Geoscience Reference
In-Depth Information
and temperature (the eddy correlation). With temperatures decreasing with height,
positive (upward) vertical wind anomalies transport positive temperature anoma-
lies upward, while negative (downward) vertical wind anomalies transport negative
temperature anomalies downward. The products are positive in both cases, yield-
ing an upward sensible heat flux. If the temperature increases with height, upward
(downward) wind anomalies transport negative (positive) temperature anomalies
and the sensible heat flux is downward. The latent heat flux is determined from
analogous relationships. A complete description of the profile and eddy correlation
methods is provided by T. Oke (
1987
).
The melt term M represents a heat gain by the column underlying the atmo-
sphere as it requires energy to transform water from its solid form to its liquid form
that represents a higher latent heat state. Unless the surface is melting (i.e., a phase
change is occurring), M is zero. It can never be negative. The melt term is con-
sidered separately from Q
E
, as the latter deals with the energy associated with the
change from the liquid (or solid) to the vapor phase (or vice versa if condensation
is occurring).
The conductive heat flux C into or out of the column underlying the atmosphere
can be expanded as −K∂T /∂Z, where K is the thermal conductivity and ∂T/∂Z is the
vertical temperature gradient, with Z increasing upward. If temperature decreases
upward, the flux is directed toward the surface and hence has a positive sign (the
term is a heat loss from the underlying column and a heat gain by the overlying
atmospheric column). The opposite is true when temperature increases upward. If
the temperature gradient is zero, there can be no conductive flux. Over a snow-free
tundra, for example, −K(∂T/∂Z) refers to the conductive flux between the surface
and the soil column. Expressions for the heat conduction through water, sea ice,
snow, and glacier ice must use thermal conductivity coefficients appropriate to each
material. Thermal conductivities of some natural materials are given in
Table 5.1
.
Fresh snow is a rather poor conductor (hence a good insulator). By comparison, ice
is a much better conductor. Note that the conductivities of ice and water depend on
temperature and salinity.
Building again on
Chapter 3
, over the Arctic Ocean, the conduction term repre-
sents two processes. First, there can be a conduction of heat in open water areas.
Second, there can be a conduction of heat through the sea ice and any overlying
snow cover. Upward heat conduction through sea ice and any overlying snow cover
implicitly includes the effects of sea ice growth. If sea ice is growing (bottom accre-
tion of ice), there is a release of latent heat to the bottom of the ice. This appears as
a heat gain by the ice, which helps to maintain an upward conductive flux (see later
discussion and
Chapter 7
).
Regarding the bulk heating term B, recall that solar radiation can penetrate to a
considerable depth through liquid water, sea ice, and snow. Although some (or in
the case of a snowpack, most) of this penetrating radiation will be scattered back out
of out of the subsurface column, that which is absorbed represents a heat gain by the
column underlying the atmosphere. This is an important part of seasonal heat gain
in the upper part of the Arctic Ocean (Frey et al.,
2011
).
