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Produced by the NASA Center for Aerospace Information (CASI)
1 ^

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 

Grant period: 05/15/80 - 05/15/82
This 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 not

' 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 

these areas follows.

(1) Improved Spectral Bandpass 

In order to insure that the measurements were limited to the

visible portion of the spectrum an optical filter was placed in front 

each detector. Figure 1 shows the relative responses of the system w

and without the filter. Although insertion of the filter resulted in 

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

j
CRIONAL PAGE 19

OF POOR QUALITY

Original Response

--- Filtered Response

3

a

0.8

O.7 % I

CL 0.6..

a

> 0.4

0 0.3

o. 

0J

I

) 0.3 0.4 0.5 0.6 0.7 O.e 0.9 LO Id

Wavelength in Microns

Figure 1. Relative spectral response of the original

and filtered CFO$ photodiode.

2
ORIGiNA . iI#'Z_IIn' 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 plan

by a horizontally semi-infinite cloud, and second, the visual appearan

of the clouds simulated from the material. Based on these criteria, 

materials were found to be superior to all others tested; they are

surgical quality sterilized cotton and decorative billet st yrofoam wh

is marketed b y Dow Chemical. Figures 2 and 3 compare measured princip

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 

μG 1.0 and μ a = 0.5 using the C.1 

droplet distribution at a wavelength of 0.7 μm. All radiances have

been normalized b y 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

materials is discussed in the next section. 

Both materials do well in simflating the visual appearanc es of 

clouds. If ,we add a third criterion, namely thr, practicality of work

with the materials, the styrofoam becomes preferable for a number of 

reasons. First, the styrofoam material is rigid and will maintain a

constant ratio of geometric to optical depth, and the shape of the

simulated clouds will not distort due to gravit y with rotation of the

3 

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5
OF POOR QUALITY

CFOS apparatus about the simulation zenith. Second, simulated styrof

clouds are easily exchanged from one cloud scene to another or moved

about within a cloud scene. Finally the styrofoam simulated clouds a

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 

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 base

on the bulk radiative properties of a 'semi-infinite' sheet of the cl

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 g ure q shows a plot of the

ratio of R/T generated from the parameterization of Stephens (1978) f

water clouds in the visible portion of the spectrum. Also shown is t

same ratio from a simple Eddington-model which employed the

Henyey-Greenstein approximation for various cases and also a few resu

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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ORIGINAL PAGE ES

OF POOR QUALITY

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

I

,

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
I

OF 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 origina

'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 cras

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 cotto

an approximate visible optical depth ma y be assigned by weighing the

cloud and measuring the cross sectional area to determine the area ma

densit y then using the R/T line as a transfer function to

e' The additional measurement is yet another 

limitation to the suitability of using cotton as the cloud simulation

material.

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 

finite clouds as measured on CFOS and the same quantities predicted by

the theoretical Monte Carlo model as described in McKee and Cox (197

The radiances in each case have been normalized by the radiance measu

or calculated at the zenith. The CFOS profiles have been retrieved fr

measurements over a field of simulated (iF,t yrofoam) cubic clouds cut f

a slab of material whose R/T ratio corresponds to an equivalent optic:A

8
ORIGINAL, PAC,- fj OF 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).

9
OF Poaa QUALIrY

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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 assum

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 

Is given as

N

Nc)d 0cld + Ncir ()

clr + 0 0sdw

• 

In the above formula the subscripts cld, clr and sdw • stand for

cloud, clear and shadow, respectively, and n represents solid angle.

It is assumed that there is no radiance contribution from the shadow 

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 i

either considered completely within or exterior to the field of view. 

Nevertheless, smoothed versions of the curves are in good agreement.

The apparent disagreement between measured and calculated radiances a

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. Simila

disagreement is seen in the radiance comparisons for infinite clouds 

00 zenith.

12
•

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 b

of the domain for which CFOS was ori g inall y intended. Larger clouds

smaller cloud spacing and more realistic simulated surfaces will all

tend to increase the radiance signal. In addition, the above experim

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 mea

of exploring the effects of realistic cloud geometries on the radian

reflected by opticall y thick clouds and cloud ,fields.

Cr

1

13

A
a r ORIGINAL E AG`! 

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.

14