Geoscience Reference
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In addition to the relative particle size, scattering also depends on the absorption of
radiation by particles: absorption strongly modiies the Mie scattering process (Petty,
2004 ).
Absorption
Absorption by gases is a spectrally very selective process, in contrast to the scattering
of radiation by particles, which is a rather continuous function of wavelength. The
most active absorbing gas in the atmosphere is water vapour, which absorbs both in
the shortwave and in the longwave part of the spectrum (see Figure 2.2b ). Ozone is
active at the ultraviolet (UV) side of shortwave radiation. Oxygen plays a minor role
in the far-infrared region and in the medium UV part of the spectrum. Finally, meth-
ane, nitrous oxide and carbon dioxide play an important role as greenhouse gases
(next to water vapour) at near-infrared wavelengths.
The presence of an absorption lines at a given wavelength for a given molecule is
a relection of the fact that the photon with the given wavelength has an energy that
corresponds to a transition in the internal energy of that single molecule. But because
the interactions between molecules (through collision) inluence the energy state of
molecules, the exact location, strength and width of the absorption lines also depend
on local pressure and temperature (Petty, 2004 ). Hence, the extinction will be height-
dependent for two reasons: the concentration of an absorber may vary with height,
and the absorption lines differ with height.
Apart from absorption by gases, radiation may also be absorbed by particles. This
absorption depends both on the material of the particle and the wavelength. In the case
of liquid water - relevant for the radiative properties of clouds and fog - the absorp-
tivity is low for the wavelength region of visible light, whereas it is high throughout
the thermal infrared (Hale and Querry, 1973 ). The ultimate effect of absorption by
particles also depends on the size of the particle and the wavelength of the radiation,
as absorption modiies Mie scattering.
2.2.2 Downwelling Shortwave Radiation
For a given location, the downwelling shortwave radiation varies on two predictable
time scales: the yearly cycle and the diurnal cycle. This temporal variation in solar radi-
ation is the main driving force in the temporal variation of all terms in the surface energy
balance at a given location. On a larger scale the location dependence of the insolation,
in particular the variation with latitude, is one of the drivers for the general circulation
in the atmosphere. The predictable part of this latitudinal, yearly and diurnal variation
of the solar radiation is outside of the scope of this topic. The equations describing the
variation of solar radiation at the top of the atmosphere can be found in Appendix A .
The inal result of all factors that inluence the amount of solar radiation at the top of
the atmosphere ( K 0 ), at a given moment and at a given location, can be summarized as:
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