Download AA2 FALL 2005

Photosynthetically-active radiation
(spectral portion,0.3-0.4 CI)
0400-0500h
0500-0600h
0600-0700h
0700-0800h
0800-0900h
0900-1000h
1000-1100h
1100-1200h
Radiation receipt varies considerably in
any mountainous environment
1600
Aspect
TOA K on
A 45° slope
On Julian Day
352
45
1400
90
13 5
Solar
1200
radiation
(W/m2)
1000
180
2 25
2 70
3 15
800
0 deg re es
600
400
200
0
0
4
8
12
Hour
16
20
24
Longwave Radiative Exchange
The atmosphere absorbs long-wave radiation (L)
from the Earth, clouds and gases at all altitudes
Absorption greatest in lower portion of the atmosphere,
where H20 and CO2 concentrations are highest
The atmosphere absorbs effectively from 3-100 m,
except in the atmospheric window (8-11 m)
Most longwave loss to space occurs through this
window, but clouds can partially close it
L is greater in magnitude and more variable than L
L = 0 (T0)4 + (1 - 0) L
Amount of L reflected
(slight adjustment)
L* = L - L (usually negative)
NET ALL_WAVE RADIATION
DAYTIME:Q* = K - K + L - L
Q* = K* + L*
NIGHT:
Q* = L*
Radiation Measurements
L
K
PAR
UV-A
K (not visible)
L
More radiation sensors…
Source: University of Colorado
K in tropical forests of Colombia/Ecuador
a)
1000
LRF
TMCF dry
TMCF wet
Solar radiation
(W m-2)
800
600
400
200
0
08h00
10h00
12h00
Solar Time
14h00
16h00
Radiation Balance Components
Negative in Oke
Clouds
Reduce K because of absorption and reflection
from cloud tops (may eliminate S)
Increase D by scattering incoming solar radiation
Strongest K  under partly cloudy skies with sun
in clear patch
Absorb much of L and re-emit it as L
(low cloud emits more)
Reduce diurnal temperature variation
Global
Energy
Balance
Source: NOAA
Q* -
positive in daytime
almost always negative at night
Any Q* imbalance is accounted for by
convective exchange or conduction
Q* = QH + QE + QG + S
where
QH = sensible heat flux
QE = latent heat flux
QG = conduction to or from ground
(See Figure 1.10)
Conduction, Convection and Advection
Conduction is the process through which heat is diffused to cooler
materials as radiation is absorbed. Land surfaces heat quickly,
while water bodies can mix and have higher heat capacity. Solids
(land) are better conductors than gases (atmosphere).
Convection is physical mixing with a strong vertical motion in
gaseous or liquid media. As heat is absorbed by the ground, the air
immediately above is heated. Warm air is less dense and, thus,
rises, while cooler air falls.
Advection is the term used to describe lateral heat transfers. Winds
carry heat from absorbed radiation from one area to another.
Recall the First Law of Thermodynamics
ENERGY IN = ENERGY OUT
Qin > Qout (flux convergence)
Net storage gain leads to warming
Qout > Qin (flux divergence)
Net storage energy loss leads to cooling
Qin = Qout
No net change in energy storage
Water: H2O
•High heat capacity
•Exists in all states at Earth’s
temperatures
•Heat required/released during phase changes:
Latent heat of fusion (Lf = 0.334 MJ kg-1)
Latent heat of vaporization (Lv = 2.45 MJ kg-1)
Latent heat of sublimation (Ls = Lf + Lv)
Water Balance
p = E + r + s
Where
p is precipitation
E is evapotranspiration
r is net runoff
s is soil moisture* storage content
QE = Lv E
QM = Lf M
Where E and M are in kg m-2 s-1
See Fig. 1.13
Sensible and Latent Heat Fluxes
Eddy correlation (later)
•Sonic anemometer
measurements of vertical
velocity and temperature
•Krypton hygrometer
measurements of water
vapour density
Advection and Winds
Air flow at local scale can affect energy balance
as can air flow at scales larger than boundary layer
At the micro-scale, horizontal temperature variation
causes horizontal pressure differences
Why ? Warm air is lighter than cold air
This leads to winds (kinetic energy)
Energy transferred to smaller and smaller scales
before being dissipated as heat
(details next class)
DAYTIME:
Both sides of equation are positive:
surface radiative surplus
Surplus partitioned into ground and atmosphere
Convection is the most important means of
daytime heat transport from surface
QE is greater when soil moisture is high
QH is greater when water is more restricted
NIGHT:
Both sides of equation are negative:
surface radiative deficit
Deficit partitioned into heat gain from ground
and atmosphere
Q* loss is partially replenished by QG
QE and QH of less importance as convective
exchange is dampened by the night-time
temperature stratification