Earths Climate System Today

1
Earths Climate System Today

• Heated by solar energy
• Tropics heated more than poles
• Imbalance in heating redistributed
• Solar heating and movement of heat by oceans and
  atmosphere determines distribution of
• Temperature
• Precipitation
• Ice
• Vegetation

2
Electromagnetic Spectrum

• Electromagnetic energy travels through space
• Energy heating Earth mostly short-wave radiation
• Visible light
• Some ultraviolet radiation

3
Incoming Solar Radiation

• Radiation at top of Earths atmosphere 1368 W
  m-2
• If Earth flat disk with no atmosphere, average
  radiation 1368 W m-2
• Earth 3-dimensional rotating sphere,
• Area 4?r2
• Average solar heating 1368 ? 4 342 W m-2

4
30 Solar Energy Reflected

• Energy reflected by clouds, dust, surface
• Ave. incoming radiation 0.7 x 342 240 W m-2

5
Energy Budget

• Earths temperature constant 15?C
• Energy loss must incoming energy
• Earth is constantly receiving heat from Sun,
  therefore must lose equal amount of heat back to
  space
• Heat loss called back radiation
• Wavelengths in the infrared (long-wave radiation)
• Earth is a radiator of heat
• If T gt 1?K, radiator of heat

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Energy Budget

• Average Earths surface temperature 15?C
• Reasonable assumption
• Surface of earth radiates heat with an average
  temperature of 15?C
• However, satellite data indicate Earth radiating
  heat average temperature -16?C
• Why the discrepancy?
• What accounts for the 31?C heating?

7
Energy Budget

• Greenhouse gases absorb 95 of the long-wave,
  back radiation emitted from Earths surface
• Trapped radiation reradiated down to Earths
  surface
• Accounts for the 31?C heating
• Satellites dont detect radiation
• Muffling effect from greenhouse gases
• Heat radiated back to space from elevation of
  about 5 km (top of clouds) average 240 W m-2
• Keeps Earths temperature in balance

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Energy Balance
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Greenhouse Gases

• Water vapor (H2O(v), 1 to 3)
• Carbon dioxide (CO2, 0.037 365 ppmv)
• Methane (CH4, 0.00018 1.8 ppmv)
• Nitrous oxide (N2O, 0.00000315 315 ppbv)
• Clouds also trap outgoing radiation

10
Variations in Heat Balance

• Incoming solar radiation
• Stronger at low latitudes
• Weaker at high latitudes
• Tropics receive more solar radiation per unit
  area than Poles

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Variations in Heat Balance

• What else affects variation in heat balance?
• Solar radiation arrives at a low angle
• Snow and ice reflect more radiation at high
  latitudes
• Albedo
• Percentage of incoming solar radiation that is
  reflected rather than absorbed

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Average Albedo
13
Sun Angle Affects Albedo

• All of Earths surfaces absorb more solar
  radiation from an overhead sun
• Water reflects lt5 radiation from an overhead Sun

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Sun Angle Affects Albedo

• Water reflects a high fraction of radiation from
  a low-lying Sun
• Earth average albedo 10

15
Pole-to-Equator Heat Imbalance

• Incoming solar radiation per unit area higher in
  Tropics than Poles
• Sun angle higher in Poles than Tropics
• Albedo higher at Poles than Tropics
• Variations in cloud cover affect heat imbalance

16
Seasonal Change in Solar Radiation Albedo

• Tilt of Earths axis results in seasonal change
  in
• Solar radiation in each hemisphere
• Snow and ice cover (albedo)

17
Seasonal Change in Solar Radiation

• Large seasonal change in solar radiation between
  the hemispheres

18
Seasonal Change in Albedo

• Increases in N. hemisphere winter due mainly to
  snow cover and to lesser degree Arctic sea ice
• Increases in S. hemisphere winter due to sea ice

19
Albedo-Temperature Feedback
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Water a Key to Earths Climate

• Water has high heat capacity
• Measure of ability to absorb heat
• Heat measured in calories
• 1 calorie amount of heat required to raise
  temperature of one gram of water by 1?C
• Heat Capacity (cal cm-3) Density (g cm-2) x
  Specific Heat (cal g-1)
• Specific heat of water 1
• Ratio of heat capacity watericeairland
  60521
• Heat capacity of air linked to water vapor

21
Differences in Heating Land Oceans

• Low latitude ocean major storage tank of solar
  heat
• Sunlight direct, albedo low, heat capacity high
• Heats surface winds mix heat
• Contrast with land
• Albedo high, heat capacity conductance low
• Tropical/subtropical lands become hot, but dont
  store heat

22
Sensitivity of Land Oceans to Solar Heating

• Change in mean seasonal surface temperature
  greatest over large landmasses and lowest over
  oceans

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Thermal Response Different

• Large land masses heat and cool quickly
• Extreme seasonal temperature reached 1 month
  after Solstice
• Upper ocean heats and cools slowly
• Extreme seasonal temperature reached 2-3 months
  after Solstice

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Redistribution of Heat

• Heat transfer in Earth atmosphere
• Sensible heat
• Heat that a person directly senses
• Sensible heat T x specific heat
• Latent heat (hidden or concealed)
• Additional heat required to change the state of a
  substance
• Sensible and latent heat affected by convection

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Convection
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Sensible Heat

• Sensible heating greatest
• At low latitude
• Overhead Sun
• Over land
• Low heat conductance (air heats)
• Dry regions
• Low humidity
• Sensible heat lowest
• Over oceanic regions

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Latent Heat

• Heat is temporarily hidden or latent in water
  vapor
• Powerful process transferring heat long distances
• Transfer is two step process
• Initial evaporation of water and storage of heat
  in vapor
• Later release of stored heat during condensation
  and precipitation (typically far from site of
  evaporation)

