Surface energy balance
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Transcript Surface energy balance
Surface energy balance (2)
Review of last lecture
–
What is energy? 3 methods of energy transfer
–
The names of the 6 wavelength categories in the
electromagnetic radiation spectrum. The wavelength range
of Sun (shortwave) and Earth (longwave) radition
–
Earth’s energy balance at the top of the atmosphere.
Incoming shortwave = Reflected Shortwave + Emitted longwave
–
Earth’s energy balance at the surface.
Incoming shortwave + Incoming longwave = Reflected shortwave
+ Emitted longwave + Latent heat flux + Sensible heat flux
+ Subsurface conduction
Surface energy balance
Incoming shortwave + Incoming longwave = Reflected shortwave + Emitted longwave
+ Latent heat flux + Sensible heat flux + Subsurface conduction
SWdn
SWup
LWdn
LWup
LH
SH
dT/dt
Fc
Incoming solar radiation
SWdn = S cos
where
S is solar constant S=1366 Watts/m2
is solar zenith angle, which is the angle between the
local zenith and the line of line of sight to the sun
Reflected solar radiation
SWup = SWdn
where is albedo, which is the ratio of
reflected flux density to incident flux density,
referenced to some surface.
Global map of surface albedo
Incoming and surface emitted
longwave radiation
• Incoming longwave radiation can be
estimated from air temperature using the
blackbody approximation
• Surface emitted longwave radiation can be
estimated from surface temperature using
the blackbody approximation
Physical Representation of
Radiation
• Blackbodies: purely hypothetical bodies that absorb
and emit the maximum radiation at all wavelengths
• The Earth and the sun are close to blackbodies.
• The atmosphere is not close to blackbody, but it can
served as the first order approximation
Stefan-Boltzmann Law
• States that radiation emitted from a blackbody is
a function ONLY of temperature
• Hotter bodies emit more energy than colder bodies
I=T4
where I is the intensity of the radiation, T is the
temperature in K, and is the Stefan-Boltzmann
constant, 5.67 x 10-8 W m-2 K-4)
• So, double T, 16x more radiation
• Earth (290K)= 401 Wm-2, Sun (6000K) = 7.3 x 106
Wm-2. So ISun >> Iearth
• Incoming LW (air-emitted):
LWdn = Tair4
• Surface emitted LW:
LWup=Ts4
Net longwave radiation
( LWdn - Lwup = Tair4 - Ts4 )
• Is generally small because air temperature is often
close to surface temperature
• Is generally smaller than net shortwave radiation
even when air temperature is not close to surface
temperature
• Important during the night when there is no
shortwave radiation
Surface “Sensible” and
“Latent” heat transfers
First, recall 2 other methods of
energy transfer in addition to
radiation:
1. Conduction
–
This is how excess heat in ground
is transferred to the atmosphere
via an extremely thin layer of air in
contact with the surface
2. Convection
–
Once the heat is transferred from
the surface to the air via
conduction, convection takes over
from here via “sensible” and
“latent” heat transfers
Sensible heat flux
• Sensible heat: heat energy which is readily detected
• Sensible heat flux
SH = Cd Cp V (Tsurface - Tair)
Where is the air density, Cd is flux transfer coefficient, Cp is
specific heat of air (the amount of energy needed to increase
the temperature by 1 degree for 1 kg of air), V is surface wind
speed, Tsurface is surface temperature, Tair is air temperature
• Magnitude is related surface wind speed
– Stronger winds cause larger flux
• Sensible heat transfer occurs from warmer to cooler areas
(i.e., from ground upward)
• Cd needs to be measured from complicated eddy flux
instrument
Latent Heat
• Energy required to induce changes of
state in a substance
• In atmospheric processes, invariably
involves water
• When water is present, latent heat of
evaporation redirects some energy
which would be used for sensible heat
– Wet environments are cooler
relative to their insolation amounts
• Latent heat of evaporation is stored in
water vapor
– Released as latent heat of
condensation when that change of
state is induced
Latent heat flux
• LH = Cd L V (qsurface - qair)
Where is the air density, Cd is flux transfer coefficient, L is
latent heat of water vapor, V is surface wind speed, qsurface
is surface specific humidity, qair is surface air specific
humidity
• Magnitude is related surface wind speed
– Stronger winds cause larger flux
• Latent heat transfer occurs from wetter to drier areas (i.e.,
from ground upward)
• Cd needs to be measured from complicated eddy flux
instrument
Seasonal variation of surface energy
budget
Storage change = net radiation - latent heat flux sensible heat flux
Summary: Surface energy balance
Incoming shortwave + Incoming longwave = Reflected shortwave + Emitted longwave
+ Latent heat flux + Sensible heat flux + Subsurface conduction
SWdn
=Scos
SWup
=SWdn
LWdn
LWup
=Tair4 =Ts4
LH=CdLV(qsurface- qair)
SH=CdCpV(TsurfaceTair)
dT/dt
Fc = - dT/dz