Chapter 2: Global Energy BalanceChapter 2: Global Energy Balance

This chapter discusses:This chapter discusses:

1.1. Earth’s emission temperatureEarth’s emission temperature

2.2. Greenhouse effectsGreenhouse effects

3.3. Global radiative energy balance and its Global radiative energy balance and its distributionsdistributions

Why do we have seasons?Why do we have seasons?

Earth’s Tilt and Seasonal RadiationEarth’s Tilt and Seasonal Radiation

Orbit around Sun: slightly elliptical Tilt of spin axis at 23.5°

Rotation on axis (day&night) in 24 hrsSpeed (eq 1038 mi/hr, 0 at axis)Axis points toward North Star (Polaris)

Summer Solstice (NH) on June 21/22Axis toward Sun

Vertical rays on Tropic of Cancer (23.5°N Lat)24-hr day above Arctic Circle (66.5°N Lat)24-hr night below Antarctic Circle (66.5°S Lat)

Winter Solstice (NH) on Dec. 21/22Axis away from SunVertical rays on Tropic of Capricorn (23.5°S Lat)24-hr night above Arctic Circle (66.5°N Lat)24-hr day below Antarctic Circle (66.5°S Lat)

Fall Equinox (NH) on Sept 22/23 (day = night length at all points) Spring Equinox (NH) on March 20/21

(day=night length at all points)

Earth’s Orbit Today and the Seasons

Earth’s Orbit Parameters

Eccentricity (shape of the orbit: varies from being elliptical to almost circular)

Obliquity (tilt of the axis of rotation)Precession (wobbling of the axis of rotation)

Eccentricity: Earth’s orbit around the sun

Orbit = Circle(Eccentricity = 0)

Orbit = Ellipse

(Eccentricity = 0.05 shown0.0167 today

0.0605 maximum)

Varies from near circle to ellipse with a period of 100,000 yearsDistance to Sun changes insolation changes

Obliquity: Tilt of the Earth’s rotational axis

• Cycle of ~ 41,000 years• Varies from 22.2 to 24.5°(The current axial tilt is 23.5°)

If Earth’s orbit were circular,No tilt = no seasons90° tilt = largest seasonal differences at the poles (6 mon. darkness, 6 mon. overhead sun)

• Greater tilt = more intense seasons

Precession: positions of solstices and equinoxes in the eccentric orbit slowly change

Period of about 23,000 years

Wobbling of the axis

Turning of the ellipse

Earth’s Orbit Changes Through Time

Changes in Insolation Received

on Earth

• Precession dominates at low and middle latitudes

• Tilt is more evident at higher mid-latitudes.

• Eccentricity is not significant directly, but modulates the amplitude of the precession cycle.

• Summer changes dominate over winter at polar latitudes.

Energy from the SunEnergy from the Sun

1. Characteristics

Travels through space (vacuum) in a speed of light

3. Importance to climate and climate change

In the form of waves:

In stream of particles

Electromagnetic waves

(Photons)

Primary driving force of Earth’s climate Primary driving force of Earth’s climate engineengine

2. Electromagnetic spectrum

From short wavelength, high energy, gamma rays to long wavelength, low energy, radio waves

Releases heat when absorbed

Ultraviolet, Visible, InfraredUltraviolet, Visible, Infrared

Sun’s Electromagnetic SpectrumSun’s Electromagnetic Spectrum

Solar radiation has peak intensities in the Solar radiation has peak intensities in the shorter wavelengthsshorter wavelengths, , dominant in the region we know as dominant in the region we know as visiblevisible, thus , thus shortwave shortwave radiationradiation

Blackbody Radiation Curves

Any object above Any object above absolute zero absolute zero radiates heat, radiates heat, as as proportional to proportional to TT44

Longwave & Shortwave RadiationLongwave & Shortwave Radiation

The The hot sunhot sun radiates at radiates at shortershorter wavelengths wavelengths that carry that carry more more energyenergy, and the , and the fraction fraction absorbed by the absorbed by the cooler earthcooler earth is is then re-radiated then re-radiated at at longer longer wavelengths.wavelengths.

