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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
The diurnal (daily) cycle in net radiation at the base of the atmos.The diurnal (daily) cycle in net radiation at the base of the atmos.
Q* = K* + L* = KQ* = K* + L* = K - K - K + L + L - L - L
-L*
L* is typically negative unless there is low cloud cover
Surface energy budgetSurface energy budget Q* = QQ* = QHH + Q + QEE + Q + QGG
Q*Q*
QQHH
QQGG
QQEE(shows sign convention only… each flux can have either sign)
(= ground/lake/ocean heat flux)
an arbitrary example of a duirnal cycle
Understanding the diurnal (daily) cycle in temperatureUnderstanding the diurnal (daily) cycle in temperature (similar principles apply to understanding the seasonal cycle)
Fig. 3-22a
Night-time near-ground temperature profile… “stable stratification”
z
T=T(z)
Daytime near-ground temperature profile…“unstable stratification”
z
T=T(z)
Diurnal cycle in near-ground stratificationDiurnal cycle in near-ground stratification
Upward heat flow,vertical mixing enhanced (p65)
Inversion Inversion … downward heat flow, mixing damped
The atmospheric boundary layer and the depth (The atmospheric boundary layer and the depth () of mixing) of mixing
zz
““free atmosphere”free atmosphere”
• no frictionno friction• vertical velocities steady and of ordervertical velocities steady and of order cm scm s-1-1 except except
in clouds/over mountainsin clouds/over mountains
““friction layer” or “boundary layer”friction layer” or “boundary layer”
• friction reduces windspeedfriction reduces windspeed• variation of wind with height, instability (warm air variation of wind with height, instability (warm air underneath cold), and flow around obstaclesunderneath cold), and flow around obstacles
produce turbulenceproduce turbulence• vertical velocities fluctuate and are of order vertical velocities fluctuate and are of order m sm s-1-1
Depth (Depth () of mixing varies in time/space) 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
dawn dusk
Order 100 m
Order 1 km
winter
summer
Nocturnal Radiation InversionNocturnal Radiation Inversion
Cause … ground cooling: Q* < 0, ie. outgoing longwave radiation exceeds incoming longwave
then air above cools by convection (stirring), QH < 0
Conditions for severest inversion …
clear sky, dry air
long night with light wind
Result: radiation frost?
Photo :Keith Cooley
Complexity of local (site-Complexity of local (site-specific) effects on local specific) effects on local radiation and energy radiation and energy balance… producing balance… producing “micro-climates” that “micro-climates” that can be manipulated (eg. can be manipulated (eg. windbreaks)windbreaks)
Figs. 3-21
Latitudinal variation in net Latitudinal variation in net allwave radiationallwave radiation
Averaged over a long period, latitudinal heat advection by ocean (25%) and atmosphere (75%) rectifies the imbalance
a Sa S00 +L+L
( 1-a ) S
( 1-a ) S00 , a the albedo
, a the albedo
Fig. 3-15
Why do we consider earth’s global climatological temperature Why do we consider earth’s global climatological temperature TTeqeq
to be at equilibrium (Sec. 3-2)? to be at equilibrium (Sec. 3-2)?
Because there is a stabilizing feedback...Because there is a stabilizing feedback...
T
tR a S R T
eq
eq 20
2 41 4( )
Let eq be the change in eq over time interval t. Then:
Where R is earth’s 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 eq ).
Rate of change gains - losses
area of earth’s shadow area of earth’s surface
At earth’s equilibrium temperature, there is balance...At earth’s equilibrium temperature, there is balance...
Set a =0.3 and =1 to obtain earth’s (radiative) equilibrium temperature (Sec. 3-2).
R a S R Teq2
02 41 4 0( )
Common factor cancels
Factors controlling temperature on regional & global time & Factors controlling temperature on regional & global time & space scalesspace 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
Fig. 3-18a
• latitudinal temperature gradient is greatest in the winter hemisphere• in summer (winter) temperature over land warmer (cooler) than over ocean
Why are water bodies “more conservative” in their temperature?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”
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