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Bài giảng khí hậu học chương 3

G304 – Physical Meteorology and Climatology

Chapter 3
Energy balance and
Temperature

By Vu Thanh Hang, Department of Meteorology, HUS


3.1 Atmospheric influences on Insolation
• The atmosphere absorbs some radiation directly and
thereby gains heat
• Another portion of radiation disperses as weaker rays
going out in many different directions through a process
called scattering
• Some of the scattered radiation is directed back to
space
• The energy that is scattered is not absorbed by the
atmosphere Æ not contribute to its heating
• The remaining insolation passes through the
atmosphere without modification, reaching the surface

as direct radiation


3.1 Atmospheric influences on Insolation (cont.)
• Absorption:
- Atmospheric gases, particulates, and droplets all reduce
the intensity of solar radiation (insolation) by absorption, a
process in which radiation is captured by a molecule.
- Absorption represents an energy transfer to the
absorber.
- This transfer has two effects: the absorber gains energy
and warms, while the amount of energy delivered to the
surface is reduced.
- The gases of the atmosphere are not equally effective at
absorbing sunlight & different wavelengths of radiation are
not equally subject to absorption.


3.1 Atmospheric influences on Insolation (cont.)
• Absorption (cont.):
- If the atmosphere were able to absorb all the incoming
solar energy, the sky would appear completely dark
- Ultraviolet radiation is almost totally absorbed by ozone in
the stratosphere
- Visible radiation passes through the atmosphere with only a
minimal amount of absorption
- Near infrared radiation is absorbed mainly by two gases:
water vapor & carbon dioxide


3.1 Atmospheric influences on Insolation (cont.)
• Reflection & scattering:
- The reflection of energy is a process whereby radiation
making contact with some material is simply redirected
away from the surface without being absorbed
- All substances reflect visible light, but with vastly differing
effectiveness
- Objects do not reflect all wavelengths equally
- The percentage of visible light reflected by an object or
substance is called its albedo.

- When light strikes a mirror, it is reflected back as a beam
of equal intensity, in a manner known as specular
reflection


3.1 Atmospheric influences on Insolation (cont.)
• Reflection & scattering (cont.):
- When a beam is reflected from an object as a larger
number of weaker rays traveling in different directions, it
is called diffuse reflection, or scattering
- Large solid surfaces, gas molecules, particulates, and
small droplets scatter radiation.
- The scattered energy reaching Earth’s surface is thus
diffuse radiation, which is in contrast to unscattered
direct radiation


3.1 Atmospheric influences on Insolation (cont.)
• Reflection & scattering (cont.):
- In a scattering process, the radiation is redirected but
not absorbed
- The characteristics of radiation scattering by the
atmosphere depend on the size of the scattering agents
relative to the wavelength of the incident
electromagnetic energy
- Three general categories of scattering exist: Rayleigh
scattering, Mie scattering and nonselective scattering


3.1 Atmospheric influences on Insolation (cont.)
• Reflection & scattering (cont.):
+ Rayleigh scattering:
- Scattering agents smaller than about one-tenth the
wavelength of incoming radiation disperse radiation known
as Rayleigh scattering, which is performed by individual
gas molecules in the atmosphere
- It primarily affects shorter wavelengths
- Rayleigh scattering disperses radiation both forward and
backward
Æ blue sky on a clear day, the blue tint of the atmosphere
when viewed from space, the redness of sunsets and
sunrises


Fig. 3-3

The sky appears blue because gases and particles in the atmosphere
scatter some of the incoming solar radiation in all directions. Air molecules
scatter shorter wavelengths most effectively. Thus, we perceive blue light,
the shortest wavelength of the visible portion of the spectrum.


Fig. 3-5

Sunrises and sunsets appear red because sunlight travels a longer path
through the atmosphere. This causes a high amount of scattering to remove
shorter wavelengths from the incoming beam radiation. The result is sunlight
consisting almost entirely of longer (e.g., red) wavelengths.


3.1 Atmospheric influences on Insolation (cont.)
• Reflection & scattering (cont.):
+ Mie scattering:
- Microscopic aerosol particles are considerably larger than
air molecules and scatter sunlight by a process known as
Mie scattering
- It is predominantly forward, diverting relatively little energy
backward to space
- On hazy or polluted days the sky appears gray, as the
whole range of the visible part of the spectrum is effectively
scattered toward the surface
- It causes sunrises and sunsets to be redder than they
would due to Rayleigh scattering alone, so episodes of
heavy air pollution often result in spectacular sunsets


3.1 Atmospheric influences on Insolation (cont.)
• Reflection & scattering (cont.):
+ Nonselective scattering:
- The water droplets in clouds are considerably larger than
suspended particulates reflecting all wavelengths of
incoming radiation about equally Æ clouds appear white or
gray
- Scattering by clouds is sometimes called nonselective
scattering
- An isolated water droplet affects various wavelengths of
solar radiation differently Æ a rainbow involves each
wavelengths being refracted a different amount Æ the
bands of individual colors


