Retrieval of cirrus properties from high spectral resolution IR measurements

We have been investigating the potential of high spectral resolution infrared measurements (600-2500 cm(-1)) such as those made by the HIS aircraft instrument and anticipated from the Infrared Atmospheric Sounding Interferometer instrument (IASI), to be launched on the METOP-1 satellite by the end of 2001, for the retrieval of cirrus cloud properties. Simulations investigating the sensitivity of optical properties to particle size and shape have been carried out for a number of different particle distributions. Ray tracing for randomly oriented hexagonal columns, Mie theory for spheres and the T-matrix formulation for spheroids of different aspect ratios, have been used. The effect of particle size, shape, and cloud ice water path (IWP) on the exitant radiance has been investigated. Comparisons of the exitant radiance for clouds of equal IWP and differing microphysics are shown. From these studies some spectral regions showing sensitivity to cloud microphysics are identified and spectral difference plots are used to display the different behavior exhibited by different particle distributions. As a comparison, some examples of aircraft data obtained with the WS instrument are also shown. It is concluded that particle size and shape have distinct spectral signatures and there appears to be some potential for using particular spectral variations to retrieve radiatively important microphysical and physical cloud parameters.