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The data base on Optical
Properties of Aerosols and Clouds (OPAC) and associated FORTRAN
code has been developed by a German group from Meteorolgisches Institut
der Universitat Munchen and from Max-Plank-Institut fur Meteorologie (Hess
et al., 1998).
The properties of clouds and aerosol particles are
highly variable, both in time and space, that is why it is impossible to
model aerosols and clouds in detail. It is necessary to reduce the variability
of naturally occuring aerosols and clouds to typical cases. This goal is
achieved in OPAC by the use of a dataset of typical clouds and aerosol
components listed in Tables 1 and 2. Table 1 lists the names and types
of clouds (first column), and corresponding references (second column).
The basic aerosol components and their composition are listed in one-column
Table 2.
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Stratus (continental) (water clouds) | Diem, 1948; Hofmann and Roth, 1989; Tampieri and Tomasi 1976. |
Stratus (maritime) (water clouds) | Tampieri and Tomasi 1976; Stephens et al., 1978. |
Cumulus (continental, clean) (water clouds) | Tampieri and Tomasi 1976; Squires 1958; Leatich et al., 1992. |
Cumulus (continental, polluted) (water clouds) | Tampieri and Tomasi 1976; Diem, 1948; Fitzgerald and Spyers-Duran, 1973. |
Cumulus (maritime) (water clouds) | Tampieri and Tomasi 1976. |
Fog | Tampieri and Tomasi 1976. |
Cirrus 1 (T=-25° C) (ice clouds) | Heymsfield and Platt, 1984; Strauss et al., 1997; Hess and Wiegner, 1994. |
Cirrus 2 (T=-50° C) (ice clouds) | Heymsfield and Platt, 1984; Strauss et al., 1997; Hess and Wiegner, 1994. |
Cirrus 3 (T=-50° C)+ small particles (ice clouds) | Heymsfield and Platt, 1984; Strauss et al., 1997; Hess and Wiegner, 1994. |
Table 2. Aerosol components used in the OPAC software as they were described in Deepak and Gerber, 1983; Shettle and Fenn, 1979; d'Almedia et al., 1991; Koepke et al., 1997.
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(*) Two sea-salt modes are given to allow for a different
wind-speed-dependent increase of particle number for particles of different
size (Koepke et a., 1997).
(**) Three mineral modes are given to allow one to consider
the increase of relative amount of large particles for increase of turbidity
(Hess et al., 1998).
OPAC consists of two parts:
The radiative properties in OPAC are modeled on the basis of components of aerosols and clouds. Each component is described by an individual particle size distribution and spectral reflectance index. Its radiative properties are modeled with Mie theory (see e.g. Van de Hulst, 1981) in the case of water clouds and aerosols, and with geometric optics in the case of ice crystals, which are assumed to be hexagonal comumns for calculations in the solar spectrum range (Hess and Weigner, 1994). In the terrestrial spectral range, ice crystals are considered to be spheres because the geometric optics assumption is only valid for particles, which are considerably larger than the wavelength of the incident radiation (Hess et al., 1998). Radiative properties of a real atmospheric aerosol is modeled as weighted sums of the radiative properties of its components listed in Table 2 (see paragraphe "Modeled optical properties of aerosols and clouds"). Microphysical properties of aerosols and clouds are described below.
Microphysical properties of aerosol components
Aerosol particles result from different sources and processes. To describe the wide range of possible composition the aerosol particles are modeled as components (Deepak and Gerber, 1983), each of them meant to be an internal mixture of all chemical substances that have a similar origin. These components, listed in Tables 2and 3, that were referred as basic aerosol constituents on the LITMS aersol data page, may be externally mixed (it means that there is no physical or chemical interaction between particles of different components) to form aerosol types.
Microphysical properties of aerosol components mean the following characteristics (as it was referred in Hess et al., 1998):
Microphysical properties of clouds
Five water-cloud models, one fog
model, and three ice-cloud models have been considered
in OPAC.
The modified Gamma function is used as the size
distribution function of water clouds and fog (Deirmendjian, 1969):
Optical properties of aerosols, water clouds, and fog are calculated with Mie theory (Quenzel and Muller, 1978). Optical properties of ice clouds are calculated with geometric optics (Hess and Weigner, 1994). In the OPAC software package, the optical properties values are stored in separate files (listed in second column of Tables 3, 4, 5) for each component and relative humidity. The following optical properties have been archived (see, e.g. Van de Hulst, 1981 for explicit formulas and definitions):
ONLINE ACCESS TO THE OPAC SOFTWARE PACKAGE DATA
Table 3. Microphysical and optical(*) properties of aerosol components in dry state (from Hess et. al., 1998)
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micrometers |
micrometers |
micrometers |
micrometers |
![]() g cm-3 |
(milligram m-3)/(part. cm-3) |
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(*) Optical properties of aerosol components as well as their
complex refractive indices are stored in the data files listed in column
2 of the table.
(**) Click with the mouse left button on a file of interest
to view its content. To download a file, click it with the mouse right
button and select the "Save as" item of the pop-un menu.
(+) For aerosol components which are able to take up water,
the data for eight values of relative humidity are available.
Table 4. Microphysical and optical(*) properties of the water-cloud and fog models (from Hess et al., 1998)
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(micrometers) |
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(micrometers) |
(cm-3) |
(g m-3) |
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(*) Optical properties of water clouds components and of
the fog model are stored in the data files listed in column 2 of the table.
(**) Click with the mouse left button on a file of interest
to view its content. To download a file, click it with the mouse right
button and select the "Save as" item of the pop-un menu.
Table 5. Microphysical and optical(*) properties of ice cloud model (from Hess et al., 1998)
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(micrometers) |
(cm-3) |
(g m-3) |
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5.352 x 1010 |
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-50° C + small particles(***) |
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(*) Optical properties of ice clouds components are stored
in the data files listed in column 2 of the table.
(**) Click with the mouse left button on a file of interest
to view its content. To download a file, click it with the mouse right
button and select the "Save as" item of the pop-un menu.
(***) Cirrus 3 is the same distribution as cirrus 2 between
20 and 2000 micrometers. Additionally, there are 0.169 particles m-3
between 2 and 6 micrometers and 0.387 particles m-3 between
6 and 20 micrometers.