2.5 Net Solar Flux on a Sloping Surface
The net solar flux on a sloping surface is given as
∆I s = ( I sc↓iff + I sc↓ir ) - I sc↑ .
d
d
(2.20)
2.6 Shortwave Solar Flux Model Validation
The Smart Weapons Operability Enhancement (SWOE) field program data were used to
evaluate the shortwave solar flux model. The SWOE field programs were conducted
during the fall of 1992 (Grayling I-Grayling, MI), the spring/summer of 1993 (Yuma,
AZ), and the winter/spring of 1994 (Grayling II-Grayling, MI). Each program ran for
approximately 44 days and meteorological information was collected at several locations
in a 2-km-square area with a temporal frequency of one minute. Hourly average values of
the meteorological information were used to evaluate the solar flux model. The hourly
average values were computed using the one-minute values for a period of 30 minutes
before and after the hour. The exception was the cloud amount information. During
Grayling I, observations of the cloud cover were made on the hour from approximately
0700 to 1900 local. It should be realized that the cloud amount might not be consistent
with the hourly average values of the measured solar flux. Since the exact time of the
cloud observations is unknown, it is not possible to use the one-minute observation that
corresponds to the cloud amount. Cloud amount can have a significant impact on both the
direct and the diffuse solar flux. In general, an increase in the cloud amount will decrease
the direct component of the solar flux and will increase the diffuse (more of the direct is
scattered as diffuse) up to some value of cloud optical depth. Beyond this value the
diffuse will decrease. The analysis presented deals only with the daytime solar flux
values, that is, observations and model calculated values when the solar zenith angle is
less than 90 degrees.
Figure 2.1 is a comparison of the model calculated total solar flux and the corresponding
measured flux for a 10-day period. Some of the differences can be directly attributed to
the fact that the measured values are one-hour averages while the calculated values
basically use the instantaneous cloud observation. The phasing and the amplitudes match
fairly well for clear conditions. Differences occur mainly for cloudy conditions. The
components of the solar flux are presented in Figures 2.2 and 2.3. As the cloud amount
increases, the direct solar flux decreases while the diffuse tends to increase. The solar
flux model does not require cloud optical depth information. The dependence on optical
depth is implicit in the coefficients for transmission, reflectance, and the cloud weighting
function. As indicated above, the diffuse component should decrease when the optical
depth exceeds some critical value.
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