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Remote sensing over oceans of optically thin cirrus and its significance

From about 10 months of spectral data obtained by the Infrared Interferometer Spectrometer (IRIS) flown on Nimbus 4 satellite in 1970, we have examined the extinction characteristics of thin ice crystal clouds over the oceanic convective rain belts in the tropics and midlatitudes. The optical depth of these clouds in the IR window region is < 3 and their spectral features are consistent with cloud particle size of 10 Sum. These optically thin ice crystal clouds are extensions of the anvil clouds generated by deep convective systems. From the IRIS data the frequency of occurrence of these thin clouds is estimated to be about 30 % over the warm waters surrounding Indonesia. As a result these clouds can contribute significantly to the radiative heating of this region. The IRIS data also indicated the existence of optically thin ice clouds that extend from the upper troposphere into the lower stratosphere, in the polar regions, during winter and early spring. The spectral features of these polar cirrus clouds differ somewhat from that of the optically thin cirrus clouds in the tropics. From theoretical simulation of the infrared spectra in the 8-25 jim region we infer that these polar clouds have a vertical stratification in particle size, with larger particles (12 .,um) in the bottom of the cloud and smaller ones (< 1 um) aloft. Radiative transfer calculations also suggest that the equivalent ice water content of these polar clouds is of the order of 1 to 2 milligram per cmi. It is suggested that the 'ozone hole' in the region of Weddel Sea is linked, at least in part, to these ice crystal clouds.