The
‘clothesline’
effect
Evaporation increase
over vegetation
Soil moisture depletion
near ‘leading edge’
New equilibrium
approached with
lower temperature
and higher humidity
Do not set up tower here
QH may be directed toward surface
vegetation exerts a drag
Oasis effect
QE, QH, temp
Cold-water
advection fog
Warm-water
advection fog
‘Oasis effect’
QE>Q* for
irrigated crop
near Phoenix,
AZ
Notice the effect
of a drought (in
the absence of
irrigation) on QH
and QE
Properties of a water body
(1)Allows transmission of K
QS is a good thermal sink
(2)Transfers heat by convection and mixing
(3)Converts energy surplus to latent heat,
rather than sensible heat
QE is a good thermal sink
(4)Has a large thermal inertia (heat capacity)
Daytime
Large diurnal
temperature variation
Little diurnal
temperature variation
Night
Source: Ahrens, 2001
Variable receipt of
solar radiation
Differences in magnitude
Differences in timing
Produces local slope
winds
Plant and animal habitat
are affected (eg. cacti
and short grasses
on S-facing slopes,
shrubs and tall grasses on
N and E-facing slopes
in the Lethbridge coulees)
Also geomorphological
and hydrological
implications
1600
Aspect
45
1400
90
135
Solar
1200
radiation
(W/m2)
1000
180
225
270
315
800
0 deg re es
600
400
200
0
0
4
8
12
16
20
24
Hour
Top-of-atmosphere solar radiation (KTOA) on plane surfaces of 45 slope at 2.3 N latitude,
77.0W
on Julian Day 352.
Figurelongitude,
5.4
Top-of-atmosphere
solar radiation on plane surfaces of 45 degree slope at 2.3°N,
Simulated
hourly, net canopy
photosynthesis
from 0900h to
1000h, in August
Tambito Reserve,
El Tambo,
Cauca, Colombia
Slope and aspect
effects are
superimposed on the
altitudinal gradient
of productivity
Mountain Valley Breezes
Daytime
The sun heats the
hillslope, causing
air to move up the
slope
Night
Night radiation cools
the slopes
Cooler, denser air
moves downslope
Source: http://apollo.lsc.vsc.edu
Similar to sea breeze
circulation pattern
~2-4 m/s
Valley warmer than air at same level over
surrounding plain or further downstream
~1 m/s
Gravity
drainage
wind
(katabatic)
Long-wave radiation
Emission (cooling)
Destruction of Valley Inversion
Radiation fog and
frost pockets likely
Coldest air settles
Where would you grow fruit at a site with marginal growing season ?
The Urban Boundary Layer
LEADING EDGE
INTERNAL
BOUNDARY
LAYER
URBAN ‘CANOPY’: An amalgam
of variable urban microclimates
Urban
Energy
Balance
In reality, the picture is more
complex than shown:
Q*+ QF = QH+QE +QS+ QS
p + F + I = E + r + S + A
QF = combustion heat source
F = water released from combustion
I = water supply from reservoirs etc.
S = water storage change
A = net moisture advection
Table 8.1
QF may exceed net radiation in winter
QH and
QS are
larger in
cities
Urban Heat Island
Sacramento, California, USA
Credit: NASA/Marshall Space Flight Center and
Global Hydrology and Climate Center
thermal and mechanical
convection enhancement
Temperature
at Montreal, PQ
7AM
March 7, 1968
Urban
Precipitation
Enhancement
St. Louis,
Missouri, USA
rural:urban ratio
of summer rainfall
Stability
affects
pollution
patterns
Pollution
dispersion most
effective under
unstable
conditions
Inversions due
to advection