Premium
Time‐series of zenith radiance and surface flux under cloudy skies: Radiative smoothing, optical thickness retrievals and large‐scale stationarity
Author(s) -
von Savigny C.,
Davis A. B.,
Funk O.,
Pfeilsticker K.
Publication year - 2002
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2001gl014153
Subject(s) - radiance , radiative transfer , zenith , radiative flux , smoothing , environmental science , atmospheric radiative transfer codes , scale (ratio) , flux (metallurgy) , solar zenith angle , remote sensing , physics , atmospheric sciences , optics , geology , materials science , mathematics , statistics , quantum mechanics , metallurgy
Cloudy sky zenith radiance time‐series covering spatial scales a few meters up to 200–400 km measured by a ground‐based photometer at 753 nm are investigated with 2nd‐order structure functions, and compared to broad‐band short‐wave column transmittance. A previously reported scale break occurs at scales on the order of the vertical cloud extension due to radiative smoothing (i.e., lateral photon transport by diffusion in optically thick clouds). We use simulated radiance and flux fields for 3D clouds to explain why optical depths can be extracted with reasonable accuracy from surface fluxes by using 1D radiative transfer theory at large‐enough scales. We also show clear evidence of a transition from nonstationary to stationary behavior, i.e., a scale break, occurring at spatial scales of a few tens of kilometers. We argue that this qualitative change in the correlations of remotely observed radiation fields is likely to carry over to the most highly variable inherent cloud property, namely optical depth.