Influence of cirrus clouds on the VISSR atmospheric sounder-derived sea surface temperature determinations

Using a more realistic cirrus cloud model, the characteristics of transmittance, emittance, and optical thickness and their relationships to cirrus in a diverse set of cases are studied by solving the equation of transfer of IR radiation. The doubling method is employed in the multiple scattering calculation. The satellite-observed brightness temperatures for different cases are computed, and stepwise regression analyses are performed to yield retrieval equations for sea surface temperature (SST). It is shown that the radiative properties of cirrus depend strongly on particle concentration, thus on the optical thickness of clouds. For clear atmospheres, channel 8 (11.2 microm) is more transparent than other channels. For cirrus clouds only, when the optical thickness of cirrus tau(c) is <0.10, channel 8 is still more transparent, while, with tau(c) increasing from 0.2 to between 4 and 8, channel 12 (4 microm) becomes the most transparent. When tau(c) >/= 8, the transparency of channel 12 decreases and those of other channels increase. For a very large r, the transparency of VAS channels will become almost equal. In addition, the IR absorption emittance of cirrus and the brightness temperatures also have sensitivities to different cloud optical thicknesses. The general retrieval equation for the determinations of SST, which is suitable for the clear air model as well as for the cirrus cloud atmospheres (with our definition of cirrus), is obtained through a combination of channels 12, 8, 6 (4.5 microm), and 5 (13.3 microm).The retrieval error is <1.0 K. The error analyses indicate that the clear air retrieval equations should not be used for SST determination in cirrus conditions.