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NOAA-NESDIS
Regional And Mesoscale Meteorology
Team
Satellite Interpretation Discussion
Thursday August 27, 1998
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Water Vapor Difference Product
This product was generated in two steps. A current image was differenced with the image from the hour before to determine the changes in temperature that occurred.
Figure 1 shows an image developed from two images of the 7.43 um channel (the 9:46Z image subtracted from the 10:46Z), and the figure 2 image is developed from the
7.02 um channel. The color table show reds to display a warming of temperatures
over the hour time frame, while the blues display a cooling. The ends of the color bar show a maximum value of +or- 13 degrees centigrade.
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| Figure 1 |
Figure 2 |
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Figure 1
Sounder 7.43 um ch.,18 Aug. 1998 (10:46Z - 9:46Z)
click image to enlarge |
Figure 2
Sounder 7.02 um ch.,18 Aug. 1998 (10:46Z - 9:46Z)
click image to enlarge |
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| Taking a difference between these two images gives the final product. This method could give some idea about vertical motions or advection that is occurring. When a high cloud top is present, the two channels give similar images, and the product produces a white or neutral result. When a high cloud isn't present the two channels see water vapor at different levels in the atmosphere. The 7.43 um channel typically is most sensitive around 700 mb, while the 7.02 um channel is more sensitive at the 500mb level. To compare with the product is an IR image (fig. 3), and the normal water vapor image produced by the 7.43 um and 7.02 um channels (figs. 4 & 5).
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Fig. 3 (GOES Sounder IR)
click picture to enlarge
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| Figure 4 |
Figure 5 |
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Figure 4
GOES Sounder 7.43 um ch.,18 Aug. 1998 (11:46Z)
click image to enlarge |
Figure 5
GOES Sounder 7.02 um ch.,18 Aug. 1998 (11:46Z)
click image to enlarge |
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Interesting features to note in this loop of the water vapor difference product are the alternating warming and cooling bands
the are seen through Canada and upper Michigan. Also the relative cooling aloft (a blue
band) seen propagating out of the convective activity over Illinois. The possibility that
the bands are thermally induced gravity waves has been poised. The waves over Canada and
upper Michigan have a wavelength of approximately 140km and a propagation rate of 20 m/s.
Using a vertical wavelength of about 8 km which is similar to the distance between a 300 mb
pressure level and the tropopause, the internal gravity wave equation is satisfied;
however, this conjecture is still unconfirmed.
This product is still in the earliest stages and interpretation is still being developed. Its interpretation
is open to discussion and reader's suggestions and comments are welcome..
Ben Ruston
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