R. L. HULSTROM* Martin Marietta Ae~ospace Denver, Colorado 80201 The Cloud Bright Spot

Clouds can cause an increase in solar radiation that can affect a remote-sensing technique which utilizes reflected energy.

VERYONE familiar with remote sensing, E whether simple photography or a com- The measurements presented in the fol- plex multispectral scanner, is aware of the lowing sections are of global solar radiation, common influences of clouds. These more defined as the downward direct and diffuse common influences are, of course, shadows solar radiation as received on a horizontal and/or complete obscuration of ground surface from a solid angle of 2.r. The appro- targets. The following sections present some priate equation for this relationship is: field measurements and analyses that show a third cloud effect resulting in increased, as KJ (Ah)= S (Ah)+ D (AA) (1) compared to nominal clear conditions, solar radiation incident at the ground. In addition where K is the global solar radiation, S is the to these measurements, Hansonl (1960) re- vertical component of direct solar radiation, ported measuring "anomalous total global D is the radiation (diffuse solar radia- radiation" on an Antarctic snowfield that ex- tion), and Ah represents some increment of ceeded the extraterrestrial radiation (solar wavelength h. The vertical component of the constant) for over an hour. For simplicity, direct solar radiation is that radiation coming this phenomenon will be referred to as cloud from the solid angle of the 's disc, as bright spots as opposed to cloud shadows. received on a horizontal surface. It is ex-

ABSTRACT:A cloud effect, culled "cloud bright spot," is presented in terms of field measurements, theory, and significance to remote sensing. Field measurements of global solar radiation show that clouds can cause ground "bright spots" having a 30 percent higher incident solar radiation flux than surrounding areas. The possible cause and recommendations con- cerning ground truth nzeasurements and instrumentation are also pre- sented.

One important aspect of a bright spot, in pressed as addition to having abnormally high values of S(Ah) = I (Ah) cos 6, incident solar radiation, is that an interpreter (2) of the remote sensor data probably cannot where I is the solar radiation coming from readily identify such bright spots as com- the solid angle of the sun's disc on a surface pared to the more obvious shadows. For ex- perpendicular to the axis of the solid angle, ample, such bright spots might be inter- and 6, is the solar zenith angle. These quan- preted as areas of high surface reflectance. tities and relationships are illustrated in The following discussion also addresses Figure 1. the significance of bright spots to various The direct solar beam I(Ah) can further be remote sensing techniques. Recommen- dations are made for simple field monitoring * This research was partly supported by sub- instrumentation to be used during remote contract to the Colorado School of Mines under sensing missions such as the NASA ERAP, NASA Grant NGL 06-001-015, The Bonanza Re- ERTS,and EREP programs. mote Sensing Project. As T~(AA)is inversely proportional to wavelength to the fourth power, much more molecular of solar radiation occurs in the shorter wavelengths. This is readily apparent to the human eye on clear days; the sky radiation appears blue in color. As the sky becomes hazy, the sky radiation may become whitelgray in color, indicating that the degree of scattering is more uniform with respect to wavelength, i.e., T,(AA) and Ds(Ah) = l/A1.". However, if the aerosols have a small radius (on the order of the FIG. 1. Illustration of global solar radiation, sky radiation, and direct solar radiation. wavelength of the incident solar radiation) they can scatter in a bluish manner. The spectral distribution of Dc(Ah) is of a white represented in terms of atmospheric effects nature, as is readily apparent upon ob- () by serving normal clouds. The global solar radiation can no$ be I(Ah) = (R2/Rm2)Io(Ah)e - T(AA) sec 0, (3) given in more detail as where I,(AA) is the solar radiation on a sur- K (Ah) = face perpendicular to the solar beam outside I, (Ah) cos 0, e - (TR + TS + TA) sec 0, the 's atmosphere when the Earth is at DR(Ah) DS(AA) D,(AA). its mean distance Rmfrom the sun (solar con- + + + (6) stant), sec O0 represents the optical path With this equation the following measure- length through the atmosphere; R is the sun- ments can be interpreted and analyzed. Earth distance at the time of measurement of I(Ah). R21RmZis commonly referred to as the INSTRUMENTATION AND MEASUREMENTS reduction factor for mean sun-Earth dis- The following measurements of global tance. The expression T(AA) is commonly and sky radiation were made with two referred to as the extinction coefficient Eppley Model 2 spectral pyranometers which is composed of the molecular scat- (Figure 2). The Eppley instruments have tering (Rayleigh) extinction coefficient calibration constants of 5.03 and 5.20 T~(AA),the aerosol scattering extinction coef- millivolts (mv) per cal. cm.? min.-'. ficient T~(AA),and the absorption extinction The instruments were set up at the Wa- coefficient T~(AA);thus, terton Test Site (Figure 3), located approxi- mately 18 miles southwest of Denver, Colo. The site elevation is approximately 6,200 In this paper, only a qualitative descrip- feet above sea level; its coordinates are tion of these extinction coefficients is 105.13' longitude and 39.50" latitude. An needed, which is: insignificant influence from the Rocky

