Geoscience Reference
In-Depth Information
Figure 12.5
Average diurnal variation of the energy balance
components in and above the tropical Atlantic Ocean during the
period 20 June to 2 July 1969.
Source
: After Holland. From Oke (1987). By permission of Routledge
and Methuen & Co, London, and T.R. Oke.
assumption that the horizontal advective term due to
heat transfer by currents is zero and that the total energy
input is absorbed in the upper 27 m of the ocean. Thus,
between 06:00 and 16:00 hours, almost all of the net
radiation is absorbed by the water layer (i.e. ∆
W
is
positive) and at all other times the ocean water is heating
the air through the transfer of sensible and latent heat of
evaporation. The afternoon maximum is determined
by the time of maximum temperature of the surface
water.
Figure 12.4
Diurnal temperatures near, at and below the surface
in the Tibesti region, central Sahara, in mid-August 1961. (A) At
the surface and at 1 cm, 3 cm and 7 cm below the surface of a
basalt. (B) In the surface air layer, at the surface, and at 30 cm and
75 cm below the surface of a sand dune.
Source
: After Peel (1974).
Surface properties modify the heat penetration, as
shown by mid-August measurements in the Sahara
(Figure 12.4). Maximum surface temperatures reached
on dark-coloured basalt and light-coloured sandstone
are almost identical, but the greater thermal conductivity
of basalt (3.1 W m
-1
K
-1
) versus sandstone (2.4 W m
-1
K
-1
) gives a larger diurnal range and deeper penetration
of the diurnal temperature wave, to about 1 m in the
basalt. In sand, the temperature wave is negligible at
30 cm due to the low conductivity of intergranular air.
Note that the surface range of temperature is several
times that in the air. Sand also has an albedo of 0.35,
compared with about 0.2 for a rock surface.
3 Snow and ice
Surfaces that have snow or ice cover for much of the
year present more complex energy budgets. The surface
types include ice-covered ocean; glaciers, tundra; boreal
forests, steppe, all of which are snow-covered during
the long winter. Rather similar energy balances charac-
terize the winter months (Figure 12.6). An exception
is the local areas of ocean covered by thin sea ice and
open leads in the ice that have 300 W m
-2
available -
more than the net radiation for boreal forests in summer.
The spring transition on land is very rapid (see Figure
10.38). During the summer, when albedo becomes a
critical surface parameter, there are important spatial
contrasts. In summer, the radiation budget of sea ice
more than 3 m thick is quite low and for ablating glaciers
is lower still. Melting snow involves the additional
energy balance component (∆
M
), which is the net latent
heat storage change (positive) due to melting (Figure
2 Water
For a water body, the energy fluxes are apportioned very
differently. Figure 12.5 illustrates the diurnal regime for
the tropical Atlantic Ocean averaged for 20 June to 2
July 1969. The simple energy balance is based on the
