.JPG

Convection moves energy between surface and atmosphere:

Free convection:

Mixing related to differential buoyancy

C. Surface – Atmosphere Energy Transfer

Convection moves energy between surface and atmosphere:

Forced convection:

= disorganized flow

22Hurricane Ike at landfall, Huston/Galveston, 13 Sep. 2008

http://en.wikipedia.org/wiki/Image:HGX_N0R_Legend_0.png

C. Surface – Atmosphere Energy Transfer

How does the surface energy surplus get to the atmosphere?

1. Sensible heat: Readily detected heat energy Magnitude of change related to object’s

specific heat (J kg-1 K-1) and mass

2. Latent Heat: Energy required to change the phase of

a substance23

C. Surface – Atmosphere Energy Transfer

When radiation hits water (e.g., ocean, lake, moist soil, plants that can transpire), energy that could have gone to sensible heating is redirected to evaporate some water.

Evaporation of water makes energy available to the atmosphere that otherwise would warm the surface, thus acting as an energy transfer mechanism.

There is no net energy loss

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C. Surface – Atmosphere Energy Transfer

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Surface surplus offset by transfer of sensible heat(8 units) and latent heat (21 units) heat to atmosphere.

C. Surface – Atmosphere Energy Transfer

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Latent heat

(21 units) is a bigger factor than sensible heat (8 units):

C. Surface – Atmosphere Energy Transfer

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Latitudinal variations: Between 38°N and S = net energy surpluses Poleward of 38o = net energy deficits Winter hemispheres - Net energy deficits

poleward of 15o

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Earth’s Heat Budget

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Aerosols - Aerosol = liquid or solid particle suspended in the atmosphere.- Large quantity = concentration of 1000 / cm3. (1 breath = 1000cm3 = 1 million aerosols)- Tiny = micrometers = 1 millionth of a meter. Aerosols are formed by human and natural causes (e.g., sea salt from ocean waves; fine soil; smoke and soot from fires, vehicles, and aircraft; volcanic eruptions). 

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Sulfate particles produced from volcanic eruptions causes a cooling of the surface.

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Earth's surface is 5 million kilometers further from the sun in Northern Earth's surface is 5 million kilometers further from the sun in Northern summer than in winter, indicating that seasonal warmth is controlled by summer than in winter, indicating that seasonal warmth is controlled by more than solar proximity.more than solar proximity.

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Seasons & Solar IntensitySeasons & Solar Intensity

Solar intensity, defined as the energy per area, governs earth's seasonal Solar intensity, defined as the energy per area, governs earth's seasonal changes.changes.

A sunlight beam that strikes at an angle is spread across a greater surface area, A sunlight beam that strikes at an angle is spread across a greater surface area, and is a less intense heat source than a beam impinging directly.and is a less intense heat source than a beam impinging directly.

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Earth's annual energy Earth's annual energy balance between solar balance between solar insolation and insolation and terrestrial infrared terrestrial infrared radiation is achieved radiation is achieved locally at only two locally at only two lines of latitude.lines of latitude.

A global balance is A global balance is maintained by excess maintained by excess heat from the heat from the equatorial region equatorial region transferring toward the transferring toward the poles.poles.

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Northern Northern hemisphere hemisphere sunrises are in sunrises are in the southeast the southeast during winter, during winter, but in the but in the northeast in northeast in summer.summer.

Summer noon Summer noon time sun is also time sun is also higher above the higher above the horizon than the horizon than the winter sun.winter sun.

Local Solar ChangesLocal Solar Changes

C. Surface – Atmosphere Energy Transfer

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Latitudinal variations: Energy surplus at low latitudes is offset by advection (horizontal

heat movement) of heat poleward by global wind (75%) and ocean (25%) currents

Global Sea Surface Temperatures: Climatology:http://www.cpc.ncep.noaa.gov/products/GODAS/clim_movie.shtml

C. Surface – Atmosphere Energy Transfer

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Ocean currents:1. Climate change could cause a shift in the position of some

ocean currents, through a variety of mechanism. Can you identify any land regions where climate could be vulnerable to shifts in nearby currents?

2. Are there any localities whose climate could cool even if the average global temperature were to warm?

• Daily Temp (NYC)

• Gases in DRY AIR

• Atmospheric Pressure and Altitude

• Thermal Structure of the atmosphere• Determined by energy source, density of layers and composition of layers

• Daily path of the sun for • a location at ? latitude

• Effect of Sun’s angle of incidence

• Annual variation in daily duration of available insolation

Relationship between

mean monthly temperature

and latitude

• Continental effect and Marine effect