The purpose of this study is to assess the performance of the plane parallel model in analyzing satellite measurements of clouds, and to examine what role 3D cloud effects may play in explaining any discrepancies. Direct comparisons between one year of Earth Radiation Budget Satellite (ERBS) scanner radiances and plane parallel model calculations are carried out under different Sun-Earth-satellite viewing configurations over ocean between 30$ sp circ$N and 30$ sp circ$S. When the plane parallel model calculations are matched to observations at nadir on a pixel-by-pixel basis by adjusting cloud fraction and cloud optical depth, the resulting frequency distributions of cloud optical depth show a systematic shift towards larger values with increasing solar zenith angle, regardless of what assumptions are made in the calculations. On average, this increase in cloud optical depth is extremely large for solar zenith angles $ sbsp{ tilde}{>}$63$ sp circ.$ For the thinnest 50% of the clouds, the increase only occurs at very oblique sun angles, whereas it is observed at all solar zenith angles for the thickest 10% of clouds. The cause is traced to a fundamental flaw in plane parallel theory when applied to real clouds: at nadir the solar zenith angle dependence of model reflectance is opposite to that of the observations. / On average, differences between observed and plane parallel model reflectances are found to be less sensitive to view and relative azimuth angle than to solar zenith angle. For solar zenith angles less than ${ approx}63 sp circ,$ plane parallel model reflectances are within $ approx$10% of the observations. As solar zenith angle increases, differences between the observations and calculations increase at all view angles. At lower sun elevations, observed reflectances exceed plane parallel values by a constant amount at all view angles in the backscattering direction, while plane parallel model reflectances show a different view angle dependence from that observed in the forward direction. When comparisons are performed as a function of relative azimuth angle, no appreciable dependence in the reflectance difference is observed. Violation of the principle of reciprocity applied to real observations is shown to be mainly caused by the systematic difference in the solar zenith angle dependence between observations and plane parallel calculations. / Monte Carlo simulations involving stochastic, isotropic, scale-invariant broken cloud fields are carried out in order to show that, qualitatively, differences between observed and plane parallel reflectances are generally consistent with 3D theory. While much of the discrepancy between 3D and 1D reflectances can be attributed to the presence of cloud sides, affecting the illuminated cloud area, it is shown that the slope of the illuminated cloud top surfaces may also play an important role.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.40181 |
| Date | January 1996 |
| Creators | Loeb, Norman Gary |
| Contributors | Davies, Roger (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Atmospheric and Oceanic Sciences.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001486392, proquestno: NN12420, Theses scanned by UMI/ProQuest. |
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