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

$\displaystyle n(r) = n_0 r^{\alpha} \exp\left(-\frac{(\alpha+3)r}{r_e}\right),$ (0.1)

where $ n_0$ is found by normalization and $ \alpha$ is set to 1. In case of spherical particles the parameter $ r_e$ would be the effective radius. For aspherical particles, the parameter $ r_e$ 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

$\displaystyle r_{\rm eff} = {{3}\over{4}}{{\int V(h) n(h) dh}\over{\int A(h) n(h) dh}}$ (0.2)

where $ h$ is the maximum dimension of an ice crystal, $ n(h)$ is the number of particles with maximum dimension $ h$ in the size distribution, and $ V$ and $ A$ 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

$\displaystyle r_{\rm eff} = {{\int D^2 L n(L) dL}\over{2 \int (D L + {\sqrt{3}\over{4}} D^2) n(L) dL}}$ (0.3)

where $ D$ is the width of the ice crystal (that is, the maximum diameter of the hexagonal area) and $ L$ 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 $ 3 \sqrt{3} / 4 = 1.299$ 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$ \mu m$ with ic_properties fu and ic_fu_reff fu and 1.299 $ \cdot$ 20$ \mu m$ = 26$ \mu m$ 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