NOAA-NESDIS
Regional And Mesoscale Meteorology Team
Daily Satellite Discussion
Friday June 13, 1997

    Visible satellite images of warm, tropical oceans, typically show plentiful small, shallow, low-level cumulus clouds. With 10.7 micron IR images, those clouds may not be seen or are not nearly as evident. This is primarily due to the lower resolution, the small thermal contrast between the clouds and the sea, and water vapor attenuation, which is quite large over warm oceans. In addition, many of the enhancements used with 10.7 micron IR images have not been designed for optimum viewing of the low-level cumulus. Satellite analysts have traditionally used the day-time visible images to obtain good depictions of low-level winds in the otherwise clear regions and simply gave up this capability at night, when the visible data is unavailable.

    The improved quality of the 3.9 micron IR data from GOES-8 & 9 offers the opportunity to recover some of the information lost by not having visible data available at night. The night-time 3.9-micron imagery, at full resolution (2.6 x 4.0 km), can be viewed with an enhancement which "stretches" the data in the narrow radiance range which includes both the low-level cumulus and the sea-surface. The 3.9-micron imagery then takes on an appearance much like the day-time visible imagery, where deep convection and cirrus clouds are absent.

    The two figures shown are nighttime 10.7 and 3.9 micron images. They are centered on Jamaica in the Caribbean Sea. Notice the resemblance that the 3.9 micron image has to a Visible image.

    Carol Vaughn

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