The Cloud Field Optical Simulator project includes work in the following areas: (1) improvement in the shape of the desired (visible) spectral response of the measurement, (2) selection of two usable materials for cloud simulation, (3) a means of assigning a visible optical depth to the simulated clouds, and (4) confirmation that the apparatus is capable of detecting basic finite cloud characteristics. A brief description of the accomplishments in each of these areas is presented

Cox, S. K.

Mckee, T. B.

English

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https://ntrs.nasa.gov/search.jsp?R=19840009687 2020-03-20T23:14:19+00:00Z
i1 ^
Annual Report
for
NASA Grant NA65-56
entitled	 flln,	 ,,n4
"A Laboratory Investigation of the Reflective Properties
of Simulated, Opticall y Th'ck Clouds"
(N AS A-CR - 175 3 E2) A LABORATORY INVESTIGATION 	 N84-17755
OF THE REFLECTIVE -PROPEIYTIES OI SIMULATED,
OPTICALLY THICK CLOUDS Annual Report, 15
May 1,980 - 15 May 1982 (Colorado stave 	 Unclas
U,Div.)	 15 p HC A02/MF A01	 CFSCL 04D 0;3/47 11624
Principal Investigator:
	 Thomas B. McKee
Co-Investigator:
	 Stephen K. Cox
Department of Atmospheric Science	 }
Colorado State Un+versity
Fort Collins, Colorado 80523
	
