Geoscience Reference
In-Depth Information
Fig. 11.5
Dust optical properties ((
a
) mass-specific extinction coefficient, MEC. (
b
) Single-
scattering albedo) in the terrestrial spectrum for different refractive indices (
colours
)andsize
distributions, as shown in Fig.
11.1
for desert dust (
solid lines
) and transported dust (
dashed lines
)
Also shown are refractive indices from the Optical Properties of Aerosols and
Clouds (OPAC) database (Hess et al.
1998
), intended to represent a mixture of clay
and quartz, and from the World Meteorological Organization (WMO) (WCP
1986
)
as these are frequently used in numerical prediction and climate models. All four
representations show different values across the spectrum. This is likely to be due to
differing compositions of dust, for example, the peak in absorption between 9 and 10
m is caused by the absorption properties of various minerals with high absorption
here, such as quartz, silicates, gypsum and calcite (e.g. Hansell et al.
2011
), whilst
the peak displayed in the OPAC refractive indices (green, Fig. 14b) from 12 to 13
m is dominated by quartz.
As with the SW spectrum, LW spectral optical properties can be calculated
using a Mie scattering code, assuming the dust particles are spheres, for the two
different size distributions shown in Fig.
11.1
and the four refractive indices shown
in Fig.
11.4
. Figure
11.5
shows the contrasting optical properties that result. Some
features can be traced back to the shape of the real part of the refractive index,
such as peak extinction at 9.5 m. Differences in SSA are related to the imaginary
part of the refractive index that controls absorption. The lower WMO (black) values
cause notably higher SSA values across the spectrum, whilst the reverse is true of
the Fouquart data (red), though here the lower real part of the refractive index also
contributes. The low real part of the Fouquart refractive index also creates lower
mass-specific extinction coefficients compared to the other refractive indices. The
transported size distribution containing fewer coarse particles (dashed line) results
Search WWH ::
Custom Search