T,( a l/h4 (Rayleigh's- theory) 7.y a l/ha when 0 < a* < 4 (Mie scattering). The expression^.,, (Ah) is highly selective depend- ing on wavelength, i.e., 7~(Ah)is high if h corresponds to an absorption band of water vapor, , , etc. The sky radiation D is composed of dif- fuse, scattered solar radiation from the mo- lecular atmosphere () DR(Ah), the diffuse, scattered solar radiation from atmospheric aerosols () Ds(Ah), and solar radiation reflected from and/or transmitted by clouds D,-(Ah); thus, FIG.2. Eppley Model 2 pyranometer and ICA * According to Angstrom2 (1930), and Volz' Model 400 recorder used to measure global solar (19561, a = 1.3 iq a reasonable value. radiation. THE CLOUD BRIGHT SPOT

6

5

4

3

2

1

0

12 llam loam 9am 8 am Mounrain STD Time FIG.3. Waterton Test Site and solar radiation measuring instrumentation. FIG.5. Global solar radiation in the 0.700-2.8 pm region. Mountains front range exists with respect to global and sky radiation. This is because one of the mountains substends a solar elevation 0.295 to 0.700 pm (by deduction), 0.295 to angle of greater than 5" (see IGY Instruction 2.80 pm, and 0.700 to 2.80 pm. The type of Manual, Part VI); actually it substends 7". cloud that caused the global radiation envi- The corresponding azimuth angle sub- ronment shown in Figures 4 and 5 was docu- stended is only 7", making any influence mented with a fisheye photograph (Figure even less likely. 6). The outputs of the Eppleys were recorded The measurement shown in Figure 7 is on two ICA Model 400 recorders. This com- simply global radiation in the 0.295 to 2.80 bination results in an accuracy of 0.010 cal. pm region, but it represents the effects of a cm.-%in.-', a sensitivity of 0.005 cal. cm.? different cloud type (Figures 8 and 9) than min.-' and a three-second, full-scale re- the one shown in Figure 6. sponse time. The measurements shown in Figure 10 The resultant measurements shown in were made by continuously shading one Figures 4 and 5 were obtained by equipping Eppley while allowing the other to measure one Eppley (5.20 mv calibration constant) global solar radiation. This resulted in a with a WG-7 filter, transparent from 0.295 to simultaneous record of the effects of a cloud 2.80 micrometers (pm) and the other Eppley on the diffuse sky radiation, D (0.295 to with an RG-8 filter transparent from 0.700 to 2.80 pm), and the global radiation K J (0.295 2.80 pm. This combination resulted in mea- to 2.80 pm). surements of global solar radiation from

Mountain STD Time

FIG.4. Global solar radiation in the 0.295-2.80 FIG. 6. Fisheye photograph of cloua tnat pm region. caused bright spots shown in Figures 4 and 5. 8.0

7.0 --

Global Solar Radiation (1 ca1.m.-%in.-'- 5.03 w)

Ill IIIII 19.3.1 '23.9!1 33.1' 44. 1 I 1 Solar Zenith Angles 12 11 am 10 am 9 am Mountain Standard Time FIG.7. Global solar radiation showing effects of clouds in the 0.295-2.8 pm region.

Solar Zenith Angle 10 am. 9 em. 8 am. Mountain Standard Time FIG. 10. Simultaneous recordings of global and diffuse radiation.