t
t
'	 4
Grant period: 05/15/80 - 05/15/82
Aw
VThis report summarizes the progress toward the specific research
goals of the cloud field simulation project which has been made since
June 1981. Accomplishments prior to this date were reported in our
first semi-annual progress report in February 1981 and at the Fourth
Conference on Atmospheric Radiation, Toronto.'
Gu y ing the reporting period stated above, progress toward the
research goals of CFOS (the Cloud field Optical Simulator) may be noted
'	 in the following areas: (1) improvement in the shape of the desired
(visible) spectral response of the measurement, (2) selection of two
usable materials for cloud simulation, (S) a means of assigning a
"visible" optical depth to the simulated clouds and*(4) confirmation
that the apparatus is capable of detecting basic finite cloud
characteristics. A brief description of the accomplishments in each o•r
these areas follows.
(1) Improved Spectral Bandpass 	 j
In order to insure that the measurements were limited to the
visible portion of the spectrum an optical filter was placed in front of
each detector. Figure 1 shows the relative responses of the system with
t
and without the filter. Although insertion of the filter resulted in a
relatively poorer S/N ratio it ensures that the measurements will not
include spectrally dependent effects which are visually unobservable.
'McKee, T. B., J. M. Davis, and S. K. Cox, 1981: Design and
verification of a cloud field optical simulator. Preprints
Fourth Conference Atmospheric Radiation, Toronto, Amer. Meteor.
Soc., pg. 227.
ORIGINAL PAGE 19
i or.. POOR QUALITY
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CRIONAL PAGE 19
OF POOR QUALITY
Original Response
--- Filtered Response
3
a
0.8
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	 %	 I
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	 0.9
	 LO	 Id
Wavelength in Microns
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Figure 1. Relative spectral response of the original
and filtered CFO$ photodiode.
2
ORIGiNA . iI#'Z_I n' GJ
OF POOR QUALI11
2. Selection of Suitable Materials for Cloud Simulation
Several materials have been examined in a search for one which
adequate)>- simulates the visible reflective properties of optically
thick clouds. The criteria which were used to rate the materials were
first, the behavior of the radiances reflected into the principal plane
by a horizontally semi-infinite cloud, and second, the visual appearance'
of the clouds simulated from the material. Based on these criteria, two
materials were found to be superior to all others tested; they are
surgical quality sterilized cotton and decorative billet st yrofoam which
is marketed by Dow Chemical. Figures 2 and 3 compare measured principal
plane reflected radiances of the two materials with theoretical curves
for semi-infinite clouds from Monte Carlo radiative transfer
calculations. The model was run for two solar zenith an g les	 F
µG	 1.0 and µ a = 0.5 using the C.1 	 f
droplet distribution at a wavelength of 0.7 µm. All radiances have
been normalized by the dividing by reflected radiance at the zenith.
The materials are almost equivalent in satisfying the first criteria.
However, the styrofoam is slightly better in the backscatter direction,
The assignment of an equivalent optical depth to the simulation
t
F
materials is discussed in the next section.
	 i
Both materials do well in simflating the visual appearanc es of	 1
clouds. If ,we add a third criterion, namely thr, practicality of working
	 t
with the materials, the styrofoam becomes preferable for a number of	 t
reasons. First, the styrofoam material is rigid and will maintain a
constant ratio of geometric to optical depth, and the shape of the
3
simulated clouds will not distort due to gravit y
 with rotation of the
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CFOS apparatus about the simulation zenith. Second, simulated styrofoam
clouds are easily exchanged from one cloud scene to another or moved
about within a cloud scene. Finally the styrofoam simulated clouds are
easily made into regular shapes, i.e. cubes, c y linders, inverted
paraboloids etc. for comparison with theoretical results for finite
clouds.
3. A Method o•f Assigning a Visible Optical Depth to the
Simulated Clouds
In order to relate the reflective properties-, of simulated clouds to
real water clouds it is necessary to have an accurate estimate of the
optical depth of the simulated clouds. One means of doing so is based
on the bulk radiative properties of a 'semi-infinite' sheet of the cloud
material at 0 0
 zenith. Specifically theory pra-dicts that the ratio
of spectral reflectance R  to spectral transmittances
T%
 of a semi infinite cloud over a non reflecting surface
is nearly a linear function of optical depth, see for example Coakley
aand Chy'lek (1975) and Stephens (1978). Fi gure q shows a plot of the
ratio of R/T generated from the parameterization of Stephens (1978) for
water clouds in the visible portion of the spectrum. Also shown is the
same ratio from a simple Eddington-model which employed the
Henyey-Greenstein approximation for various cases and also a few results`
of the R/T ratio from Monte Carlo calculat',ons. The linear dependancc>
between R/T and K the visible optical depth is apparent.
An equivalent visible optical depth ('re) may be
assigned to the CFOS cloud simulation material by measurement of the
ratio of R/T for a sheet of the material with a large ratio of
6
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,
on, C,5, ,X=O,7Um
on, Best , X 0,7um
grlo, C, I, X e 0,7um
on, C, 1, a t 0.7 um
ws, (1978), visible
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v.
20	 30	 40	 50	 60	 70	 90	 90	 100
Visible Optical Depth
Figure 4. A plot of the ratio of reflectance to
transmittance derived from various
calculations as a function of optical
depth.
7
a:J
IOF POOR QUALITY
horizontal to vertical dimension. once It	 is
e
established for a large slab of Styrofoam several clouds  may be
'sculptured' from the slab while maintaining in each the vertical
dimension of the original slab. The procedure requires an additional
step in the case of cotton since the vertical structure of the original
'slab' (surgical cotton is availabiQ in 12" by 60" sheets> can not be
maintained while forming clouds of a realistic shape. However, it was
found that the mass of the material in a vertical column of unit crass
section is also related to the ratio of the R/T in a nearly linear
manner; see figure 5. Thus, for a cloud simulated from surgical cotton
an approximate visible optical depth ma y be assigned by weighing the
cloud and measuring the cross sectional area to determine the area mass
densit y then using the R/T line as a transfer function to
e'
The additional measurement is yet another	 1
limitation to the suitability of using cotton as the cloud simulation
material.
v
4. Retrieval of Basic Finite Cloud Features
The most .-ecent accomplishment with the CFOS concerns the ability
to measure the reelected radiances of finite clouds. Fi gures 6 and 7
are comparisons between radiances reflected into the principal plane by
finite clouds as measured on CFOS and the same quantities predicted by
the theoretical Monte Carlo model as described in McKee and Cox (1970.
The radiances in each case have been normalized by the radiance measured
or calculated at the zenith. The CFOS profiles have been retrieved from
measurements over a field of simulated (iF,t yrofoam) cubic clouds cut from
a slab of material whose R/T ratio corresponds to an equivalent optic:A`s
8
ORIGINAL, PAC,- fjOF POOR 
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Area Mass Density ( 9 / cm2)
Figure 5. A plot of the ratio of reflectance to
transmittances measured by CFOS for a
'semi-infinite" cotton simulated c:oud
plotted as a function of area mass density
and equivalent visible optical depth (Te).
If
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depth 1
e - 76. The clouds were evenl y spaced on a
horizontal grid such that the true fraction cloud cover (f) was 0.04,
Figures 6 and 7 each show two measured profile retrievals. One assumes
that as the nadir angle W increases, ,
 the solid angle of cloud
field relative to the underl y ing surface increase. as (i + tan 0)
resulting in a modified fractional cloud cover f' - f 0 + tan 0);
see Davis, Cox, and McKee (1979). The second treatment utilizes the
exact geometry of the cloud  field and the CFOS de tec tors, counts the
clouds in the field of view and calculates the total solid angle
subtended by the clouds and their shadows. The radiance measurement (N)
Is given as
N	
Nc)d 0cld + Ncir () clr + 0 0sdw
•	 F
In the above formula the subscripts cld, clr and sdw • stand for
R
cloud, clear and shadow, respectively, and n represents solid angle.
r
It is assumed that there is no radiance contribution from the shadow t
regions. The Nclr measurements were made over the surface
on which the clouds were mounted. The albedo of the surface was 0.02.
The second treatment results, in a 'noisier' curve because the cloud is
	
