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”