Available Energy: Net Radiation and Soil Heat Flux - Transport in the Atmosphere-Vegetation-Soil Continuum - page 37

that the magnitude of the downwelling longwave radiation is smaller during winter
than during summer. This is due to the lower temperature of the atmosphere. The
envelope of the radiation is a Planck curve for a temperature that is representative
of the lower 100 m of the atmosphere. Thus, it appears that most of the 'clear-sky'
longwave radiation received by the surface originates from the irst hundred meters
of the atmosphere. This is due to the fact that longwave radiation emitted at higher
altitudes in the atmosphere is completely absorbed by the layers below and hence
the downwelling longwave radiation does not contain radiation emitted in the higher
atmosphere. The atmospheric window is clearly visible in the downwelling radiation:
hardly any radiation at wavelengths within the atmospheric window is emitted by the
atmosphere.
The spectrum for the winter conditions shows more dents where the radiation is
lower than the Planck curve. This is due to the lower water vapour concentration dur-
ing winter: not all water vapour emission lines are fully used.
In contrast to the clear-sky emission that occurs in distinct molecular absorption
lines, clouds emit (and absorb) radiation like a black body in the longwave wave-
length region. Because, usually, the cloud base is higher than a few hundred meters,
all radiation emitted by clouds outside the atmospheric window is absorbed by the
atmosphere between the clouds and the surface. Hence only the radiation by the
clouds inside the atmospheric window (denoted by L c ) will reach Earth's surface, that
is, that part of the radiation emitted by clouds that is not absorbed by the atmosphere
between the clouds and the surface. This effect is illustrated in Figure 2.12b . Note,
that as the emissivity (and hence the absorptivity) of clouds is close to unity, clouds
do not relect longwave radiation. Hence, longwave radiation originating from Earth's
surface (see next section) is absorbed by clouds: the downward longwave radiation
originating from clouds is not due to the relection of upwelling longwave radiation
from the surface.
Figure 2.14a shows observations of downwelling longwave radiation for a clear
and a cloudy day. For most of the day, L ↓ is 50-100 W m -2 higher on the cloudy day as
compared to the clear day, owing to extra radiation that is emitted by the clouds, and
passes through the atmospheric window (if L 0 ↓ would be identical for both days, this
50-100 W m -2 would represent L c ↓ ). Also note that the diurnal cycle of L ↓ is rather
limited.
Complex models have been designed to describe L 0 ↓ and L c ↓ in detail as a func-
tion of vertical proiles of temperature and atmospheric composition. These models
can either make a line-by-line calculation (using the absorption spectra as depicted
in Figure 2.2b and yielding spectral luxes as shown in Figure 2.13 ) or the absorption
spectra can be summarized as absorption bands, hence speeding up the calculation. It
is outside the scope of this topic to treat these complex models.
A commonly used empirical model for incoming longwave radiation is based on
the observation that incoming longwave radiation comes mainly from the lowest few