Incoming Solar Radiation (Insolation)Incoming Solar Radiation (Insolation)

At the top of the At the top of the atmosphereatmosphere

Warming Earth's AtmosphereWarming Earth's Atmosphere

Solar radiation passes first through the upper atmosphere, but only Solar radiation passes first through the upper atmosphere, but only after absorption by earth's surface does it generate sensible heat to after absorption by earth's surface does it generate sensible heat to warm the ground and generate longwave energy.warm the ground and generate longwave energy.This heat and energy at the surface then warms the atmosphere This heat and energy at the surface then warms the atmosphere from below.from below.

Earth’s Radiation Budget (Global Annual Average)Earth’s Radiation Budget (Global Annual Average)

Earth reflects 30% directly back to space, absorbs Earth reflects 30% directly back to space, absorbs about 20% in the atmosphere, and absorbs about 50% about 20% in the atmosphere, and absorbs about 50% at the surface.at the surface.Earth’s lower atmosphere is warmed by radiation, Earth’s lower atmosphere is warmed by radiation, conduction, convection of sensible heat and latent conduction, convection of sensible heat and latent heat.heat.

Kiehl and Trenberth (1997) BAMS Kiehl and Trenberth (1997) BAMS (Fig. 7)(Fig. 7)

Comparison of Different EstimatesComparison of Different Estimates

Kiehl and Trenberth (1997) BAMS Kiehl and Trenberth (1997) BAMS (Fig. 7)(Fig. 7)

Incoming Solar RadiationIncoming Solar Radiation

Solar radiation is scattered and reflected by the atmosphere, clouds, Solar radiation is scattered and reflected by the atmosphere, clouds, and earth's surface, creating an average albedo of 30%.and earth's surface, creating an average albedo of 30%.Atmospheric gases and clouds absorb another 19 units, leaving 51 Atmospheric gases and clouds absorb another 19 units, leaving 51 units of shortwave absorbed by the earth's surface.units of shortwave absorbed by the earth's surface.

Earth-Atmosphere Energy BalanceEarth-Atmosphere Energy Balance

Earth's surface absorbs the 51 units of shortwave and 96 more of Earth's surface absorbs the 51 units of shortwave and 96 more of longwave energy units from atmospheric gases and clouds.longwave energy units from atmospheric gases and clouds.These 147 units gained by earth are due to shortwave and longwave These 147 units gained by earth are due to shortwave and longwave greenhouse gas absorption and emittance.greenhouse gas absorption and emittance. Earth's surface loses these 147 units through conduction, Earth's surface loses these 147 units through conduction, evaporation, and radiation.evaporation, and radiation.

Earth’s Average Radiating Temperature?Earth’s Average Radiating Temperature?

T= 15°C (59°F)

T= –18°C (0°F)

Greenhouse Effect!Greenhouse Effect!

Ground-based Measurements

Space-based Measurements

Caused by Greenhouse Gases!Caused by Greenhouse Gases!

Water vapor accounts for 60% of the atmospheric greenhouse effect, CO2 26%, and the remaining greenhouse gases 14%.

Greenhouse Gases

Water vapor (H2O)

Carbon dioxide (CO2) Methane (CH4)

Nitrous oxide (N2O)Ozone (O3)Chlorofluorocarbons

(CFC’s)

What are they?

CO2 contributes most (55-60%) to the anthropogenic greenhouse effect, and methane is a distant second (16%).

CFCs cause the strongest greenhouse warming on a molecule-for-molecule basis.

Lecture OutlineLecture Outline

1.1. More on radiation, greenhouse effects, and More on radiation, greenhouse effects, and greenhouse gasesgreenhouse gases

2.2. Forms of heat transfer in the atmosphere and Forms of heat transfer in the atmosphere and oceansoceansa.a. RadiationRadiationb.b. ConductionConductionc.c. ConvectionConvectiond.d. TurbulenceTurbulence

3.3. How radiation is distributed with latitudeHow radiation is distributed with latitude

4.4. Wind patterns and general circulationWind patterns and general circulation

Radiation - Heat TransferRadiation - Heat Transfer

Radiation Radiation travels as waves travels as waves of photons that of photons that release energy release energy when absorbed.when absorbed.

