Geoscience Reference
In-Depth Information
2
Available Energy: Net Radiation and Soil Heat Flux
2.1 Introduction
In
Chapter 1
three different modes of energy transport were identiied: radiation,
conduction and convection. In the context of the surface energy balance, luxes of
sensible and latent heat are often taken together, as both are transported by turbulent
motion. Then net radiation and soil heat lux are summed under the name of
available
energy
, in the sense that
Q*- G
is the amount of energy available for sensible and
latent heat lux. This can be expressed by a rewritten version of the energy balance
equation:
QGHLE
*−=+
v
(2.1)
Once the available energy has been determined, the main question is how this energy
is partitioned between
H
and
L
v
E
.
This chapter irst deals with the net radiation lux. Second, the soil heat lux is con-
sidered. This includes a brief discussion on surfaces covered by snow, ice and water,
as energy transport there takes place by conduction as well. Basic radiation laws are
given in
Appendix A
for reference.
2.2 Net Radiation
Radiative luxes as they are relevant in the study of the surface energy balance can be
split on the basis of their origin (and hence wavelength):
•
Shortwave radiation is radiation originating from the Sun (either direct, or after inter-
action with the atmosphere). It covers the wavelength range of about 0.15-3 μm, cor-
responding to a surface temperature of the Sun of about 5800 K (the link between the
black-body temperature and the peak wavelength of the Planck curve is given by Wien's
displacement law; see
Figure 2.2a
). The spectrum of solar radiation can be subdivided
into near-ultraviolet radiation (0.2-0.4 μm), visible light (0.4-0.7 μm), and near-infrared
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