Infrared transmission through cirrus clouds: A radiative model for target detection

An IR transmission model for thin and subvisual cirrus clouds composed of hexagonal ice crystals with a specific use for target detection has been developed. The present model includes parameterizations of the ice crystal size distribution and the position of cirrus clouds in terms of ambient temperature. To facilitate the scattering and absorption calculations for hexagonal column and plate crystals in connection with transmission calculations, we have developed parameterized equations for their single scattering properties by using the results computed from a geometric ray-tracing program. The successive order-of-scattering approach has been used to account for multiple scattering of ice crystals associated with a target-detector system. The direct radiance, path radiance, and radiances produced by multiple scattering and background radiation involving cirrus clouds have been computed for 3.7- and 10-μm wavelengths. We show that the background radiance at the 3.7-μm wavelength is relatively small so that a high contrast may be obtained using this wavelength for the detection of airborne and ground-based objects in the presence of thin cirrus clouds. Finally, using the present model, including a simple prediction scheme for the ice crystal size distribution and cloud position, the transmission of infrared radiation through cirrus clouds can be efficiently evaluated if the target-detector geometry is defined.