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teaching:radiative_transfer:rs_tau_reff

Remote sensing of cloud optical thickness and effective radius

Nadir radiances (TOA, viewing direction 'umu 1', straight downwards) at the two wavelengths 750 nm and 2160 nm

In the following we simulated the backscattered radiation for a given optical thickness of a cloud and a given effective radius. From it we try to find a connection between radiation and optical properties of the cloud.

Looking at backscattered radiance at 750 nm we can conclude that there is no dependence on the effective radius. Therefore it should be possible to get a quite good estimate of the optical thickness.

Looking at 2160 nm there is a strong dependency on the effective radius. So given the optical thickness you can estimate the effective radius.

The classical method for cloud remote sensing of optical thickness and effective radius is descibed Nakajima and King (1990).

Calculate nadir radiances (viewing direction 'umu 1', straight downwards) at the two wavelengths 750 nm and 2160 nm respectively for effective radii between 1 µm and 25 µm and for cloud optical thicknesses between 1 and 30 and plot the results. When you have radiance measurements at both wavelengths, would it be possible to retrieve cloud optical thickness and effective radius?

Use a shell script to do the calculations. It could for example look like this:

runit.sh
lambda=2160
 
rm result$lambda\nm.dat 
 
for tau in `seq 1 30`; do
    for reff in `seq 1 25`; do
 
       	sed 's/LAM/'$lambda'/' disort.tmplt | sed 's/TAU/'$tau'/' > disort.inp
	sed 's/REFF/'$reff'/' wc.tmplt > wc.dat
 
	uvspec15 < disort.inp | gawk '{print '$tau', '$reff', $1}' \
	    >> result$lambda\nm.dat 
 
    done
done

The file disort.tmplt is a template for a uvspec input file where the wavelength and the cloud optical thickness are replaced by the shell script. The template looks as follows:

uvspec.tmplt
                         # specify libRadtran data path
data_files_path          /local/emde/libRadtran/data
 
sza 30                   # solar zenith angle
 
                         # extraterrestrial solar radiation 
source solar               kurudz_1.0nm.dat
 
                         # Location of atmospheric profile file. 
atmosphere_file          us-standard
 
albedo 0.0               # Surface albedo
 
rte_solver disort2       # Radiative transfer equation solver 
                         # disort2 with intensity correction, disort1 without
 
nstr 16                  # number of streams used in DISORT
 
wavelength  LAM          # Wavelength range [nm]
 
mol_abs_param  LOWTRAN   # gas absorption parameterization
 
wc_file 1D               wc.dat
wc_properties mie interpolate
wc_modify set tau  TAU
 
zout TOA                 # position
umu 1                    # viewing direction
phi 0
 
output_user uu           # Output radiance value
 
quiet

A second template file is needed to specify the effective radius of the cloud (wc.tmplt).

The best is to plot the result as a contour plot, here is a python example:

plot_radiance.py
result750=loadtxt('result750nm.dat')
 
figure(1)
contourf(result750[:,2].reshape(30,25))
colorbar()
xlabel('effective radius [um]')
ylabel('cloud optical thickness')
title('radiance [mW / (m^2 nm sr)] @ 750 nm')
teaching/radiative_transfer/rs_tau_reff.txt · Last modified: 2018/05/04 08:40 (external edit)