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Chapter 5: Atmospheric Structure and Energy BalancePowerPoint Presentation

Chapter 5: Atmospheric Structure and Energy Balance

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### (II) Radiation Energy

### (III) Modelling Energy Balance

(I) Characteristics of the Atmosphere

- Thickness, air pressure, density
- Air pressure and density decrease with altitude
- 90% of its mass (5.1 x 1018 kg) is within 16 km (10 mi) of the
surface (about 0.0025 times the radius of the Earth)

97% of air in first 29 km or 18 mi; 99% 32 km (18 mi); 99.9% 47km (30mi)

- Atmospheric motions can therefore be considered to occur
“at” the Earth’s surface

- The greatest and most important variations in its composition involve
water in its various phases

- Water vapor
- Clouds of liquid water
- Clouds of ice crystals
- Rain, snow and hail

Argon (.93%) and

Carbon Dioxide (.03%)

Ozone (.000004%)

Composition of the Atmosphere

Dry Air

Solid particles (dust, sea salt,

pollution) also exist

Water vapor is constantly being

added and subtracted from the

atmosphere, and varies from near

0% (deserts) to 3-4% (warm,

tropical oceans and jungles)

Vertical Structure of the Atmosphere

4 distinct layers

determined by

the change of

temperature

with height

- Extends to 10 km in the extratropics, 16 km in the tropics
- Contains 80-90% of the atmospheric mass, and 50% is
- contained in the lowest 5 km (3.5 miles)
- It is defined as a layer of temperature decrease
- The total temperature change with altitude is about 72°C
- (130°F), or 6.5°C per km (lapse rate)
- It is the region where all weather occurs, and it is kept
- well stirred by rising and descending air currents
- The transition region of no temperature change is the
- “tropopause”: it marks the beginning of the next layer

Vertical Structure of the Atmosphere

4 distinct layers

determined by

the change of

temperature

with height

- Extends to about 50 km
- It is defined as a layer of temperature increase and
- is stable with very little vertical air motion – a good place to fly!
- Why does temperature increase?
- The major heating is the UV of sunlight absorbed by O3.. When the sunlight
- travel down, the UV will become less and less available, so the temperature
- increase with height…
- The transition region to the next layer is the “stratopause”

Atm. vertical structure

- Air pressure p at sea level is 1 atm. = 1.013 bar = 1013 mb
- p decr. with altitude by factor of 10 every 16 km.
- T decr. with altitude in troposphere,
rises in stratosphere

drops in mesosph.

rises in thermosph.

Objectives:

Electromagnetic (EM) radiation & spectrum

Energy flux

Blackbody radiation -- Wien’s Law & Stefan-Boltzmann Law

Planetary energy balance

UNBC

EM radiation

wavelength

later t

- EM radiation includes visible light, ultraviolet, infrared, microwaves.
- wavelength
- period T, frequency = 1/T
- wave speed or phase speed c = /T =
- Speed of light in vacuum: c = 3.00108 m/s

UNBC

Energy flux

- Power = energy per unit time (watt W = J/s)
- Flux F = power per unit area (W/m2)

less flux

high lat.

=> less F

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Blackbody radiation

- Absolute temperature in degrees Kelvin (K)
- 0 K = -273°C (coldest possible T)

- All bodies emit EM radiation
- e.g. humans emit mainly infrared (IR)

- “Blackbody” emits (or absorbs) EM rad. with 100% efficiency.

UNBC

Wien’s Law

Planck function (blackbody rad. curve)

Rad. flux

max

wavelength

max = const./T

Temp. T in K

const. = 2898 m

max refers to the Wavelength of energy radiated with greatest intensity.

UNBC

Stefan-Boltzmann Law

Planck function (blackbody rad. curve)

Rad. flux

wavelength

F = T4

= 5.67 x 10-8 W/m2/K4

total F = area

under curve

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Planetary energy balance

- Earth is at steady state:
Energy emitted by Earth =

Energy absorbed ..(1)

- E emitted = (area of Earth) Te4
= 4 Re2 Te4

(Te= Earth’s effective rad. temp., Re= Earth’s radius)

- E absorbed = E intercepted - E reflected
- Solar E intercepted = S Re2 (solar flux S)
- Solar E reflected = AS Re2 (albedo A)
- E absorbed = (1-A) S Re2

- (1) => 4 Re2 Te4 = (1-A) S Re2

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Magnitude of greenhouse effect

- Te4 = (1-A) S/4
- Te = [(1-A) S/(4 )]1/4 (i.e. fourth root)
- Te = 255K = -18°C, very cold!
- Observ. mean surf. temp. Ts = 288K = 15°C

- Earth’s atm. acts as greenhouse, trapping outgoing rad.
- Ts - Te = Tg, the greenhouse effect
- Tg = 33°C

UNBC

Greenhouse effect of a 1-layer atm.

