Optimization of MAS and MODIS Polar ocean cloud mask
Memmen, Sean P.
Durkee, Philip A.
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With the reduction of funding for sea ice reconnaissance flights, the National/Naval Ice Center needs to capitalize on the improvements in satellite technology. Imaging sensors such as AVHRR, DMSP/OLS, SSM/I and RADARSAT have been used to detect the presence of sea ice, but with the exception of SSM/I and RADARSAT, clouds are a major obstacle to viewing the surface. With NASA's development of the Moderate-resolution Imaging Spectroradiometer (MODIS) and MODIS Airborne Simulator (MAS), there is finally a sensor capable of using multi-spectral techniques to detect the presence of clouds. A group at the Space Science and Engineering Center (S SEC), University of Wisconsin - Madison lead by Dr. Steve Ackerman has developed a mask for MAS/MODIS. The technique determines a level of confidence that a given pixel is clear based on a series of multi-spectral tests. By combining the confidence level from all tests, it is possible to detect the presence of clouds at different altitudes in the atmosphere. Based on the Ackerman et al. (1997) scheme, threshold optimizations were made on the T(beta)(ll micrometers) and T(beta)(3.9 micrometers) - T(beta) (ll micrometers) tests, while the T(beta)(ll micrometers) - T(beta)(l2 micrometers) test was removed. These are daytime modifications based on analysis of several MAS and a limited number of MODIS cases. From subjective analysis, the modifications greatly improved the detection of clouds over cold polar oceans where sub-pixel ice may be present or water temperatures might falsely indicate clouds. The number of Cloudy pixels (<0.66 clear confidence level) for a given scene was increased 12.1% on average for MAS cases. The NPS cloud mask also classified two times more Probably Clear and Undecided pixels than the original mask due to eater sensitivity to thin, small clouds
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