Radiative smoothing in zenith-scattered skylight transmitted through optically thick clouds to the ground

Abstract

A ground-based photometer was used to monitor the temporal variation of the zenith-scattered skylight radiance (at λ = 752 nm) with a temporal resolution of 2 Hz. In detail, eight data sets for different optically thick (τ(c) ≥ 1/(1-g) with τ(c) being the cloud's optical thickness and g the asymmetry factor for Mie scattering) cloud covers were analyzed with standard spectral methods. A general feature of all cloudy sky radiance power spectra is the presence of a remarkable scale break occurring at spatial scales that approximately correspond to the vertical cloud extension. This finding is consistent with theoretical estimates by Marshak et al. [1995] and Davis et al. [1997b] based on radiative smoothing by photon diffusion in vertically extended, optically thick and homogeneous clouds.