Retrieval of Tropical Cirrus Thermal Optical Depth, Crystal Size, and Shape Using a Dual-View Instrument at 3.7 and 10.8μm

Abstract

Abstract In this paper the authors derive thermal optical depth at 3.7 and 10.8 μm for tropical cirrus utilizing Along Track Scanning Radiometer data under nighttime conditions. By analytically solving the equation of radiative transfer, inclusive of scattering, a pair of nonlinear equations can be solved for the optical depth. Stable and unique solutions for the optical depth are found by combining nadir and forward (55°) views. The accuracy of the analytic solution is compared with solutions from a numerical radiative transfer model assuming an isothermal cirrus cloud. Numerical solutions for two nonisothermal cirrus clouds are also compared with the analytic solution for an equivalent mean cloud temperature. The numerical model uses appropriate hexagonal column scattering phase functions at 3.7 and 10.8 μm. The largest analytic model error is shown to occur at nadir, and it is shown that this error is not overly sensitive to crystal size and is independent of cloud-top temperature. A correction to the ...