By inspection of the image constructed from the brightness temperature difference between the 11μm and 12μm satellite data, semi-transparent cirrus clouds where brightness temperature in 11μm and visible reflectivity are relatively low are identified clearly as having a bigger brightness temperature difference area than that of cloud free areas.
Effective emissivity is determined for semi-transparent cirrus clouds using the simple cloud model where scattering is neglected and only absorption is considered. Cirrus clouds often have 'black' parts in this wavelength. The radiance of the 'black' part of the cirrus clouds whose temperature is assumed to be equal to that of semi-transparent part and the clear radiance just off the cirrus clouds which is assumed to be equal to the radiance at the bottom of the cloud are used for effective emissivity calculation.
A simple relationship between the effective emissivity for 11μm and 12μm has been determined empirically from 860 satellite measurements for eight cirrus clouds cases which have 'black' parts. The bi-spectral method has been developed to retrieve cloud temperature and effective emissivity for semi-transparent cirrus clouds using the effective emissivity relationship between 11μm and 12μm. We have compared the retrieved cloud temperature and effective emissivity by our method with those estimated from the 'black' part of the cirrus cloud. It shows reasonable agreement for the cirrus cloud whose effective emissivity is larger than 0.4.
Meteorological Society of Japan