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View angle dependence of cloud optical thicknesses retrieved by MODISThis study examines whether cloud inhomogeneity influences the view angle dependence of MODIS cloud optical thickness (tau) retrieval results. The degree of cloud inhomogeneity is characterized through the local gradient in 11 microns brightness temperature. The analysis of liquid phase clouds in a one year long global dataset of Collection 4 MODIS data reveals that while optical thickness retrievals give remarkably consistent results for all view directions if clouds are homogeneous, they give much higher tau-values for oblique views than for overhead views if clouds are inhomogeneous and the sun is fairly oblique. For solar zenith angles larger than 55deg, the mean optical thickness retrieved for the most inhomogeneous third of cloudy pixels is more than 30% higher for oblique views than for overhead views. After considering a variety of possible scenarios, the paper concludes that the most likely reason for the increase lies in three-dimensional radiative interactions that are not considered in current, one-dimensional retrieval algorithms. Namely, the radiative effect of cloud sides viewed at oblique angles seems to contribute most to the enhanced tau-values. The results presented here will help understand cloud retrieval uncertainties related to cloud inhomogeneity. They complement the uncertainty estimates that will start accompanying MODIS cloud products in Collection 5 and may eventually help correct for the observed view angle dependent biases.
Document ID
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Marshak, Alexander
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Varnai, Tamas
(Maryland Univ. Baltimore County MD, United States)
Date Acquired
September 7, 2013
Publication Date
November 1, 2005
Subject Category
Meteorology And Climatology
Funding Number(s)
PROJECT: RTOP 621-30-86
PROJECT: RTOP 622-42-57
Distribution Limits
Work of the US Gov. Public Use Permitted.
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