All objects above All objects above 0° K release 0° K release radiation, and its radiation, and its heat energy value heat energy value increases to the 4th increases to the 4th power of its power of its

temperaturetemperature ((Stefan-Boltzmann Stefan-Boltzmann

LawLaw).).

EEBB BB = = σσTT44

σσ = 5.67= 5.67××1010-8 -8 W mW m-2 -2 KK-4-4

Energy Flux, Solar Constant, Emission TemperatureEnergy Flux, Solar Constant, Emission Temperature

Energy (work) = Force Energy (work) = Force × × Length Length 1 joule (J) = 1 newton (N) 1 joule (J) = 1 newton (N) × × 1 meter (m)1 meter (m)Power = Energy / TimePower = Energy / Time watt (W) = J / swatt (W) = J / s

Solar luminosity: LSolar luminosity: L00=3.9=3.9××10102626 J/s = 3.9 J/s = 3.9××10102626 W W

Energy conservation: LEnergy conservation: L00 at the Sun’s photosphere at the Sun’s photosphere

= L= L00 at any distance d from the Sun (1 at any distance d from the Sun (1stst law of law of

thermodynamics dQ=dU – dW)thermodynamics dQ=dU – dW)

Flux density = flux / area = LFlux density = flux / area = L00 / (4 / (4ππdd22) = solar ) = solar

constantconstant

Earth’s solar constant SEarth’s solar constant S00= 1367 = 1367 ± ± 2 Wm2 Wm-2-2

Emission temperature of the Sun = 5796 KEmission temperature of the Sun = 5796 K

Emission temperature of a planet: solar radiation Emission temperature of a planet: solar radiation absorbed = planetary radiation emittedabsorbed = planetary radiation emitted

Asrar et al. (2001) BAMS (Fig. Asrar et al. (2001) BAMS (Fig. 5)5)

Energy Flux, Solar Constant, Emission Temperature Energy Flux, Solar Constant, Emission Temperature (continued)(continued)

Absorbed solar radiation = SAbsorbed solar radiation = S00 (1 – (1 – αα) ) ππ r r22

Emitted terrestrial radiation = Emitted terrestrial radiation = σσ TT4 4 44ππ rr22

Both are equal: SBoth are equal: S0 0 (1 – (1 – αα) /4 = ) /4 = σσTT44

Emission temperature: T = [(SEmission temperature: T = [(S00/4) (1 – /4) (1 – αα) /) /σσ]]1/41/4

Earth’s emission temperature = 255 K = –18Earth’s emission temperature = 255 K = –18°°CC

Absorption & EmissionAbsorption & Emission

Solar radiation is selectively absorbed by earth's surface cover.Solar radiation is selectively absorbed by earth's surface cover. Darker objects absorb shortwave and emit longwave with high Darker objects absorb shortwave and emit longwave with high efficiency.efficiency. In a forest, this longwave energy melts snow.In a forest, this longwave energy melts snow.

Atmospheric AbsorptionAtmospheric Absorption

Solar radiation Solar radiation passes rather freely passes rather freely through earth's through earth's atmosphere, but atmosphere, but earth's re-emitted earth's re-emitted longwave energy longwave energy either fits through a either fits through a narrow window or is narrow window or is absorbed by absorbed by greenhouse gases greenhouse gases and re-radiated and re-radiated toward earth.toward earth.

Wavelength

Abs

orpt

ion

(100

%)

Nitrous Oxide

Methane

Ozone

Water Vapor

Carbon Dioxide

Total Atmo

IR

UV

Absorption, Scattering and Reflectance (Albedo)

Temperature increases

All directions

Backwards

Absorption, Scattering and Reflectance (Albedo)

Scattered LightScattered Light

Solar radiation Solar radiation passing through passing through earth's earth's atmosphere is atmosphere is scattered by scattered by gases, aerosols, gases, aerosols, and dust.and dust.

