A Three-Dimensional Radiative Transfer Model to Investigate the Solar Radiation within a Cloudy Atmosphere. Part I: Spatial Effects

Abstract

A new Monte Carlo?based three-dimensional (3D) radiative transfer model of high spectral and spatial resolution is presented. It is used to investigate the difference in broadband solar radiation absorption, top-of-the-atmosphere upwelling, and surface downwelling solar radiation in a cloudy atmosphere between 3D and 1D calculations. Spatial variations of these same radiation components (absorption, upwelling, and downwelling), together with pathlength distributions, are analyzed for different wavelengths to describe the main physical mechanisms at work. The model contains all of the important atmospheric and surface radiative constituents. It includes Rayleigh scattering, absorption and scattering by both cloud droplets and aerosols, and absorption by the major atmospheric gases. Inputs include 3D liquid water fields, aerosols, and gas distribution, type, and concentrations. Using satellite imagery of a tropical cloud field as input, model results demonstrate that various plane-parallel (1D) assumptions can underestimate atmospheric absorption when compared to 3D computations. This discrepancy is caused by a complex interaction of gaseous absorption, cloud droplet absorption, and the solar zenith angle. Through a sensitivity analysis, the authors demonstrate that the most important factor is the morphology of the cloud field, followed by the vertical stratification of water vapor.