S/4

AS/4

Te4

Te

Atm.

Te4

(1-A)S/4

Ts4

Ts

Earth

- Energy balance at Earth’s surface:
- Ts4 =(1-A)S/4 +Te4 ..(1)

- Energy balance for atm.:
- Ts4 = 2Te4 .. (2)

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Te4=(1-A)S/4 ..(3) (same eq. as in last lec.)

Divide (2) by ; take 4th root:

Ts= 21/4Te = 1.19 Te

For Te = 255K, Ts = 303K. (Observ. Ts = 288K)

Tg = Ts- Te = 48K,

15K higher than actual value.

- Overestimation: atm. is not perfectly absorbing all IR rad. from Earth’s surface.

UNBC

Objectives:

Effects of clouds

Earth’s global energy budget

Climate modelling

Climate feedbacks

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Climatic effects of clouds

- Without clouds, Earths’ albedo drops from 0.3 to 0.1.
By reflecting solar rad., clouds cool Earth.

- But clouds absorb IR radiation from Earth’s surface (greenhouse effect) => warms Earth.
- Cirrus clouds: ice crystals let solar rad. thru, but absorbs IR rad. from Earth’s sfc.
=> warm Earth

- Low level clouds (e.g. stratus): reflects solar rad. & absorbs IR => net cooling of Earth

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- IR rad. from clouds at T4
- High clouds has much lower T than low clouds
=> high clouds radiate much less to space than low clouds.

=> high clouds much stronger greenhouse effect.

UNBC

Climate Modelling

- “General circulation models” (GCM):
Divide atm./oc. into 3-D grids.

Calc. variables (e.g. T, wind, water vapor, currents) at grid pts.

=> expensive.

- e.g. used in double CO2 exp.

GFDL, Princeton

UNBC

- Weather forecasting also uses atm. GCMs. Assimilate observ. data into model. Advance model into future => forecasts.
- Simpler: 1-D (vertical direction) radiative-convective model (RCM):
Doubling atm. CO2 => +1.2°C in ave.sfc.T

- Need to incorporate climate feedbacks:
- water vapour feedback
- snow & ice albedo feedback
- IR flux/Temp. feedback
- cloud feedback

UNBC

Water vapour feedback data into model. Advance model into future => forecasts.

Atm. H2O

Ts

Greenhouse effect

- If Ts incr., more evap. => more water vapour => more greenhouse gas => Ts incr.
- If Ts decr., water vap. condenses out => less greenhouse gas => Ts decr.
- Feedback factor f = 2.
- From RCM: T0 = 1.2°C (without feedback)
=> Teq = f T0 = 2.4°C.

(+)

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Snow & ice albedo feedback data into model. Advance model into future => forecasts.

snow &

ice cover

Ts

planetary albedo

- If Ts incr. => less snow & ice => decr. planetary albedo => Ts incr.
- As snow & ice are in mid-high lat. => can only incorp. this effect in 3-D or 2-D models, not in 1-D RCM.

(+)

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IR flux/Temp. feedback data into model. Advance model into future => forecasts.

Ts

Outgoing IR flux

- So far only +ve feedbacks => unstable.
- Neg. feedback: If Ts incr. => more IR rad. from Earth’s sfc. => decr. Ts

(-)

- But this feedback loop can be overwhelmed if Ts is high & lots of water vap. around
=> water vap. blocks outgoing IR

=> runaway greenhouse (e.g. Venus)

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Uncertainties in cloud feedback data into model. Advance model into future => forecasts.

- Incr. Ts => more evap. => more clouds
- But clouds occur when air is ascending, not when air is descending. If area of ascending/descending air stays const.
=> area of cloud cover const.

- High clouds or low clouds? High clouds warm while low clouds cool the Earth.
- GCM’s resolution too coarse to resolve clouds => need to “parameterize” (ie. approx.) clouds.
- GCM => incr. Ts => more cirrus clouds => warming => positive feedback.
=> Teq = 2 -5°C for CO2 doubling

UNBC

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