On the Bispectral Method for Cloud Parameter Determination from Satellite VISSR Data: Separating Broken Cloud and Semitransparent Cloud

Abstract

Using a physical model, bispectral curves of visible albedo versus infrared brightness temperature are derived for various idealized cloud layers. These layers vary between those which are unbroken with variable optical depth (cirrus) to those which are broken, but the clouds have a uniform high optical depth (strato-cumulus). It is shown that the above two cases can be distinguished by the different shapes of the bispectral curves. The effects of variable solar angle and satellite viewing angle on the shapes of the bispectral curves are investigated. The effects of infrared scattering and deep cloud layers are also investigated and found to be important for high clouds with variable optical depth, such as cirrus. The predicted curves are compared with two-dimensional bispectral histograms obtained from the VISSR radiometer of the GMS-1 satellite. The theoretical curves compare quite well qualitatively. Two detailed quantitative comparisons for clouds which are assumed to be semitransparent high clouds give good agreement. The limitatins of the model are discussed.