Energy Sources and their Importance (1 of 3)

Figure 2a shows the Planck blackbody radiance curves for the sun (6000 K) and the earth (300 K). The energy received from the sun at the top of the atmosphere is represented by the area under the left-hand curve, and energy emitted by the earth is represented by the area under the right-hand curve. If all the sun's energy reaching the earth were reflected back to the satellite, a satellite detector would sense the values represented by the solar curve (the left side of Fig. 2a). However, about 50% of the sun's energy is selectively absorbed by various atmospheric constituents (ozone, water vapor, molecular oxygen, carbon dioxide, certain aerosols) and the earth's surface. The remainder is scattered back to space by aerosols and reflected by clouds and the earth's surface. That scattering and reflection is a function of wavelength and the particular constituent (cloud phase/droplet size, soil type, etc.) with which the interaction is occurring. This reflected and back scattered solar energy can be detected by a satellite sensor. The vertical lines in the figure locate the spectral region sensed by GOES in the 3.9 um band. Satellite detectors do not measure energy at a single wavelength, the GOES imagers' 3.9 um channel extends from 3.78 - 4.04 um. In the figure, notice that satellite measurements in the 3.9 um band are a combination of earth emitted and solar reflected radiation.