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The Cloud Bright Spot

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The Cloud Bright Spot 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 sky 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 sun'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 scattering 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- (attenuation) 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 Earth'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, ozone, oxygen, etc. The sky radiation D is composed of dif- fuse, scattered solar radiation from the mo- lecular atmosphere (Rayleigh scattering) DR(Ah), the diffuse, scattered solar radiation from atmospheric aerosols (Mie scattering) 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).
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