Transcript Earth`s Climate System Today

Earth’s 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
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Electromagnetic Spectrum
Electromagnetic
energy travels
through space
 Energy heating
Earth mostly shortwave radiation
 Visible light
 Some ultraviolet
radiation
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Incoming Solar Radiation
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Radiation at top of
Earth’s 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
30% Solar Energy Reflected
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Energy reflected by clouds, dust, surface
 Ave. incoming radiation 0.7 x 342 = 240 W m-2
Energy Budget
Earth’s 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 > 1K, radiator of heat
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Energy Budget
Average Earth’s 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?
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Energy Budget
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Greenhouse gases absorb 95% of the long-wave,
back radiation emitted from Earth’s surface
 Trapped radiation reradiated down to Earth’s
surface
 Accounts for the 31C heating
 Satellites don’t 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 Earth’s temperature in balance
Energy Balance
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

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

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
Sun Angle Affects Albedo
All of Earth’s
surfaces absorb
more solar
radiation from an
overhead sun
 Water reflects <5%
radiation from an
overhead Sun

Sun Angle Affects Albedo
Water reflects a high fraction of
radiation from a low-lying Sun
 Earth average albedo = 10%
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Pole-to-Equator Heat Imbalance
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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
Seasonal Change in Solar Radiation & Albedo
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Tilt of Earth’s axis results in seasonal change in
 Solar radiation in each hemisphere
 Snow and ice cover (albedo)
Seasonal Change in Solar Radiation
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Large seasonal change in solar radiation
between the hemispheres
Seasonal Change in Albedo
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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
Albedo-Temperature Feedback
Water a Key to Earth’s Climate
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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
-3
-2
 Heat Capacity (cal cm ) = Density (g cm ) x
Specific Heat (cal g-1)
 Specific heat of water = 1
Ratio of heat capacity water:ice:air:land 60:5:2:1
 Heat capacity of air linked to water vapor
Differences in Heating Land & Oceans
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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
don’t store heat
Sensitivity of Land & Oceans to Solar Heating
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Change in mean seasonal surface temperature greatest
over large landmasses and lowest over oceans
Thermal Response Different
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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
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
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
0°C-100°C, 1 calorie
of heat energy
of water releases
-1
HCondensation
2O(l)  H
2O(g) requires 540 cal g
-1
needed to increase540
1gH
cal
latent heat of vaporization
2Og by– 1°C
80
watertransformation,
freezes – latentice
heat
of melting
80cal
calgg-1-1heat
heatreleased
requiredwhen
for phase
 water
Latent Heat of Vaporization
Important – evaporation occurs at any
temperature between 0-100°C
 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
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Saturation vapor density
 Warm air holds 10X more water than cold
Redistribution of Latent Heat
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Evaporation in warm equatorial region
Stored energy carried vertically and horizontally
Condensation and precipitation releases energy
Water Vapor Feedback
Unequal Heating of Tropics and Poles
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Latitudes <35° have excess incoming solar
radiation over outgoing back radiation
Excess heat stored in upper ocean drives general
circulation of oceans and atmosphere
Atmospheric Circulation
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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?
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
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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
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Horizontally moving air undergoes
turbulence
 Air is forced to mix vertically through
eddy motions because of
 High velocity
 Depth of atmosphere
 Low viscosity
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
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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
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
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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
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
-1
 Dry adibatic lapse rate = 10°C km
 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
10°C km-1
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Dew Point Lapse Rate
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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 2°C km-1
Over 1 km, air cools by 10°C
 Air temperature rapidly approaches dew point
as parcel rises
 As air temperature approaches dew point,
cloud forms
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
 4°C km-1 minimum (rapid condensation)
 9°C 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
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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