Regional And Mesoscale Meteorology Team
Daily Satellite Discussion
Friday July 11, 1997
Today's imagery demonstrates one method of cloud-clearing to determine Sea Surface Temperatures (SST) from satellite. GOES 3.9 micron imagery at night will reveal small changes in temperatures over the cloud free ocean and is used in SST determination, for several reasons. GOES can observe an area as frequently as once every 15-to-30 minutes, providing a greater probability of cloud free observations. At 3.9 um, moisture contamination is less than it is in the Longwave IR and since it is a cleaner window; diffraction is less, which effectively means higher resolution; and lastly,cloud sensitivity is less. Above (left) is one of the nighttime 3.9 micron images that was used as input into the cloud-clearing algorithm. The data has been "stretched" in the narrow radiance range which includes both the low-level cumulus and the sea surface, in order to show all the cloudy areas.
Thirty minute interval imagery, over the course of one night was chosen to do the cloud clearing. Clouds were eliminated from each image by disgarding pixels on an image which were determined colder than a threshold value. The nighttime images were then combined to maximize the cloud-free areas. Above (right)is the resulting "cloud-cleared" image. Since cloud detection is a trade-off between retaining some cloudy pixels but not removing valid SST pixels, or vice versa, it should be stressed that some cloudy pixels may still remain.
The last image (left) has been enhanced to accentuate trends, such as the warmer waters of the Gulf Stream. Even when there is an abundance of clouds over the ocean, as in this case, cloud-clearing is a way to highlight the clear areas to be able to resolve SST trends and tendencies.
Carol Vaughn