Chapter 5:  Atmospheric Structure and Energy Balance
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Chapter 5: Atmospheric Structure and 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 10 18 kg) is within 16 km (10 mi) of the

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Chapter 5: Atmospheric Structure and Energy Balance

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Chapter 5 atmospheric structure and energy balance

Chapter 5: Atmospheric Structure and Energy Balance


Chapter 5 atmospheric structure and 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


Chapter 5 atmospheric structure and energy balance

TRACE GASES

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)


Chapter 5 atmospheric structure and energy balance

Vertical Structure of the Atmosphere

4 distinct layers

determined by

the change of

temperature

with height


Chapter 5 atmospheric structure and energy balance

  • 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


Chapter 5 atmospheric structure and energy balance

Vertical Structure of the Atmosphere

4 distinct layers

determined by

the change of

temperature

with height


Chapter 5 atmospheric structure and energy balance

  • 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

Temperature

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.


Ii radiation energy

(II) Radiation Energy

Objectives:

Electromagnetic (EM) radiation & spectrum

Energy flux

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

Planetary energy balance

UNBC


Em radiation

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 = 3.00108 m/s

UNBC


C t ct c

c = /T =>  = cT = c/ 

longer period waves => ? wavelength

longer

UNBC


Energy flux

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

UNBC


Em spectrum

EM spectrum

  • EM radiation classified by their wavelength or freq.

UNBC


Inverse square law

Inverse-square law

S

  • Solar flux S falls off as

e.g. if r = 2r0

=> S = S0/4

UNBC


Blackbody radiation

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

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


Blackbody rad curves for sun earth

Blackbody rad. curves for Sun & Earth

max = const./T

Temp. T in K

const. = 2898 m

UNBC


Stefan boltzmann law

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

UNBC


Planetary energy balance

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

UNBC


Magnitude of greenhouse effect

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

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 = 2Te4 .. (2)

UNBC


Chapter 5 atmospheric structure and energy balance

Subst. (2) into (1):

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


Iii modelling energy balance

(III) Modelling Energy Balance

Objectives:

Effects of clouds

Earth’s global energy budget

Climate modelling

Climate feedbacks

UNBC


Chapter 5 atmospheric structure and energy balance

Cumulus

Cumulonimbus

Stratus

Cirrus

UNBC


Climatic effects of clouds

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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Chapter 5 atmospheric structure and energy balance

  • 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


Earth s global energy budget

Earth’s global energy budget

UNBC


Climate modelling

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


Chapter 5 atmospheric structure and energy balance

  • 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

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Water vapour feedback

Water vapour feedback

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

Snow & ice albedo feedback

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.

(+)

UNBC


Ir flux temp feedback

IR flux/Temp. feedback

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)

UNBC


Uncertainties in cloud feedback

Uncertainties in cloud feedback

  • 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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