NOAA-NESDIS
Regional And Mesoscale Meteorology Team
Daily Satellite Discussion
Friday December 12, 1997
The topic of today's images is the interpretation of the water vapor (6.7 um) channel. Figure 1 is a 6.7 um image with 16 km resolution valid at 0100 UTC on Dec 5. This image combines the 6.7 um images from GOES 8 and GOES 9. Note that the brightness temperatures over MN are low, which is usually interpreted as an abundance of mid-and upper-level moisture, while the FL panhandle is indicating higher brightness temperatures, usually interpreted as drier. In this case the typical interpretation is wrong, as shown by radiosonde profiles in Figure 2.

Figure 1
6.7 micron water vapor image
(click to enlarge)
Figure 2
Skew-T diagrams from MN and FL
(click to enlarge)
Given a temperature profile, increased moisture at mid and upper levels will result in a decreased 6.7 um brightness temperature. This is because the additional moisture reduces the 6.7 um signal originating at the lower, warmer levels. In this case, however, the temperature profiles are different. Here the 6.7 um brightness temperature over MN is colder, not because there exists more moisture, but because the mid and upper level water vapor is colder. When using 6.7 um imagery, both the temperature and moisture profiles must be taken into account.

Eric Hilgendorf and Jack Dostalek

We welcome your comments and discussion at ramsdis@comet.ucar.edu

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