Geoscience Reference
In-Depth Information
α = fractional albedo of the surface, and
Ln
= the net (outgoing) longwave radiation.
heat from the air (
H
) (
Figure 12.2A
). Occasion-
ally, condensation may contribute heat to the
surface.
Commonly, there is a small residual heat
storage (
Rn
is usually positive by day, since the absorbed
solar radiation exceeds the net outgoing longwave
radiation; at night, when
S
= 0,
Rn
is determined
by the negative magnitude of
Ln
. since the
outgoing longwave radiation from the surface
invariably exceeds the downward component
from the atmosphere.
The surface energy flux terms are defined as
positive away from the surface interface:
S
) in the soil in spring/summer and a
return of heat to the surface in autumn/winter.
Where a vegetation canopy is present there may
be a small additional biochemical heat storage,
due to photosynthesis, as well as physical heat
storage by leaves and stems. An additional energy
component to be considered in areas of mixed
canopy cover (forest/grassland, desert/oasis),
and in water bodies, is the horizontal transfer
(
advection
) of heat by wind and currents (Δ
A
; see
Figure 12.2B
). The atmosphere transports sensible
and latent heat both vertically and horizontally.
Δ
G
= heat flux into the ground,
H
= turbulent sensible heat flux to the
atmosphere,
LE
= turbulent latent heat flux to the atmosphere
(
E
= evaporation;
L
= latent heat of
vaporization).
B NON-VEGETATED NATURAL
SURFACES
By day, the available net radiation is balanced
by the outgoing turbulent fluxes of sensible heat
(
H
) and latent heat (
LE
) into the atmosphere and
by conductive heat flux into the ground (
G
). At
night, the negative
Rn
caused by net outgoing
longwave radiation is offset by the supply of
conductive heat from the soil (
G
) and turbulent
1 Rock and sand
The energy exchanges of dry desert surfaces are
relatively simple. A representative diurnal pattern
of energy exchange over desert surfaces is shown
in
Figure 12.3
. The 2m air temperature varies
between 17 and 29
C, although the surface of the
dry lake-bed reaches 57°C at midday.
Rn
reaches
a maximum at about 13:00 hours when most of
the heat is transferred to the air by turbulent
convection; in the early morning the heat goes
into the ground. At night this soil heat is returned
to the surface, offsetting radiative cooling. Over a
24-hour period, about 90 percent of the net
radiation goes into sensible heat, 10 percent into
ground flux. Extreme surface temperatures
exceeding 88
°
(A)
Day
Night
R
n
=GH
+LE
R
n
=GH
+LE
(B)
R
n
LE
H
Lower
atmosphere
F) have been measured in
Death Valley, California, and it seems that an
upper limit is about 93°C (200°F). Record maxi-
mum air temperatures are 56.7
°
C (190
°
Canopy +
stem layer
Δ
S
Δ
S
Soil
C at Greenland
Ranch, Death Valley, California, and 57.8°C
(136
°
Δ
A
F) at El Azizia, Libya.
Surface properties modify the heat penetration,
as shown by mid-August measurements in
the Sahara (
Figure 12.4
). Maximum surface
°
Figure 12.2
Energy flows involved in the energy
balance of a simple surface during day and night (A)
and a vegetated surface (B).
Source: After Oke (1978). Courtesy of Routledge and Methuen.
