Determination of three-dimensional cloud structures from high-resolution radiance data

Abstract

The three-dimensional structure and the inhomogeneity of clouds poses a field of challenges. The characterization of their spatial structure, their microphysical properties, and their variability is difficult. On the other hand this kind of knowledge is crucial to any investigation on the impact of clouds on the radiation budget or on the reliability of cloud remote sensing data. In this article the characteristics of horizontal transport of radiation in inhomogeneous clouds are studied using three-dimensional (3D) simulations of radiative transport and the independent pixel approximation (IPA). The opposing 3D effects of radiative smoothing and sharpening due to horizontal photon transport are examined in terms of the Green’s function which describes the interrelation of the radiance fields calculated using IPA and 3D radiative transport. Based on these considerations a novel method was developed allowing the determination of realistic 3D stratocumulus structures from high spatial resolution radiance fields observed by a compact airborne spectrographic imager (CASI, 15 m resolution). An initial 3D distribution of liquid water content and effective droplet size retrieved using the IPA assumption and an adiabatic microphysical model is adjusted with the objective to reproduce the observation by a 3D forward radiative transfer simulation. Therefor an approximate Green’s function is utilized in an iterative routine to remove 3D effects from the observation. A set of 27 stratocumulus cloud structures is provided for subsequent investigations. The capability of the method is characterized through a test case and the comparison of average cloud properties to in-situ data of various field campaigns.