Spectral Solar Radiation Data Base Documentation, Vol. I

Table of Contents

3.0 Approach


A total of 3,364 spectral data sets (16 MB) are included in the data base:

If the data acquisition systems were operating, but the atmospheric conditions were too unstable to record a spectrum, the broadband and meteorological data were included in the data base, but no quality-control processing was applied.

FSEC normally operated two spectroradiometers simultaneously, one in the direct-normal mode and the other in one of the global modes. PG&E operated one spectroradiometer, and rotated through the different measurement modes. SERI collected direct-normal, global-horizontal, and diffuse-horizontal spectra, mostly during cloudless-sky conditions.

Examples of the spectra are shown in Figures 4-1 through 4-5. Figure 4-1 (a,b) shows examples of spectra measured at FSEC in the direct-normal and global modes on a clear day. As air mass increases, the direct-normal spectra (a) shift toward longer wavelengths (the red end of the spectrum). The global spectra (b) are slightly less affected by the red shift because the radiation scattered out of the direct beam is added back into the global spectra as diffuse radiation, thus returning a portion of the short-wavelength (blue) radiation. Figure 4-1(c) shows the broadband solar radiation corresponding to the spectra.

Figure 4-2 shows examples of spectra measured under cloudy skies. The direct-normal spectra (a) are almost zero, and the global-tilt spectra (b) have low values. The graph of corresponding broadband solar radiation (c) shows nearly zero direct normal; the global values approach each other because all of the irradiance is diffuse under cloudy skies.

Figure 4-3 shows examples of spectra measured on a partly cloudy day. The graph also shows an example of data dropout during data transfer. The shape of the spectral curve is quite different for low-intensity spectra measured under cloud cover (10:31, air mass 1.88; 13:31, air mass 1.73) versus the low-intensity spectrum at higher air mass values (16:31, air mass 5.48).

Figure 4-4 shows two examples of spectra that appear to correspond with sun reflections off of clouds, which causes a "bright spot" [17] and a focusing effect on the solar radiometers. The broadband solar radiation values can actually approach or exceed the extraterrestrial value of about 1338 on day number 132/133. The global solar radiation values approach each other, while the direct-normal radiation drops indicating that the solar disk is partially blocked by clouds.

Figure 4-5 shows examples of direct-normal, global-horizontal, and diffuse-horizontal spectra measured by SERI. The diffuse radiation is mostly concentrated in the short-wavelength (visible) region of the spectrum (blue sky). The global-horizontal spectrum is equal to the direct-normal spectrum multiplied by the cosine of the zenith angle, plus the diffuse spectral radiation.

The cumulative number of these spectra versus air mass; the atmospheric descriptors Kt, Kn, D/GH; and relative humidity are shown in Figures 4-6 (a-f). The large number of spectra at low air mass values is caused by the operation of the spectroradiometers during normal working hours at mid-latitudes, and by the larger air mass values occurring near sunrise and sunset which occupy a relatively small fraction of the day. The large number of spectra during relatively clear-sky conditions is due to a selection of clear days for the data taken at SERI, the requirement for atmospheric stability during spectral scans, and perhaps climate types at the three sites. Figure 4-6(c) shows that relative humidity measurements were missing for about 25 global-normal spectra. The diffuse spectra associated with D/GH ratios near 1.0 indicate very clear conditions where diffuse measurements are probably within the measurement uncertainty.

Included on the data tape are daily field notes, quality control information, and measurement uncertainty. This information alerts the user to problems such as data dropouts (as seen in Figure 4-3) and noise in the spectrum (Figure 4-7). This information should always be reviewed when any spectral data sets are used.

Photographs or slides corresponding to many of the spectral data sets are available at SERI. The daily field notes that are included with the data document the existence and quality of the photos/slides.

5.0 Measurement Uncertainty

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