i
r
either considered completely within or exterior to the field of view.
	 i
Nevertheless, smoothed versions of the curves are in good agreement.
The apparent disagreement between measured and calculated radiances at
lar ge nadir in the 0 0 solar zenith angle case is believed to be doe
primarily to the basic scattering properties of the material. Similar
disagreement is seen in the radiance comparisons for infinite clouds at
00 zenith.
12
f^
•
It should be noted that retrieval of the finite cloud reflected
radiances, as described above, represents a rigorous test of the CFOS
concept. Tho low surface albedo combined with the extremely small
fractional cloud cover result in reflected radiances at the lowest bound
of the domain for which CFOS was ori g inall y intended. Larger clouds,
smaller cloud spacing and more realistic simulated surfaces will all
x
tend to increase the radiance signal. In addition, the above experiment
represents the retrieval radiances reflected from only part of the
scene. Measurement of total scene radiances are of a more stable
nature. Thus, it appears that the CFOS concept is indeed a valid means
of exploring the effects of realistic cloud geometries on the radiances
reflected by opticall y thick clouds and cloud ,fields.
d
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13
A
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OF POOR QUALITV
References
Coakley, J. A. and P. Ch y'lek, 1975; The two stream approximation
In radi at i +,e transfer: 	 Including  the angle of the incident
radiation. J. Atmos. Sci.., 32, 409-418.
Stephens, G. L., 1978: Radiation profiles in extended water clouds.
II: Parameterization Schemes. J. Atmos. Sci., 35, 2123-2132.
McKee, T. B. and S. K. Cox, 1974: Scattering of visible radiation
by finite clouds. J. Atmos. Sci., C1, 1885-1892.
Davis, J. M., S. K. Cox, and T. B. McKee, 1979: Vertical and
horizontal distributions of solar absorption in finite clouds.
J. Atmos. Sci., 36, 1976-1984.
vY
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14


A laboratory investigation of the reflective properties of simulated, optically thick clouds

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