ASPECT EFFECTS

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ASPECT EFFECTS
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Photosynthetically-active radiation (spectral
portion,0.3-0.4 CI)
0400-0500h
0500-0600h
0600-0700h
0700-0800h
0800-0900h
0900-1000h
1000-1100h
1100-1200h
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TERRESTRIAL
RADIATION
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Longwave Radiative Exchange
The atmosphere absorbs long-wave radiation (L)
from the Earth, clouds and gases at all
altitudes Absorption greatest in lower portion
of the atmosphere, where H20 and CO2
concentrations are highest The atmosphere
absorbs effectively from 3-100 ?m, except in the
atmospheric window (8-11 ?m) Most longwave loss
to space occurs through this window, but clouds
can partially close it
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NET
RADIATION
BALANCE
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L? is greater in magnitude and more variable than
L?
?L? ?0 ??(T0)4 (1 - ?0) L?
Amount of L? reflected (slight adjustment)
L L? - L? (usually negative)
NET ALL_WAVE RADIATION DAYTIME Q K? - K?
L? - L? Q K L NIGHT Q L
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Radiation Measurements
PAR
L?
K?
UV-A
K? (not visible)
L?
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More radiation sensors
Source University of Colorado
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K? in tropical forests of Colombia/Ecuador
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Radiation Balance Components
Negative in Oke
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Clouds Reduce K? because of absorption and
reflection from cloud tops (may eliminate
S) Increase D by scattering incoming solar
radiation Strongest K ? under partly cloudy
skies with sun in clear patch Absorb much of L?
and re-emit it as L? (low cloud emits
more) Reduce diurnal temperature variation
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Global Energy Balance (SIMPLIFIED)
Source NOAA
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SURFACE
ENERGY
BALANCE
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Q - positive in daytime - almost always
negative at night Any Q imbalance is accounted
for by convective exchange or conduction Q
QH QE QG ?S where QH sensible heat
flux QE latent heat flux QG conduction to
or from ground
(See Figure 1.10)
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Recall the First Law of Thermodynamics ENERGY IN
ENERGY OUT Qin gt Qout (flux convergence) Net
storage gain leads to warming Qout gt Qin (flux
divergence) Net storage energy loss leads to
cooling Qin Qout No net change in energy
storage
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WATER BALANCE
LATENT HEAT
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Water H2O

• High heat capacity
• Exists in all states at Earths
• temperatures
• Heat required/released during phase changes
• Latent heat of fusion (Lf 0.334 MJ kg-1)
• Latent heat of vaporization (Lv 2.45 MJ kg-1)
• Latent heat of sublimation (Ls Lf Lv)

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Water Balance
p E ?r ?s Where p is precipitation E is
evapotranspiration r is net runoff s is soil
moisture storage content QE Lv E ?QM Lf
M Where E and M are in kg m-2 s-1
See Fig. 1.13
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• Sensible and Latent Heat Fluxes
• Eddy correlation (later)
• Sonic anemometer
• measurements of vertical
• velocity and temperature
• Krypton hygrometer
• measurements of water
• vapour density

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Advection and Winds
Air flow at local scale can affect energy
balance as can air flow at scales larger than
boundary layer At the micro-scale, horizontal
temperature variation causes horizontal pressure
differences Why ? Warm air is lighter than cold
air This leads to winds (kinetic energy)
Energy transferred to smaller and smaller
scales before being dissipated as heat
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DIURNAL PATTERNS
OF SENSIBLE AND
LATENT HEAT FLUXES
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• DAYTIME
• Both sides of equation are positive
• surface radiative surplus
• Surplus partitioned into ground and
• atmosphere
• Convection is the most important means of
• daytime heat transport from surface
• QE is greater when soil moisture is high
• QH is greater when water is more restricted

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• NIGHT
• Both sides of equation are negative
• surface radiative deficit
• Deficit partitioned into heat gain from ground
• and atmosphere
• Q loss is partially replenished by QG
• QE and QH of less importance as convective
• exchange is dampened by the night-time
• temperature stratification