A multispectral technique for detecting low-level cloudiness near sunrise

A technique using the Geostationary Operational Environmental Satellite ( GOES) sounder radiance data has been developed to improve detection of low clouds and fog just after sunrise. The technique is based on a simple difference method using the shortwave (3.7 mu m) and longwave (11.0 mu m) window bands in the infrared range of the spectrum. The time period just after sunrise is noted for the difficulty in being able to correctly identify low clouds and fog over land. For the GOES sounder cloud product this difficulty is a result of the visible reflectance of the low clouds falling below the "cloud" threshold over land. By requiring the difference between the 3.7- and the 11.0-mu m bands to be greater than 5.0 K, successful discrimination of low clouds and fog is found 85% of the time for 21 cases from 14 September 2005 to 6 March 2006 over the GOES-12 sounder domain. For these 21 clear and cloudy cases the solar zenith angle ranged from 87 to 77; however, the range of solar zenith angles for cloudy cases was from 85 degrees to 77 degrees. The success rate further improved to 95% ( 20 out of 21 cases) by including a difference threshold of 5.0 K between the 3.7- and 4.0-mu m bands, requiring that the 11.0-mu m band be greater than 260 K, and limiting the test to fields of view where the surface elevation is below 999 m. These final three limitations were needed to more successfully deal with cases involving snow cover and dead vegetation. To ensure that only the time period immediately after sunrise is included the solar zenith angle threshold for application of these tests is between 89 and 70 degrees.