The plane parallel homogeneous (PPH) bias is defined as the difference between the plane parallel cloud albedo, calculated for homogeneous cloud optical depth distributions, and the independent pixel (IP) albedo, which allows for optical depth variability, but assumes that individual cloudy columns transfer solar radiation as plane parallel slabs (horizontal photon transport is neglected). Estimates of the PPH bias from extensive Advanced Very High Resolution Radiometer (AVHRR) observations are provided for areas similar in size to weather and climate model gridboxes. The goal is to investigate conditions and assumptions influencing the PPH bias, and to suggest methods to correct for it. / Visible PPH biases vary from about 0.02 to 0.30, depending on area size, view/sun geometry, and other factors influencing optical depth retrievals and albedo calculations. Broadband PPH biases are slightly smaller than visible biases; broadband absorptance biases are about an order of magnitude smaller. Approximate estimates of the bias in broadband reflected flux at cloud top often exceed 30 Wm$ sp{-2}$ for near-nadir measurements, suggesting that the assumption of cloud homogeneity produces errors that cannot be ignored in climate studies. / Solar geometry affects the PPH bias not only through the direct dependence of albedo on solar zenith angle, but also through systematic changes in the apparent mean and variance of optical depth arising from the neglect of 3-d effects in satellite radiance inversions. PPH biases decrease with data resolution, increase when atmospheric radiative effects are accounted for in optical depth retrievals, and are only slightly affected by water cloud microphysics. PPH biases are also shown to differ substantially between the forward and backward scattering directions, and between large-scale models with and without provision for fractional cloudiness. / A large fraction of the PPH bias is removable by: (1) adjusting regionally averaged optical depths with the reduction factor of Cahalan et al. (1994a), and (2) fitting observed optical depth distributions with lognormal and (to a lesser degree) gamma distributions. These methods require the logarithmic mean and variance of optical depth, which are parameterized as a function of regional mean optical depth and cloud fraction, quantities routinely available in climate models.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.40417 |
| Date | January 1996 |
| Creators | Oreopoulos, Lazaros. |
| 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: 001537901, proquestno: NN19760, Theses scanned by UMI/ProQuest. |
Page generated in 0.0017 seconds