Geoscience Reference
In-Depth Information
of energy by the atmospheric column. If F
sfc
is negative, it contributes to a loss of
energy in the atmospheric column and a gain of energy in the underlying column.
Recall from Equation
3.5
that the net surface flux represents the sum of the net
radiation at the surface and the turbulent sensible and latent heat fluxes. For an
ocean column, the contribution of F
sfc
to changes in heat storage is manifested in
terms of ice melt, heat conduction, and absorbtion of solar radiation within the
column (we can consider this as a bulk heating term). Regarding melt of snow or
ice, a unit mass of liquid water at 0
o
C possesses a higher latent heat content than a
unit mass of ice at the same temperature. Attaining the higher latent heat content
requires an absorption of energy (e.g., surface net radiation directed downward),
which, in the framework adopted here, counts as a heat loss from the atmospheric
column. If temperature decreases from the surface downward into the underlying
column, there will be a conduction of heat into the underlying column which again
counts as a heat loss from the atmospheric column. If the temperature gradient is the
reverse, the underlying column loses energy, which the atmospheric column gains.
Sea ice growth is manifested in the conduction term. Ice growth, which occurs at the
bottom ice, is associated with a latent heat release, which maintains a temperature
gradient from the ice-ocean interface to the (colder) surface, and hence an upward
conductive flux through the ice.
Solar radiation can penetrate to a considerable depth through liquid water and
snow. Although some (or in the case of a snowpack, most) of this penetrating radia-
tion will be scattered back out of the underlying column, that which is absorbed rep-
resents a heat gain. Absorbtion of solar radiation in low-albedo open water areas is
a key part of seasonal heat gain in the upper part of the Arctic Ocean. Transmission
of solar radiation through sea ice is highly variable, depending on the wavelength of
the radiation, ice thickness, and the distribution of melt ponds and bare ice surface
(Frey, Perovich, and Light,
2011
).
More than 50 percent of the total solar radiation can be absorbed in the top 10 cm
of ice. Essentially, all the ultraviolet and infrared radiation is absorbed in the upper
50 cm of ice.
If the underling column represents land (terrestrial, T; variously comprising soil,
soil with vegetation on the top [which may in turn be covered by snow], an ice sheet
or a glacier), the situation is simpler:
∂T
E
/∂t = ∂/∂t (L
T
+ S
T
) = - F
sfc
(3.7)
where L
T
and S
T
are time changes in the terrestrial storage of latent heat and sen-
sible heat. This would include latent heat storage in the form of ice (snow, glacier
ice) and liquid water, sensible heat storage in the form of ice and liquid water, and
sensible heat storage in the soil column. Because lateral heat transport divergences
for a terrestrial column are small and can be ignored, the tendency in terrestrial heat
storage is, to a good approximation, equal to the negative of the terrestrial net sur-
face heat flux F
sfc
. It is recognized, however, that lateral heat transports by rivers can
be locally significant (Su et al.,
2006
).