FIG.8. Fisheye photograph of cloud that caused OF MEASUREMENTS bright spots shown in Figure 7. All of the measurements shown in Figures 4,5,7, and 10 are given in terms of millivolts of sensor output as plotted against zenith angle 0,. To obtain absolute units of cal. cm.? min.-', simply multiply the sensor output by the reciprocal of the specific sensor's calibration constant. Each set of measurements (except those in Figure 10) are referenced to a specific cloud type and cover by use of a fisheye photograph (Fig- ures 6,8,9). For each set of measurements a nominal clear day curve is given for comparison with the cloud effects. For Figures 4, 5, and 10, this clear day curve was generated by com- puter extrapolation of the actual measured clear day conditions that existed prior to the onset of cloud influences. Such an extrapola- tion is possible by performing measure- FIG.9. Same cloud shown in Figure 8, slightly ments during the cleur conditions in order to later in time. derive the atmospheric extinction coeffi- THE CLOUD BRIGHT SPOT cient (given in Equation 6), and the ratio of between the direct solar beam, the cloud(s), sky radiation to global radiation. Knowing the ground-based sensor, and the nature of these two parameters, and by calculating 8, the cloud(s) in terms of height, thickness, for various times of day, an extrapolation can etc. From measurements of all these vari- be made for the remainder of the day based ables one could perhaps determine what set on actual measurements during the "clear" of conditions result in the occurrence of conditions by using Equation 6. For Figure cloud bright spots. 7, conditions were never "clear" and there- One candidate explanation of the cloud fore the extrapolation method could not be bright spot is the reflection of the direct solar used. Instead, nominal values characteristic beam off the cloud and onto the ground of the test site for T, and the ratio of sky to (Figure 11). global radiation were picked and then used Salomonson4 (1968) performed aircraft to compute the extrapolation. measurements of cloud reflectance and con- The results shown in Figures 4 and 5 in- cluded that "clouds and snow at large solar dicate that the cloud bright spot shown in zenith angles exhibited pronounced reflec- Region A has about a 27-percent increase tion going away from the sun and stratus over clew conditions in the 0.295 and 2.80 clouds also exhibited a lesser increase in pm region, and a 26.5-percent increase in reflection back toward the sun." the infrared, 0.700 to 2.80 pm region. The With respect to a bright spot, the reflection cloud bright spot shown in Region B has a is not off the cloud top, but off the edge of the 23-percent increase in the 0.295 to 2.80 pm cloud. Thus, as shown in Figure 11, it is region and a 24-percent increase in the infra- likely that, at large solar zenith angles with red region. The fact that the percentage respect to the cloud's edge zenith, strong increases caused by the cloud are about reflections occur toward the ground. The equal for both wavelength regions is not magnitude of the reflected radiation would surprising because clouds are known to also be a function of the cloud edge surface scatter and/or reflect in a white manner. area. With much more detailed measure- The results shown in Figure 7 indicate ments, as mentioned above, the cloud- much more sporadic cloud bright spots bright-spot phenomenon could be compared which were caused by the nature of the to the reflectance measurements off the clouds and their rapid movement across the cloud tops as reported by Salomonson. How- sky as compared to the cloud shown in Fig- ever this would be difficult because the ure 6, which had a very slow movement edges of clouds tend to change rapidly both across the sky. The percentage increases in orientation (zenith) and size. caused by the cloud shown in Figures 8 and The cloud shown in Figure 6 seems to be a 9, vary widely but do cause increases as high case in point of the reflection phenomenon. as 21 percent. It is shown that the sun, cloud, and sensor Measurements in Figure 10 show simulta- are in the geometrical relationship illus- neous records of the sky radiation D (Ah)and trated in Figure 11. The bright spots labeled the global radiation KJ,(Ah) for the 0.295 to as A and B in Figures 4 and 5, are caused by 2.80 pm region. The results indicate a 0.28 reflection off the leading edge of the cloud. cal. cm.? min.-' increase (20 percent) in the In between A and B a deep shadow exists. If global solar radiation due to the cloud bright spots, and a 0.28-cal. increase (290 percent) in the sky radiation. Hence the increase in global radiation is equal to the increase in sky radiation. The magnitude of the global radiation during a bright spot is then at- tributed to the already existing direct solar beam, the normal sky radiation, plus the cloud radiation. The 290-percent increase in sky radiation attributed to D(Ah) then ac- counts for the 20-percent increase in global radiation. In order to describe fully the physical situ- ation that results in a cloud bright spot, one would have to conduct much more detailed measurements than those reported here. The major unknowns that should be mea- FIG.11. Cloud reflection causing a bright spot sured are the exact geometrical relationships on the ground. one then considers very heterogenous The cloud-bright-spot effect on a multi- clouds moving across the sky, the sporadic spectral line scanner image and photography bright spots shown in Figure 7 might be would be a false contrast recorded in the explained. The bright spot shown in Figure scan and the photograph. However, with 10 may have been caused by a very sharp widely varying contrasts that already exist in reflection off the cloud which rapidly moved natural scenes, the cloud bright spot would into the exact conditions needed for reflec- probably go undetected. tion. Again, it should be pointed out that If a cloud bright spot was detected and much more detailed measurements are measured on the ground during a multispec- needed to define the physical and geometri- tral scanner overflight, corrections to the cal relationships that cause bright spots. scanner data could possibly be made by operating on the data with a correction factor derived from the ground measurements. This factor would simply be the ratio of the Cloud bright spots can affect any remote- normal cleur conditions to the incident sensing technique that, in some manner, global radiation measures in the bright spot records the reflected solar energy (signature) area. By applying this factor to each of the off a ground target. This results from the fact scanner channel data, the bright spot data that targets under the influence of a cloud could be normalized to the cleur conditions. bright spot have an incident solar radiation If spectral measurements of the incident flux that is abnormally greater than the sur- radiation are taken in the normal clear areas, rounding areas; therefore the contrast that similar spectral measurements would not be exists between the target and its normal sur- necessary for the cloud bright spot area be- roundings is not the natural contrast. In ad- cause the incident radiation is increased dition, if the target's reflected radiance is fairly uniformly with respect to wavelength. actually being measured, the measurement Such scanner data corrections are ex- will be in error. As shown herein, this error tremely difficult to apply because of the could be as much as 30 percent. The influ- wide ground coverage normally performed ence of a cloud bright spot on an aircraft in remote-sensing missions, especially satel- and spacecraft remote sensor is shown in lite remote sensing. To use the corrections, Figure 12. one would need extensive ground measure- ments of the global solar radiation over the entire ground track. The minimal correc- tionlattention given to cloud bright spots with respect to scanner imagery and pho- tography is that the interpreter should be Satellite aware of the fact that if any shadows exist in the imagery, there is likely to be cloud bright spots as well. Figure 12 shows a narrow field-of-view satellite remote sensor measuring the re- 1 flected radiance off a ground target under the influence of a cloud bright spot. The re- sulting data would be in error as much as 30 percent. The narrow-field-of-view remote sensor is particularly susceptible to cloud bright spots because the area of coverage of both is similar. Although the exact coverage area of bright spots has not been deter- mined, they most certainly are on the order of a fraction of a kilometer or greater. spot The Skylab EREP S191 Infrared Spec- trometer can be seriously affected by cloud bright spots as it has a 0.46-km ground cover- age. Because of its narrow field of view and Nomd "Clear" pointability, it is intended to be used to collect target radiance even if broken clouds exist in the target area. Under such condi- FIG. 12. Cloud-bright-spot effect on aircraft tions, cloud bright spots would likely be and satellite remote sensing. present. However, due to the fact that the THE CLOUD BRIGHT SPOT

