Modeling Single Scattering and Radiative Properties of Cirrus Clouds

Abstract

Abstract : The specific objective of this research is the development and improvement of theoretical models to simulate the effect of nonsphericity on single-scattering properties of cirrus cloud particles in the visible and infrared spectral regions. First, we have shown that using a matrix inversion scheme based on a special LU factorization rather than on the standard Gaussian elimination significantly improves the numerical stability of T-matrix computations for nonabsorbing and weakly absorbing nonspherical particles. As a result, the maximum convergent size parameter for particles with small or zero absorption can increase by a factor of several and can exceed 100. Comparisons of T-matrix and geometric optics (GO) computations for large, randomly oriented spheroids and finite circular cylinders show that the range of applicability of the ray tracing approximation depends on the imaginary part of the refractive index and is different for different elements of the scattering matrix. Second, we use exact T-matrix computations and physical considerations based on the Kirchhoff approximation to show that the delta-function transmission peak predicted by the GO approximation for hexagonal ice crystals is an artifact of GO completely ignoring physical optics effects and must be convolved with the Fraunhofer pattern, thereby producing a phase function component with an angular profile similar to the standard diffraction component. We have performed this convolution with a simple procedure which supplements the standard ray tracing code and makes the computation of the phase function and its Legendre expansion both more physically realistic and more accurate.