Lecture 7-8: Energy balance and temperature (Ch 3)

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Lecture 7-8 Energy balance and temperature (Ch
3)

• the diurnal cycle in net radiation, temperature
  and stratification
• the friction layer
• local microclimates
• influences on regional temperature patterns

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The diurnal (daily) cycle in net radiation at the
base of the atmos.
Q K L K? - K? L? - L?
L is typically negative unless there is low
cloud cover
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Q QH QE QG
Surface energy budget
an arbitrary example of a duirnal cycle
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Understanding the diurnal (daily) cycle in
temperature (similar principles apply to
understanding the seasonal cycle)
Fig. 3-22a
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Diurnal cycle in near-ground stratification
Night-time near-ground temperature profile
stable stratification
Daytime near-ground temperature
profile unstable stratification
Inversion downward heat flow, mixing damped
Upward heat flow, vertical mixing enhanced (p65)
z
z
TT(z)
TT(z)
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The atmospheric boundary layer and the depth (?)
of mixing

• free atmosphere
• no friction
• vertical velocities steady and of order cm s-1
  except
• in clouds/over mountains

?

• friction layer or boundary layer
• friction reduces windspeed
• variation of wind with height, instability (warm
  air underneath cold), and flow around obstacles
• produce turbulence
• vertical velocities fluctuate and are of order m
  s-1

z
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Depth (?) of mixing varies in time/space

• Depth of the ABL (i.e. magnitude of ?) depends on
  the turbulence, and increases with
• stronger surface heating QH
• stronger wind
• rougher surface

summer
Order 1 km
?
winter
Order 100 m
dawn
dusk
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Nocturnal Radiation Inversion

• ground cooling Q lt 0, ie. outgoing longwave
  radiation exceeds incoming longwave
• then air above cools by convection (stirring),
  QH lt 0

Cause
Conditions for severest inversion

• clear sky, dry air
• long night with light wind

Result radiation frost?
Photo Keith Cooley
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Figs. 3-21
Complexity of local (site-specific) effects on
local radiation and energy balance producing
micro-climates that can be manipulated (eg.
windbreaks)
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Latitudinal variation in net allwave radiation
a S0L?
Averaged over a long period, latitudinal heat
advection by ocean (25) and atmosphere (75)
rectifies the imbalance
Fig. 3-15
( 1-a ) S0 , a the albedo
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Why do we consider earths global climatological
temperature Teq to be at equilibrium (Sec. 3-2)?
Because there is a stabilizing feedback...
Let DTeq be the change in Teq over time interval
Dt. Then
area of earths surface
area of earths shadow
Rate of change ? gains
- losses
Where R is earths radius, S0 is the solar
constant, a (0.3) is the planetary albeto, ?
(?1) is the planetary emissivity and ? is the
Stefan-Boltzmann constant. The proportionality
constant involves the heat capacity of the
earth-atmosphere system. (In reality a,? may
depend on Teq ).
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At earths equilibrium temperature, there is
balance...
Common factor cancels
Set a 0.3 and ? 1 to obtain earths
(radiative) equilibrium temperature (Sec. 3-2).
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Factors controlling temperature on regional
global time space scales

• Latitude
• solar radiation
• distribution of land water
• surface thermal inertia, surface energy balance
• topographic steering/blockage of winds
• Ocean Currents
• advective domination (horizontal heat transport)
• Elevation

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• latitudinal temperature gradient is greatest in
  the winter hemisphere
• in summer (winter) temperature over land warmer
  (cooler) than over ocean

Fig. 3-18a
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Why are water bodies more conservative in their
temperature?

• solar radiation penetrates to some depth so
  warms a volume
• much of the available radiant energy used to
  evaporate water
• mixing of the water in the ocean/lake mixed
  layer ensures heat deposited/drawn from a deep
  layer
• water has a much higher specific heat (4128 J
  kg-1 K-1) than land