S191 is pointed at a known target, suitable measurements of the global solar radiation would easily be made that would quantita- tively describe the error and could be used to normalize the S191 data.

RECOMMENDATIONSFOR BRIGHT SPOT GROUNDTRUTH MEASUREMENTS As mentioned previously, it would be extremely difficult to instrument completely a remote sensing site in order to detect and evaluate the cloud bright spot influences. This is true for sensors which cover large ground areas such as a scanner and conven- FIG. 13. Typical field setup of global solar tional photography. However, a continuous radiation monitoring instrumentation on the recording of the test site global radiation Bonanza test site (NASA Site No. 185, ERAP Mis- environment on position at the site would sion No. 168). indicate whether the particular clouds pass- ing over that position actually caused bright spots. This would, in addition to calibrating result from cloud bright spots and shadows; the incident global solar radiation environ- an example of this is shown in Figure 10. ment, give an indication of the probability of Also a strip chart record during clear condi- other areas of the site being similarly in- tions will directly indicate to the ob- fluenced by the same cloud type. serverlinterpreter the nature of the global If one uses a narrow field-of-view remote solar radiation conditions during the normal sensor on a specific area less than a kilome- clear state. The interpreter, using this refer- ter in size, it is almost imperative to measure ence, can readily determine if any abnormal- continuously the incident global solar radia- ities were caused by clouds in the vicinity. tion if any broken clouds were in the vi- cinity. As mentioned previously, the Skylab EREP Sl9l operates with this limited field. The author wishes to express his apprecia- Therefore it is recommended that, despite tion to Dr. A. J. Drummond and Mr. H. W. the particular discipline (geology, agricul- Greer of the Eppley Laboratories for their ture, etc.) involved, candidate S19l test sites recommendations concerning instrumenta- employ measurements of the global solar tion. radiation.

~NSTRUMENTATIONFOR CLOUDBRIGHT 1. Hanson, K. J.: "Radiation Measurement on the SPOT GROUND-TRUTH MEASUREMENTS Antarctic Snowfield, a Preliminary Report," Instrumentation for measuring the effects Journul of Geophysical Research, Vol. 65, No. of clouds on global solar radiation and re- 3, pp. 935-945, 1960. cording this data, do not have to be complex. 2. Angstrom, K.: "On the Atmospheric Transmis- All that is required is a pyranometer (180" sion of Sun Radiation, 11," Geogr. Ann. 12, 131, 1930. field-of-view) and a suitable strip chart 3. Volz, F.: "Optik des Dunstes," Handb. recorder. An example of such a combination Geophys. 8, Kap. 14, 1956. is shown in Figure 13. It is important to use 4. Salomonson, V.: "Anistropy in Reflected Solar a fast response (about 3 sec full-scale) in- Radiation," Contract NASr-147, Colorado State strument and recorder because of the rapid University, Atmospheric Sciences Paper No. fluctuations in global solar radiation that can 128, 1968.