Defines how ice water content and effective particle radius are translated
to optical properties.
ic_properties type
Possible choices for type are
- fu
- Parameterization by Fu (1996); Fu et al. (1998), see ic_file;
this is the default setting. Note that this is a parameterization
which has been created to calculate fluxes but not radiances.
Note also that the optical properties in the solar range provided by
Fu (1996) are delta-scaled properties (that is, the forward peak of
the phase function is truncated and optical thickness, asymmetry
parameter, and single scattering albedo are reduced accordingly),
whereas uvspec uses non delta-scaled properties unless the option
ic_fu_tau scaled is specified. By default the parameterization
by Fu (1996) is treated consistently with all other ice cloud
parameterizations.
For wavelengths up to 4 micrometer Fu (1996) is used while for wavelengths
larger than 4 micrometer Fu et al. (1998) is chosen. Please note that
Fu (1996) is based on ray-tracing calculations while Fu et al. (1998)
is a mixture of ray-tracing and Mie calculations (which is required for
the infrared wavelengths where the geometrical assumption does not hold).
Hence, both parameterizations are not fully consistent. Rather, differences
of some parameterizations overlap. Also, the wavelength dependence in the solar
and infrared parts is treated differently: In the solar part (Fu, 1996)
the optical properties are defined for wavelength bands - hence they
are assumed constant within each band. In the infrared (Fu et al., 1998)
they are defined at certain wavelengths and linearely interpolated
in between. If you use this option, please see also the
discussion of ic_fu_tau and ic_fu_reff.
The allowed range for the effective radius is from 9.315 - 65.120 micrometer.
- echam4
- Use the simple two-band parameterization of the ECHAM4 climate model, described
in Roeckner et al. (1996); this is probably only meaningful if you want to compare
your results with ECHAM4, the two bands are 0.2 - 0.68 micrometer and 0.68 - 4.0 micrometer.
Within the two ECHAM4 bands, the optical properties are assumed constant.
- key
- Parameterization by Key et al. (2002). This parameterization can also
be used to calculate radiances because it uses a
double-Henyey-Greenstein phase function which better represents both
forward and backward peaks. This parameterization covers the wavelength region
from 0.2 to 5.0 micrometer and is available for the following habits:
solid-column, hollow-column, aggregate, rosette-4, rosette-6, and plate.
- yang
- Parameterization similar to Key et al. (2002) but based on more recent
single scattering calculations. Below 3.4 micrometer it actually equals
the Key et al. (2002) parameterization while from 3.4 - 100 micrometer new
coefficients have been calculated with much higher wavelength
resolution and better accuracy. Hence, yang should give a reasonably
consistent approximation from 0.2 - 100 micrometer, suitable for spectrally
resolved calculations of radiance and irradiance.
The covered range for the effective radius depends on the ic_habit.
(In micrometer: solid-column [5.96, 84.22], hollow-column [4.97, 70.24],
rough-aggregate [3.55, 108.10], rosettes-4 [2.77, 45.30],
rosettes-6 [2.85, 46.01], plate [4.87, 48.18], dendrites
[0.45, 1.88], droxtal [9.48, 293.32],
spheroid [6.58, 203.39]).
- baum
- Use ice cloud parameterization from Baum et al. (2005 a); Baum et al. (2005b),
http://www.ssec.wisc.edu/baum/Cirrus/IceCloudModels.html.
In combination with the radtiative transfer
solvers disort2 and mystic, accurate phase functions are used.
- baum_hufit
- Similar to the option baum but here the phase function
is parameterized by 128 Legendre coefficients, calculated with the
delta-fit method from
Hu and Stamnes (2000). This parameterization covers the region
from 0.4 to 2.2 micrometer. If high accuracy is needed e.g. in the vicinity of the halo,
the forward peak, or the backscatter peak, ic_properties baum
is recommended.
- hey
- Use pre-calculated ice cloud optical properties including full phase
matrices. This option has newly been implemented and is not yet well
validated. Please check your results carefully!!
The parameterization
is currently only available for the spectral region from 0.2 to 5
micrometers. The single scattering
properties have been been generated by Hong Gang using the
models by Yang et al. (2000). The parameterization is based on
simple gamma distributions
 |
(0.1) |
where
is found by normalization and
is set to 1. In case of
spherical particles the parameter
would be the effective
radius. For aspherical particles, the parameter
is found
iteratively so that the size distribution yields the required
effective radius. The
parameterization is availabe for the following habits: solid-column,
hollow-column, rough-aggregate, rosette-6,
plate, and droxtal. The
default habit is solid-column. The habit can be specified using the option
ic_habit.
- mie
- Use pre-calculated Mie tables; useful for correlated_k;
the tables are expected in data_files_path/correlated_k/.../.
For spectral or pseudo-spectral (correlated_k sbdart) calculations,
a set of pre-calculated tables is also available; the wavelength grid points of these
data has been carefully selected such that the extinction cross section,
single scattering albedo, and the asymmetry parameter are accurate to 1%
(compared to the fully-resolved Mie calculation) for all wavelengths
between 250nm and 100 micrometer. For spectral or pseudo-spectral
calculations ic_properties_interpolate has to be defined explicitely to
initiate the interpolation of the optical properties to the internal wavelength grid.
Please note that this option may be extremely memory-consuming because for each
internal wavelength a full set of Legendre moments of the phase function is
stored (up to several thousands). The Mie tables are not part of the standard distribution
(because of their large size) but they are freely available from http://www.libradtran.org.
Note that a Mie calculation assumes spherical ice particles, the scattering function of
which differs systematically from non-spherical particles. Hence, ic_properties mie
is usually not representative of natural ice clouds.
- filename
- Read optical properties from specified filename; file format is as produced
by the mie tool of libRadtran (see output_user netcdf)
or by Frank Evans' cloudprp.
The default property is fu.
Please note also that, in contrast to spherical particles, there is no unique
definition of effective size for non-spherical particles. In particular, the
above parameterizations use different definitions which, however, differ only by
a constant factor.
Yang et al. (2000), citeKey2002, and Baum et al. (2005 a); Baum et al. (2005b)
use the general definition
 |
(0.2) |
where
is the maximum dimension of an ice crystal,
is the
number of particles with maximum dimension
in the size distribution,
and
and
are the volume and mean
projected area of the particles, respectively. The volume and area are
based on the spherical diameter with equivalent volume and the
spherical diameter with equivalent projected area as defined by
Yang et al. (2000). On the other hand, Fu (1996); Fu et al. (1998) use
hexagonal columns and use the following definition
 |
(0.3) |
where
is the width of the ice crystal (that is, the maximum diameter of the
hexagonal area) and
is the length. The integrand in the numerator is proportional
to the volume while that in the denominator is proportional to the projected area.
Evaluating these formulas one finds that, for the same hexagonal particle, the effective
radius would be
times larger following the
Yang et al. (2000), Key et al. (2002) definition rather than the Fu (1996); Fu et al. (1998) definition.
As an example, an effective radius of 20
with
ic_properties fu and ic_fu_reff fu and
1.299
20
= 26
with ic_properties yang would give comparable results
for hexagonal columns.
To use the original definition of the effective radius by Fu (1996); Fu et al. (1998) use
ic_fu_reff fu!
Arve Kylling
2010-03-10