Geoscience Reference
In-Depth Information
13 μm, the so-called 'atmospheric window'. If clouds are present, their main effect
is to add radiation within the atmospheric window, thus increasing the downwelling
longwave radiation considerably. The longwave radiation emitted by the surface is
determined by the surface temperature, and to some extent by the emissivity, although
the latter tends to be close to one for most natural surfaces. The temperature of the
surface is the result of a complex balance of processes that heat and cool the surface
(i.e., all terms of the energy balance together).
The soil below the surface operates as a buffer for heat: during daytime (and during
summer) heat is transported into the soil, whereas during night time (and during winter)
heat is released from the soil. Heat transport in the soil occurs predominantly through
conduction. Hence, the complexity of soil heat transport is related not to the mathemat-
ical treatment, but to the speciication of the thermal properties. As a soil is a complex
mixture of the solid soil matrix, water and air, the thermal properties can vary widely
with the composition of the matrix, the porosity and especially the water content.
The soil heat lux is mainly driven by the supply of energy at the surface. Hence
the diurnal and yearly cycle also dominate the soil heat lux. The approximation of the
temperature at the soil surface by a sinusoidal variation in time provides a powerful
framework to study the variation of both temperature and heat lux with time and depth.
An important quantity appearing in this framework is the damping depth (or e-folding
depth). As this damping depth depends on the frequency of the variation, as well as on
the thermal diffusivity of the soil, examination of the extinction of the diurnal or yearly
variations of temperature can provide information on soil thermal properties.
The presence of vegetation modiies the dynamics of soil temperatures and soil
heat lux as it moves the active surface (where interaction with the atmosphere takes
place) away from the soil surface. As canopies are largely made up of air, the general
effect of vegetation is to provide an insulating layer on top of the soil.
Snow cover on soils has a similar effect as the thermal conductivity of snow is
much lower than that of most soils. Hence, if net cooling occurs at the surface of
the snow layer, a large temperature gradient inside the snow is needed to provide the
energy lost at the surface. This leads to very low surface temperature. Another effect
of solid water (snow or ice) on the surface energy balance is related to phase changes.
Melting and sublimation of snow can consume signiicant amounts of energy. On
the other hand, when the soil surface temperature falls below 0 °C, soil moisture will
start to freeze, causing a release of latent energy from the soil. As the heat released on
freezing needs to be transported towards the soil surface, the penetration of the freez-
ing front slows down as it moves down away from the surface.
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