Surface energy balance

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Surface Energy Balance (1)
Review of last lecture
• The current status of weather and climate predictions:
(1) weather prediction good to 10 days, (2) tropical
cyclone prediction good in track but not in intensity, (3)
climate prediction good to two seasons, (4) climate
change projections have a 3-fold difference in
magnitude.
• The main reasons of the difficulties: (1) Teleconnection
problem, (2) Feedback problem, and (3) Subgrid-scale
problem.
• Importance of the ABL: (1) interface between
atmosphere and ocean/land/ice - flux transfer and
feedback, (2) the human beings are living in the ABL
and change the environment, (3) a basic subgrid-scale
process
Energy basics
• Energy: the ability to do work
• Many forms: electrical, mechanical, thermal,
chemical, nuclear, …
• Joule (J): standard unit of energy (1 J=
0.239 calories)
• Watt (W): rate of energy flow (W = 1 J/s)
Methods of
Energy Transfer
• Conduction
– Molecule to molecule transfer
– Heat flow: warm to cold
– e.g. leather seats in a car
• Convection
– transferred by vertical movement
– physical mixing
– e.g. boiling water
• Radiation
– propagated without medium (i.e. vacuum)
– solar radiation provides nearly all energy
– The rest of this chapter deals with radiation
Radiation
• Everything
continually emits
radiation
• Transfers energy in
waves
• Waves are both
electrical and
magnetic, hence
electromagnetic
radiation
Radiation Quantity and Quality
• Quantity: how much? 
Wave height (Intensity).
• Quality: what kind? 
Wavelength: distance
between crest and crest (or
trough and trough).
generally reported in μm
(microns)- one millionth of
a meter.
The Electromagnetic Spectrum
The limitations of the
human eye!
A man detected by different
instruments
Infred
device
Bare
eyes
X-ray
Microscope
Wavelength of Sun and Earth Radiation
Sun =
“shortwave”
Sunμm)
(0.4-0.7
Peak 0.5 μm
(green)
Earth =
“longwave”
(4-100 μm)
Peak 10 μm
(infrared)
Composition of sunlight
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
Intensity: 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 StefanBoltzmann 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
Wavelength: Wien’s Law
• Details the wavelength of peak emission
of a body based on its temperature
max = 2900 / T
where  is the wavelength of maximum
emission (in m) and T is temperature (K)
• Thus, the Earth (290K) = 10 m, and
Sun (6000K) = 0.5 m
• Wein’s Law tells us that hotter objects radiate
at shorter wavelengths than cooler bodies
• All bodies emit radiation over a range of
wavelengths centered about the maximum
predicted from Wein’s Law
Satellite Measurements of the
Earth’s Radiation Budget
NASA’s Earth Radiation
Budget Satellite (ERBS)
1985-1989
NASA’s Clouds and the Earth's
Radiant Energy System (CERES) on 4
satellites (TRMM, Terra, Aqua, NPP)
2000-now
Video: Monitoring Earth's Energy
Budget with CERES
• http://www.youtube.com/watch?v=D_Qmue54W14
Earth’s energy budget (averaged over the
whole globe and over a long time)
Yellow:
shortwave
Red:
longwave
Sensible
heat 7%
Net Longwave 21%
Latent heat
23%
•
At the top of the atmosphere:
Incoming shortwave = Reflected Shortwave + Emitted longwave
•
At the surface:
Incoming shortwave + Incoming longwave = Reflected shortwave + Emitted longwave
+ Latent heat flux + Sensible heat flux + Subsurface Diffusion
Latitudinal variation of surface net radiation
• Tropics – energy surplus
• High latitudes – energy deficits
• Imbalance of heating between
tropics and high latitudes
drives global atmospheric
circulation (the 3-cell model)
Seasonal variation of surface radiation
Surface sensible and latent heat fluxes
• Both are turbulent (noisy) fluxes related to conduction and
convection.
• Both proportional to surface wind speed.
• Sensible heat flux is dry flux from warm to cold regions
• Latent heat flux (also called evaporation) is wet flux from wet to
dry regions. Latent heat of evaporation is stored in water vapor
and will be released when condensation (clouds/precipitation)
happens.
Surface Wind
From
NOAA
ESRL
Seasonal variation of surface energy
budget
Storage change = net radiation - latent heat flux sensible heat flux
Summary
–
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
–
–
Intensity of radiation (Stefan-Boltzman law): I=T4
Wavelength of radiation (Wein’s law): max = b/T
–
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
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