Available Energy: Net Radiation and Soil Heat Flux - Transport in the Atmosphere-Vegetation-Soil Continuum - page 62

Figure 2.28 Snow pack on top of soil. Left: Energy balance of the snow pack at night
with L * being the only energy lux at the top and ∆ Q s being the change in heat stor-
age. Right: Temperature proile in soil and snow.

Snow Cover
If the soil is covered by snow, this layer of snow could be considered ‒ in terms of
heat transport by conduction ‒ as an extra soil layer. However, a number of complica-
tions arise owing to the special properties of the snow layer.

• Snow is partly transparent to solar radiation, so that the absorption of solar energy takes

place in the entire volume. Hence it is not unambiguous where the 'surface' of the sur-
face energy balance is located.
Phase changes may occur inside the snow layer owing to melting of the snow (which •

consumes energy) and refreezing of the snow (where energy is released).
The snow layer itself is not a ixed porous medium like the soil matrix. The mass balance •

of a snow pack is a complicated balance between input by snow fall, possibly input by
rain (which may or may not freeze in the snow pack), evaporation either from melted
snow or directly from the frozen snow (sublimation) and drainage of melt water into the
soil. Apart from input and output at the boundaries, internal movement of water (either
from melting snow or rain water) transports both water and energy internally.
The thermal properties of snow are very different from those of the underlying soil (see •

Table 2.2 ).

The latter point has a direct inluence on the energy balance and the surface tempera-
ture, especially at night. This is illustrated in Figure 2.28 . At night the only radiative
forcing is the net longwave radiation, L* . The evaporation is usually small. Further-
more, in the case of a night with no or light winds, the sensible heat lux will effectively
be suppressed by a strong surface inversion (see Chapter 3 ). Hence, the effect of the
energy extraction by L* is that heat is extracted from the snow layer so that it cools. Fur-
thermore, the snow layer in turn extracts heat from the ground, leading to an upward soil
heat lux. Owing to the very low thermal conductivity of snow (see Table 2.2 ) a large
temperature gradient inside the snow is needed to extract the needed amount of energy.
If we assume that the cooling of the snow layer is equal at all depths the proile of the
heat lux inside the snow (denoted by G snow ) must be linear and can be described as:

d − z z

Gz () =

snow d

L * +

d

G

(2.45) snow d

d d

snow snow