Properties of reflected sunlight derived from a Green's function method

The inference of optical depth and particle size of clouds and aerosols usi ng remotely sensed reflected radiance at solar wavelengths has received muc h attention recently. The information these measurements provide is path in tegrated. However, very little is known about the vertical distribution of this weighting. To characterize it, we first solve the radiative transfer e quation (RTE) by a Green's function approach, and then investigate the sens itivity of the weighting to vertical inhomogeneities in the extinction by i ntroducing a function that is closely related to the Green's function, here in called the contribution function. This function calculates the contribut ions to the radiance at the upper boundary of the medium by underlying laye rs. Three hypothetical clouds of identical optical depth but exhibiting dif ferent extinction profiles were used in this study. The contribution functi on was found very sensitive to the extinction profile. The global reflectio n and transmission matrices used to construct the Green's function, derived using an eigenmatrix method, resulted in an efficient, stable, and accurat e method for calculating the emerging radiances that can be extended to mul ti-layered media. (C) 2001 Elsevier Science Ltd. All rights reserved.