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Título: Reflection of solar radiation by inhomogeneous clouds
Autores: Várnai, Tamás.
Fecha: 1996
Publicador: McGill University - MCGILL
Fuente:
Tipo: Electronic Thesis or Dissertation
Tema: Physics, Atmospheric Science.
Descripción: The goal of this study is to improve knowledge about how cloud inhomogeneities affect the reflection of solar radiation. In particular, it addresses two main questions: what the processes through which inhomogeneities influence solar reflection are, and how this influence can be taken into account in albedo retrievals based on future satellite measurements.
The first question is important, since present methods give numerical results only about the overall radiative effect of cloud inhomogeneities, but cannot determine the degree to which various mechanisms are responsible for this overall effect. This study establishes a theoretical framework which defines and evaluates the various processes through which cloud inhomogeneities influence solar radiation. This framework is then used to examine quantitatively the inhomogeneity effects that occur in irregular cloud fields. Among other insights, it is shown and explained that identical variations in cloud optical thickness can cause much stronger inhomogeneity effects if they are due to variations in geometrical cloud thickness, and not in volume extinction coefficient (as assumed in previous studies of irregular cloud fields). The differences in albedo can exceed 0.05, and the relative differences in reflectance toward the zenith can be greater than 25% for overhead sun, and 50% for oblique sun. Also, a possible explanation is given for a phenomenon observed in previous studies: that cloud reflectivity toward the zenith increases with decreasing solar elevation.
This study also develops an albedo retrieval algorithm that considers radiative inhomogeneity effects. The algorithm takes advantage of the unique multi-view capability of the Multi-angle Imaging SpectroRadiometer (MISR) placed on the Earth Observing System-AM satellite (to be launched in 1998). This instrument will offer new possibilities for albedo retrievals since, unlike present instruments, it will measure the radiation reflected in not only one, but nine directions. The potential accuracy of the algorithm is analyzed for a dataset obtained by using a Monte Carlo model to simulate radiative transfer through a large number of irregular cloud fields. The results indicate that using multi-view measurements can improve the accuracy of satellite-based albedo retrievals by a factor of three or more.
Idioma: en