Properties of cirrus from multispectral AVHRR imagery data

The discovery that the 11 and 12 microns window channels of AVHRR could be used to detect and even characterize the properties of cirrus stimulated the present study which reexamines the general multispectral approach for retrieving cirrus cloud top temperature and emissivity. The generalized multispectral approach described compliments the CO2 slicing method used by Wylie and the bispectral threshold methods used by Minnis et al. While the results shown were for 11 and 12 micron radiances, better definition of the cloud top temperature is probably obtainable using 3.7 micron radiances in combination with the 11 and 12 micron radiances. During the day reflection of solar radiation at 3.7 micron by low level water clouds makes the analysis untenable. At night, at least with the NOAA-9 AVHRR, instrument noise in the 3.7 micron channel also makes the analysis untenable. The identification of semitransparent systems using 3.7 micron radiances has been noted elsewhere.