ParametrisedOpticalProperties

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Parametrised optical properties for clouds and aerosols

We have seen in the tutorial on prescribed optical properties how to generate optical properties for cloud droplets using the routine Cscatter. There a single size distribution was used, but in general this will vary for different atmospheric columns and it is useful (particularly for GCMs) to provide a parametrised fit of the optical properties as a function of the effective radius. This can be added to the spectral file as block 10 for cloud droplets (and similarly block 11 for aerosols and 12 for ice crystals).

Cloud droplets

First we need to run Cscatter for a range of droplet distributions. Examples are given in the Socrates User Guide (section 3.3). A script to cover a more exhaustive range of observed distributions is also available in $RAD_DIR/examples/droplets/. The output from Cscatter for each distribution is then concatenated together into a single file. For example the output from the $RAD_DIR/examples/droplets/ example can be seen here:

less $RAD_DATA/cloud/scatter_drop_sw_type5

This contains the absorption and scattering coefficients and the asymmetry (first moment of the scattering phase function) for a range of wavelengths and a range of effective radii. We now need to average these properties for the bands in the spectral file and generate the coefficients of the functional fit to effective radius. This is done with the Cscatter_average routine:

man Cscatter_average

The different functional forms available are described in the Socrates Technical Guide (section 1.1.7). For droplets in the GA7 spectral files we use -f 5 (Pade approximation) which allows a very flexible fit but has a tendency to converge on bad fitting parameters so these need to be checked with care. For the skeleton spectral file generated in the previous tutorial:

cd $RAD_DIR/examples/sp_sw_jm
rm -f sp_sw_6_skel
. ./mk_sp_sw_6_skel

we can generate the droplets parametrisation as follows:

Cscatter_average -s sp_sw_6_skel -P 1 -t -S lean_12 \
    -f 5 fit_sw_drop5_6 mon_sw_drop5_6 1.e3 \
    $RAD_DATA/cloud/scatter_drop_sw_type5

Here we have used the options:

• -P 1: only one moment of the phase function.
• -t: method of "thick averaging" appropriate for optically thick clouds.
• -S lean_12: weight with solar spectrum.
• -f 5 fit_sw_drop5_6 mon_sw_drop5_6 1.e3: Pade fit with output in the given filenames using a density of 1.0e3 kg/m3. The density is required to convert to mass units (kg/kg) used in the radiative transfer code.

The "fit" file produced can then be added to the spectral file using prep_spec:

less fit_sw_drop5_6

prep_spec << EOF
sp_sw_6_skel
a
10
5
fit_sw_drop5_6
1.50000E-06 5.00000E-05
-1
EOF

less sp_sw_6_skel

Aerosols

For aerosols a similar method can be used. In the latest Met Office climate configuration GLOMAP-Mode aerosols are used which calculate the aerosol properties outside of the radiation code. In the old CLASSIC scheme the optical properties of each individual aerosol are parametrised in the spectral file. The example file here demonstrates the options for Cscatter needed to create the monochromatic single scattering properties for the CLASSIC aerosols:

less $RAD_DIR/examples/aerosols/run_scatter

Here we only consider a single effective radius for each aerosol although it is possible to include a humidity dependence. For a given spectral file the optical properties can then be averaged over the band, for example:

Cscatter_average -s sp_sw_6_skel -P 1 -w -S lean_12 \
  -a nitrate_6.avg $RAD_DATA/aerosol/nitrate.mon

less nitrate_6.avg

and then added to the spectral file:

prep_spec << EOF
sp_sw_6_skel
a
11
nitrate_6.avg
-1
EOF

less sp_sw_6_skel

Next: Generating gas absorption coefficients (correlated-k)