At the horizon At the horizon sunlight passes sunlight passes through more through more scatterers, scatterers, leaving longer leaving longer wavelengths wavelengths and redder and redder colors revealed.colors revealed.

Heat Transfer: ConductionHeat Transfer: Conduction

Conduction of Conduction of heat energy heat energy occurs as occurs as warmerwarmer molecules molecules transmit transmit vibration, and vibration, and hence heat, hence heat, toto adjacent adjacent coolercooler molecules.molecules.

Warm ground Warm ground surfaces heat surfaces heat overlying air overlying air by conduction.by conduction.

Heat Transfer: Convection of Sensible HeatHeat Transfer: Convection of Sensible Heat

Convective turbulence

Another Example of ConvectionAnother Example of Convection

Convection is heat energy moving as a Convection is heat energy moving as a fluidfluid from from hotter to hotter to coolercooler areasareas..

Warm air at the ground surface rises as a thermal bubble expands, Warm air at the ground surface rises as a thermal bubble expands, consumes energy, and hence cools.consumes energy, and hence cools.

Convection: A Household ExampleConvection: A Household Example

Heat Transfer: Latent HeatHeat Transfer: Latent Heat

Latent HeatLatent Heat

As As water moves toward vaporwater moves toward vapor it absorbs latent ( it absorbs latent (e.g. not e.g. not sensedsensed) heat to keep the molecules in rapid motion.) heat to keep the molecules in rapid motion.

Heat Energy for StormsHeat Energy for Storms

Latent heat releasedLatent heat released from the billions of vapor droplets from the billions of vapor droplets during during condensationcondensation and cloud formation and cloud formation fuelsfuels storm energy needs, storm energy needs, warmswarms the air, and encourages the air, and encourages tallertaller cloud growth. cloud growth.

Radiation, Convection and Conduction

Basic Energy and Mass Transfers in the Atmosphere

• Processes transfer heat/mass between the Earth’s surface and the atmosphere (pages 21, 93)– Radiation

– Conduction

– Convection

– Turbulence

• Three processes affect radiation in the Earth’s atmosphere (pages 45, 296–297)– Absorption

– Scattering

– Reflectance

Basic Energy and Mass Transfers in the Atmosphere (Cont’d)

• Processes transfer heat/mass between the Earth’s surface and the atmosphere (pages 21, 93)– Radiation: the transfer of heat energy without the involvement of a

physical substance in the transmission. Radiation can transmit heat through a vacuum.

– Convection: transmits heat by transporting groups of molecules from place to place within a substance. Convection occurs in fluids such as water and air, which move freely.

– Turbulence: is the tendency for air to be turned over as it moves.

– Conduction is the process by which heat energy is transmitted through contact with neighboring molecules.

Unequal Radiation on a SphereUnequal Radiation on a Sphere

Insolation is stronger in the tropics (low latitudes) than in the polar regions (high latitudes).

Solar flux per unit surface area Q = Solar flux per unit surface area Q = SS00 (d (dmm/d)/d)22 cos cos θθ

ddmm = mean sun-earth distance = mean sun-earth distance

d = actual sun-earth distanced = actual sun-earth distanceθθ= the solar zenith angle, which = the solar zenith angle, which depends on the latitude (depends on the latitude (φφ), season ), season ((δδ), and time of day (h).), and time of day (h).

cos cos θθ = sin = sin φφ sin sin δδ + cos + cos φφ cos cos δδ cos hcos h(See Appendix A)(See Appendix A)

At sunrise or sunset, cos hAt sunrise or sunset, cos h00 = – tan = – tan φφ

tan tan δδ

Averaged daily insolation at TOA Averaged daily insolation at TOA QQdayday = (S = (S00 / / ππ ) (d ) (dmm/d)/d)22 [ h [ h00 sin sin φφ sin sin δδ + +

cos cos φφ cos cos δδ sin h sin h00 ] ]

Why do we global wind patterns (general circulation)?Why do we global wind patterns (general circulation)?

Unequal heating of tropics and polesUnequal heating of tropics and poles

General Circulation of the AtmosphereGeneral Circulation of the Atmosphere