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Latent Heat
0C-100C, 1 calorie of heat energy needed to
increase 1 g H2O by 1C
Condensation of water releases 540 cal g-1
latent heat of vaporization
H2O(l) ? H2O(g) requires 540 cal g-1
80 cal g-1 heat required for phase
transformation, ice ? water
80 cal g-1 heat released when water freezes
latent heat of melting
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Latent Heat of Vaporization

• Important evaporation occurs at any temperature
  between 0-100C
• Latent heat is associated with any change of
  state
• Therefore, during evaporation heat is stored in
  water vapor in latent form for later release

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Water Vapor Content of Air

• Saturation vapor density
• Warm air holds 10X more water than cold

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Redistribution of Latent Heat

• Evaporation in warm equatorial region
• Stored energy carried vertically and horizontally
• Condensation and precipitation releases energy

32
Water Vapor Feedback
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Unequal Heating of Tropics and Poles

• Latitudes lt35 have excess incoming solar
  radiation over outgoing back radiation
• Excess heat stored in upper ocean drives general
  circulation of oceans and atmosphere

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Atmospheric Circulation

• Atmosphere has no distinct upper boundary
• Air becomes less dense with increasing altitude
• Air is compressible and subject to greater
  compression at lower elevations, density of air
  greater at surface
• Constant composition to 80 km
• What drives atmospheric circulation?

35
Free Convection

• Atmospheric mixing related to buoyancy
• Localized parcel of air is heated more than
  nearby air
• Warm air is less dense than cold air
• Warm air is therefore more buoyant than cold air
• Warm air rises

36
Forced Convection

• Occurs when a fluid breaks into disorganized
  swirling motions as it undergoes flow
• Fluid flow can be laminar or turbulent

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Laminar vs. Turbulent Flow

• Whether a fluid flow is laminar or turbulent
  depends on
• Velocity (rate of movement)
• Geometry (primarily depth)
• Viscosity
• Turbulent flow occurs during high velocity
  movement of non-viscous fluids in unconfined
  geometries

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Forced Convection in Atmosphere

• Horizontally moving air undergoes turbulence
• Air is forced to mix vertically through eddy
  motions because of
• High velocity
• Depth of atmosphere
• Low viscosity

39
Atmospheric Circulation

• Force of gravity maintains a stable atmosphere
• Most of the mass of air near surface
• As a result of atmospheric pressure
• Dense air at surface
• Air flows from high pressure to low pressure
• Flow is turbulent
• Turbulent flow produces vertical mixing

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Mixing by Sensible Heat

• Convection driven by sensible heat
• Air parcels rise if they become heated and less
  dense than surrounding air
• As air parcels rises,
• Air expands
• Air cools
• Air becomes less dense
• Air parcels stop rising
• Heat transferred vertically, since air forced
  from high to low pressure, heat also moves
  horizontally

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Adibatic Process

• Rising and sinking air change temperature with no
  gain or loss of heat
• Consider sinking parcel of air
• As it sinks, it contracts
• Contraction takes work
• Work takes (mechanical) energy
• Temperature of air rises
• Conservation of energy
• 1st law of thermodynamics

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Thermodynamics of Air

• First law of thermodynamics
• Heat added work done rise in Temp
• But, adibatic process (no heat added)
• Heat added work done rise in Temp
• Second term is not zero
• Work of compression results in a rise in
  temperature of air parcel

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Mixing by Latent Heat

• Water vapor is less dense than mixture of gases
  composing the atmosphere
• Evaporation adds water vapor to atmosphere and
  lowers its density
• Moist air rises, expands and cools until dew
  point reached
• When air becomes fully saturated
• Condensation begins
• Air releases latent heat
• Air heats and becomes less dense causing it to
  rise further
• Eventually water vapor lost, air parcel stops
  release of latent heat and stops rising

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Which Process More Important?

• Atmospheric circulation driven by adiabatic
  processes (sensible heat) redistributes about 30
  heat
• Atmospheric circulation driven by latent heat
  redistributes about 70 heat
• Greater amount of heat stored in water
• Larger distances moist air parcels move

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Dry Adibatic Lapse Rate

• Rising and sinking dry air parcel cools and heats
  at a constant rate
• Dry adibatic lapse rate 10C km-1
• Work required to lift an air parcel
• Mix of gases
• Acceleration of gravity
• Regardless of latitude, season, altitude, etc. a
  dry parcel of air will heat or cool at 10C km-1

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Dew Point Lapse Rate

• Consider a rising parcel of air with constant
  humidity
• Dew point decreases as parcel expands
• Drop in pressure, drop in dew point
• Lapse of dew point as parcel rises
• Dew point lapse rate 2C km-1
• Over 1 km, air cools by 10C
• Air temperature rapidly approaches dew point as
  parcel rises
• As air temperature approaches dew point, cloud
  forms

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Wet Adibatic Lapse Rate

• As wet air rises, it cools, dew point reached and
  condensation begins
• Latent heat released
• Decreasing rate of cooling
• Wet adibatic lapse rate
• 4C km-1 minimum (rapid condensation)
• 9C km-1 maximum (slow condensation)
• Differences in temperature
• For same amount of cooling, warm air looses more
  water than cold air

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Summary

• Once saturation reached latent heat released as
  long as parcel continues to rise
• The saturated process assumes condensation
  products fall out of parcel, so the parcel
  maintains 100 humidity
• Upon decent, the parcel warms, relatively
  humidity falls below 100
• After decent the parcel is warmer because latent
  heat was added during ascent
• Dry adibatic process reversible
• Wet adibatic process non-reversible