Thin Cirrus Clouds: Seasonal Distribution over Oceans Deduced from Nimbus-4 IRIS

Spectral differences in the extinction between the 10.8 and 12.6 μm bands of the infrared window region, due to optically thin clouds, are observed in the measurements made by a broad-band infrared aircraft radiometer. Similar spectral properties are also revealed by the measurements made by the high-resolution infrared interferometer spectrometer (IRIS) aboard the Nimbus-4 satellite, which had a field of view of ∼95 km. These observations show that the extinction due to cloud particles at 12.6 μm is appreciably larger than that at 10.8 μm. Both water or ice particles in the clouds can account for such spectral difference in extinction provided that the particles are smaller than the wavelength of radiation. This spectral effect is demonstrated with the help of multiple scattering radiative transfer calculations. As the IRIS data reveal this spectral feature, about 100 to 200 km away from the center of high altitude cold clouds (∼230 K), it is inferred that this feature is related to the spreading of cirrus clouds. Based on this hypothesis, we have deduced mean seasonal maps of the distribution of thin cirrus clouds over the oceans from 50°N to 50°S from the IRIS data. These maps reveal that, over the oceans, such clouds are often present over the convectively active areas such as ITCZ, SPCZ, and the Bay of Bengal. These results have application to studies of earth radiation balance and climate.