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Regional And Mesoscale Meteorology Team Daily Satellite Discussion Thursday May 15, 1997 |
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Pictured above is a 4-panel image displaying the an area in Central America for channel 2 (3.9 micron; upper left), the reflectivity product (upper right), a visible image (lower left) and channel 4 (10.7 micron; lower right). Sunglint is shown in the lower right portions of the images. Since sunglint is a result of reflected energy, it shows up in the 3.9 micron and visible imagery, but not in the 10.7 micron imagery. Because of the 3.9-10.7 difference, it also shows up in the reflectivity product. A measured 10.7 micron temperature in this region of 293K (20C) had a corresponding radiance of 0.71 milliwatts/meter**2/steradian/cm-1 (written as R after this) in the 3.9 micron channel. The satellite measured radiance for the same point in 3.9 microns was 3.43 R, giving a difference of 2.72 R. Screen displayed values between 0.1 R and 1.7 R are stretched to represent brightness counts between 1 and 255. The radiance difference of 2.72 R is above the scale and set to 255 (hence bright!). The ice cloud in the center of the image has a measured temperature in the 10.7 micron band of 201 K (-72C). This temperature would have a radiance of 0.003 R in the 3.9 micron band, with the actual satellite measured value of 0.074 R, which after scaling gives a brightness count of 24 (on the dark side). Recall that ice clouds are poor reflectors. There is a fire in the upper left portion of the image (it is not in the Caribbean Sea, but on a Peninsula that does not get drawn with the map). The fire can be seen in the 3.9 micron but not in either the visible or the 10.7 micron imagery. The measured 10.7 micron temperature of 290K (17C) is close to that of sunglint, and here it would give a radiance of 0.62 R for the 3.9 micron band. The satellite measured radiance in 3.9 microns is 2.28 R giving a difference of 1.66 R scaled to 248 brightness counts. Both the fire and the sunglint appear bright in the reflectivity image due to the large difference between the 3.9 micron radiance and 3.9 micron radiance computed from the 10.7 micron temperature, but these differences are due to different physical processes; greater heat sensitivity in 3.9 than 10.7 microns for the fires and sensitivity of 3.9 microns to reflected energy for the sunglint. Bernadette Connell We welcome your comments and discussion at ramsdis@comet.ucar.edu |