Energy Sources and their Importance (2 of 3)
Figure 2b shows a close-up view of the portion of Figure 2a (previous page) where the Planck curves for the sun and the earth overlap. Curves for solar reflection at 50% and 20% are shown as well. The exact combination of solar and terrestrial energy measured by the satellite at
3.9 um depends on the time of day as well as the reflectance and emissivity of the underlying surface. This combination of emitted terrestrial energy and reflected solar energy during daytime,
combined with information on cloud and surface characteristics, is one of the things that make interpretation of imagery at 3.9 um so interesting.
The contribution to the measured radiance temperature at 3.9 um, due to the
reflected solar component, may be determined from Fig. 2c. The set of curves shows
the radiance temperature that would be measured by a satellite for cloud tops at
several temperatures (with albedo = 0 and emissivity = 1.0), with respect to increasing
amounts of reflected solar radiation. Notice that the radiance temperature measured at
3.9 um begins to converge near an upper limit, around 350 K. This convergence occurs
because as cloud albedo increases, the addition of reflected solar radiance far
outweighs the cloud's emitted radiance (evident in Fig. 2b). When
inspecting Fig. 2c, keep in mind that cloud top temperatures above 285 K are rare.
The GOES-8 3.9 um sensor gain is set to saturate at 335 K, and 3.9 um saturation
for GOES-9 occurs at 325 K. Saturation for this sensor in future GOES Imagers, like
GOES-8, will be at 335 K.
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