3.1 Atmospheric influences on Insolation (cont.)
• Transmission:
- When solar radiation travels the vacuum of outer space,
there is no modification of its intensity, direction, or
wavelength
- When it enters the atmosphere, only some of the
radiation can pass unobstructed to the surface
- There is a reduction in the amount of radiation reaching
the surface due to scattering process in cloudy days


3.2 The fate of solar radiation
• Seasonal variations in the availability of insolation, with
almost 7% more solar energy available on perihelion than
on aphelion
• Planetary albedo is 30% (25 from the atmosphere and 5
from the surface of solar radiation are scattered back to
space)
• The surface, the atmosphere, and the planetary system
must give up as much energy as they obtain
• To achieve this energy balance, huge quantities of energy
must be transferred from the Earth system and within the
system between surface and atmosphere


Fig. 3-7

Incoming solar radiation available is subject to a number of processes
as it passes through the atmosphere. The clouds and gases of the
atmosphere reflect 19 and 6 units, respectively, of insolation back to
space. The atmosphere absorbs another 25 units. Only half of the
insolation available at the top of the atmosphere actually reaches
the surface, of which another 5 units are reflected back to space.
The net solar radiation absorbed by the surface is 45 units.


3.3 Energy transfer processes between the
surface and the atmosphere
• Surface-atmosphere radiation exchange:
- Earth’s surface and atmosphere radiate energy almost in
the longwave portion of the spectrum
- Longwave radiation emitted by Earth’s surface is largely
absorbed by the atmosphere Æ increases the atmosphere
temperature
- The energy radiated by the atmosphere is transferred in all
directions, including downward Æ surface receives a portion
of this radiation Æ causes surface heating Æ increases in
longwave radiation emission from the surface… Æ an infinite
cycle of exchange with energy transferring back and forth


3.3 Energy transfer processes between the
surface and the atmosphere (cont.)
• Surface-atmosphere radiation exchange (cont.):
- Water vapor, CO2, other greenhouse gases are good at
absorbing most wavelengths of longwave radiation, a band
from 8 to 12μm can pass through the atmosphere
unimpeded Æ atmospheric window
- The difference between absorbed and emitted longwave
radiation is referred to as the net longwave radiation
- When either is absorbed, the absorber is warmed Æ
combine longwave and shortwave into net allwave radiation
(net radiation), defined as the difference between absorbed
and emitted radiation, or equivalently, the net energy gained
or lost by radiation.


Figure created by Leland McInnes from
published EPICA data


Fig. 3-10

Net radiation is the end result of the absorption of insolation and the
absorption and radiation of longwave radiation. The surface has a net
radiation surplus of 29 units, while the atmosphere has a deficit of 29 units.


3.3 Energy transfer processes between the
surface and the atmosphere (cont.)
• Sensible heat:
- When energy is added to a substance, an increase in
temperature occurs (sensible heat)
- The magnitude of temperature increase is related to two
factors, the first of which is specific heat (J/(kg.K))
- The temperature increase resulting from a surplus of
energy receipt also depends on the mass of a substance
- Sensible heat travels by conduction through the laminar
boundary layer and is then dispersed upward by convection


3.3 Energy transfer processes between the
surface and the atmosphere (cont.)
• Latent heat:
- is the energy required to change the phase of a
substance
- In the case of melting ice, the energy is called the latent
heat of fusion.
- The change of phase from liquid to gas, the energy is
called the latent heat of evaporation
- When radiation is received at the surface Æ raise
temperature of land or water
- If water exists at the surface Æ some of energy have
been used to increase the surface temp. is instead used to
evaporate some of the water


Fig. 3-14

Both the surface and atmosphere lose exactly as much energy as they
gain. The surface has a surplus of 29 units of net radiation, which is
offset by the transfer of sensible and latent heat to the atmosphere.
The atmosphere offsets its 29 units of radiation deficit by the
receipt of sensible and latent heat from the surface.


3.4 The Greenhouse effect
• The interactions that warm the atmosphere are often
referred to as the greenhouse effect
• The greenhouse gases of the atmosphere do not impede
the transfer of latent and sensible heat
• If the atmosphere had none of the “greenhouse gases”
that absorb outgoing longwave radiation Æ Earth would be
colder, temperature would oscillate wildly from day to night
• The greenhouse effect keeps Earth warmer by absorbing
most of the longwave radiation emitted by the surface Æ
warming the lower atmosphere Æ emits radiation
downward



3.5 Global temperature distributions


One of the most immediate and obvious outcomes of radiation gain or
loss is a change in the air temperature

• The map depicts differences between mean temperatures in January and
July through the use of isotherms


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