The spatial-scale dependence of the observed anisotropy of reflected and emitted radiation.

摘要

The Earth Radiation Budget Experiment (ERBE) uses Angular Dependence Models (ADMs) to convert satellite observed radiances to radiative fluxes at the top of the atmosphere. Owing to errors in scene identification and to the relationship between the spatial scales of cloud systems and the spatial resolution of the ERBE scanner, the anisotropy of the radiation fields determined from ERBE observations was suspected of exhibiting a field of view size dependence. In order to remove effects due to the spatial scale of cloud fields, ERBE scanner observations from the Earth Radiation Budget Satellite (ERBS) were averaged to construct observations having a constant size field of view for all scan angles. Comparing the anisotropy for constant size fields of view with that obtained using unaltered full-resolution scanner observations, it was found that there were significant and systematic differences of the order of 5-10% for all scene types.; The frequencies of occurrence for clear, partly cloudy, mostly cloudy and overcast cloud categories identified by the ERBE scene identification algorithm were calculated for the constant size field of view observations. It was found that the ERBE scene identification method failed to correctly identify scene types. A bispectral threshold method was developed for scene identification. In the determination of the thresholds, the ERBE scene identification method was assumed to be correct for nadir observations. The thresholds were then determined so that the population of scene types remained constant from nadir to limb for the constant size field of view observations. ADMs were developed using the threshold scene identification method. Results showed that the spatial-scale dependence of the ADMs was significantly reduced. The threshold ADMs satisfied the principle of reciprocity more closely than did the ERBE ADMs for all cloud categories. Using the threshold scene identification, the view zenith angle dependence of the global average albedo and the longwave flux were significantly reduced compared with those obtained using the ERBE scene identification. The estimated global average albedo increases from 0.282 for the ERBE algorithm to 0.299 for the threshold algorithm. There was no significant change for the value of the estimated longwave flux.