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CloudyFluxExample
Prev: Example two-stream flux calculations in clear-sky
This tutorial follows on from the clear-sky example but we will make a new directory and copy in the input profiles used for clear-sky calculations again for clarity:
mkdir example2
cd example2
cp $RAD_DIR/examples/netcdf/CIRC_case6/case6.t* .
cp $RAD_DIR/examples/netcdf/CIRC_case6/case6.surflw case6.surf
cp $RAD_DIR/examples/netcdf/CIRC_case6/case6.stoa .
cp $RAD_DIR/examples/netcdf/CIRC_case6/case6.szen .
ls
There are a number of different ways to treat cloud in the radiative transfer code, some of which require different input profiles to be provided. We'll start with the simplest method.
This is set by the -K 1 option and assumes within a given atmospheric layer a certain fraction is cloud and the rest is clear-sky. Within the cloud any ice and liquid condensates are mixed homogeneously. We require the following input files:
cp $RAD_DIR/examples/netcdf/CIRC_case6/case6.clfr .
cp $RAD_DIR/examples/netcdf/CIRC_case6/case6.lwm .
cp $RAD_DIR/examples/netcdf/CIRC_case6/case6.re .
cp $RAD_DIR/examples/netcdf/CIRC_case6/case6.iwm .
cp $RAD_DIR/examples/netcdf/CIRC_case6/case6.ire .
Where each file gives cloud properties in layers (equivalent to theta levels in the UM):
- .clfr: cloud area fractions for each layer (cloud fractions are always assumed to fill the vertical extent of the layer)
- .lwm: in-cloud liquid water mass fraction, where mass fraction is mass of condensate divided by total mass of the mixture.
- .re: cloud droplet effective radius
- .iwm: in-cloud ice water mass fraction
- .ire: ice crystal effective dimension (what this dimension relates to depends on the parametrisation used)
Before solving for the radiative fluxes we must decide how the cloud fractions in each layer overlap in the vertical. For this representation of cloud a number of options are available:
- -C 2: Maximum/random overlap
- -C 4: Random overlap
- -C 7 -dp : Partially correlated overlap (exponential random)
The most commonly used option is maximum/random where cloud in adjacent layers is maximally overlapped, while cloud separated by one or more layers is randomly overlapped.
Finally, we need to choose how the optical properties of the cloud droplets (-d) and ice crystals (-i) are determined. A number of parametrisations may be available in blocks 10 and 12 of the spectral file. For the GA7 spectral files the recommended droplet type is 5. Ice type 8 has been the standard option for operational forecasts but is now being superseded by type 11.
These are the options to include only cloud absorption and scattering in the calculation:
Cl_run_cdf -B case6 -s $RAD_DATA/spectra/ga7/sp_lw_ga7 -R 1 9 -I -C 2 -K 1 -d 5 -i 8
fmove case6 case6_lw_cloud1
Cl_run_cdf -B case6 -s $RAD_DATA/spectra/ga7/sp_sw_ga7 -R 1 6 -S -C 2 -K 1 -d 5 -i 8
fmove case6 case6_sw_cloud1
The -d and -i options may be used independently to include only cloud droplets or ice crystals in the calculation. (For this example case the ice mass fraction is actually zero so the -i 8 option is not needed.)
This is set by the -K 2 option and assumes liquid droplets and ice crystals are in separate fractions within the layer. This is the scheme used by the Met Office UKV forecast model. The liquid and ice cloud fractions are combined into a single region for the purposes of overlap between layers in the vertical, so -C can take the same values as for -K 1 above. To provide the separate liquid and ice cloud fractions for this example we need to create some new input profiles:
- Cloud fractions: .wclfr and .iclfr are required in place of a single .clfr profile.
- In-cloud mixing ratios: .lwm and .iwm should be appropriate for the new fractions. (For this example we'll leave .lwm profile the same and add some mass fraction to .iwm.)
- Effective droplet and ice-crystal dimensions: .re and .ire (keep the same)
We will use a few of the Socrates utilities to create the extra files (more explanation of these utilities can be found in the tutorial preparing input files).
The Cscale_field and Cinc_field utilities currently only work for CDL format files so we need to convert back and forth using Ccdf2cdl and Ccdl2cdf (see the man pages for more information). We first scale the cloud fraction by 0.5 for all pressures to give the liquid water cloud fraction:
Ccdf2cdl case6.clfr
Cscale_field -R 0.0,1.2e5:0.5 -o cdl_case6.wclfr -n "wclfr" -u "None" -L "Liquid Cloud Fraction" cdl_case6.clfr
Ccdl2cdf -o case6.wclfr cdl_case6.wclfr
For the ice fraction we first scale to zero for all pressures and then increment the fraction to 1.0 between a defined cloud top and base pressure:
Cscale_field -R 0.0,1.2e5:0.0 -o cdl_case6.iclfr -n "iclfr" -u "None" -L "Ice Cloud Fraction" cdl_case6.clfr
P_CLTOP=5.0e4
P_CLBASE=6.0e4
Cinc_field -R $P_CLTOP,$P_CLBASE:1.0 -o cdl2_case6.iclfr -n "iclfr" -u "None" -L "Ice Cloud Fraction" cdl_case6.iclfr
Ccdl2cdf -o case6.iclfr cdl2_case6.iclfr
Finally we set an ice mass fraction for the ice cloud layers:
IWM=0.000025
Cscale_field -R 0.0,1.2e5:$IWM -o cdl_case6.iwm -n "iwm" -u "kg/kg" -L "Ice Water Mass Fraction" cdl2_case6.iclfr
rm -f case6.iwm
Ccdl2cdf -o case6.iwm cdl_case6.iwm
We can then call the radiative transfer with the following options:
Cl_run_cdf -B case6 -s $RAD_DATA/spectra/ga7/sp_lw_ga7 -R 1 9 -I -C 2 -K 2 -d 5 -i 8
fmove case6 case6_lw_cloud2
Cl_run_cdf -B case6 -s $RAD_DATA/spectra/ga7/sp_sw_ga7 -R 1 6 -S -C 2 -K 2 -d 5 -i 8
fmove case6 case6_sw_cloud2
The resulting heating rates reflect the presence of the ice and liquid clouds:
ncplot case6_lw_cloud2.hrts &
ncplot case6_sw_cloud2.hrts &
For GCM runs there is significant unresolved variability in the cloud properties which affects the resultant fluxes in a non-linear way. The Socrates code can represent this variability by either representing two cloud regions (nominally stratiform and convective) directly in the solver or by sampling a sub-grid cloud field using the Monte-Carlo Independent Column Approximation (MCICA).
This is set by the -K 4 option. Clouds are divided into ice and water fractions within each region and into "stratiform" and convective regions, so four of each type of input file are required:
- Cloud fractions: .wclfr, .iclfr, .wccfr and .iccfr
- In-cloud mixing ratios: .lwm, .iwm, .lwmcv and .iwmcv
- Effective dimensions: .re, .ire, .recv and .irecv
There are two options for the treatment of vertical overlap of the cloud regions:
- -C 6: Maximum/random overlap in a triple column (stratiform, convective and clear-sky regions)
- -C 8 -dp +dp : Partially correlated overlap (exponential random)
Set using the option -C 10 with either -K 1 or 2 (which then require the input fields described above for these two options). Further options for the configuration of the sub-grid cloud are then required. These will not be described here, but an example calculation is given in examples/netcdf/mcica/.