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NOAA-NESDIS
Regional And Mesoscale Meteorology
Team
Satellite Discussion
Tuesday June 2, 1998
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| Today's picture focuses on southwest Texas where record
setting temperatures caused a false "low-reflectivity" signature on the
reflectivity product. This effect begins about midday when both the
3.9 um and 10.7 um radiances reach near-saturation at the warm end of the
scale. Figure 1 shows a late morning reflectivity image when temperatures
on both channels have not quite reached saturation. As heating continues
large portions of western Texas suddenly turn black on the reflectivity
imagery (see Figure 2). |
| Figure 1 |
Figure 2 |
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Figure 1
(NOAA GOES-9 Reflectivity Product at 1715 UTC)
click image to enlarge |
Figure 2
(NOAA GOES-9 Reflectivity Product at 2030 UTC )
click image to enlarge |
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| Recall that, during the day,the reflectivity product is
made by using the 10.7 temperature to calculate an equivalent radiance
at 3.9 um. This value is then subtracted from the measured radiance
at 3.9 um. In the early image the 3.9 um temperatures are warmer than those
at 10.7 um. However, when both channels reach near-saturation, the temperature
differences are negligible and may even be negative. The resulting
radiance difference is near zero. Figures 3 and 4 show the individual 10.7
and 3.9 um images that were used to produce Figure 2. In these images,
the average temperature within the boxes are 51.5C for the 3.9 um image,
and 54.6C for the 10.7 um. Therefore, the difference is negative
and the output is plotted as zero (black) in the reflectivity product. |
| Figure 3 |
Figure 4 |
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Figure 3
(NOAA GOES-9 10.7 um IR at 2030 UTC)
click image to enlarge |
Figure 4
(NOAA GOES-9 3.9 um IR at 2030 UTC)
click image to enlarge |
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