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Relationship Between Surface Reflectance in the Visible and Mid-IR used in MODIS Aerosol Algorithm-TheoryData from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument that flies in polar orbit on the Terra platform, are used to derive the aerosol optical thickness and properties over land and ocean. The relationships between visible reflectance (at blue, rho(sub blue), and red, rho(sub red)) and mid-infrared (at 2.1 microns, rho(sub 2.1)) are used in the MODIS aerosol retrieval algorithm to derive global distribution of aerosols over the land. These relations have been established from a series of measurements indicating that rho(sub blue) is approximately 0.5 rho(sub red) is approximately 0.25 rho(sub 2.1). Here we use a model to describe the transfer of radiation through a vegetation canopy composed of randomly oriented leaves to assess the theoretical foundations for these relationships. Calculations for a wide range of leaf area indices and vegetation fractions show that rho(sub blue) is consistently about 1/4 of rho(sub 2.1) as used by MODIS for the whole range of analyzed cases, except for very dark soils, such as those found in burn scars. For its part, the ratio rho(sub red)/rho(sub 2.1) varies from less than the empirically derived value of 1/2 for dense and dark vegetation, to more than 1/2 for bright mixture of soil and vegetation. This is in agreement with measurements over uniform dense vegetation, but not with measurements over mixed dark scenes. In the later case the discrepancy is probably mitigated by shadows due to uneven canopy and terrain on a large scale. It is concluded that the value of this ratio should ideally be made dependent on the land cover type in the operational processing of MODIS data, especially over dense forests.
Document ID
20020083259
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Kaufman, Yoram J. (NASA Goddard Space Flight Center Greenbelt, MD United States)
Gobron, Nadine (Institute for Environment and Sustainability Ispra, Italy)
Pinty, Bernard (Institute for Environment and Sustainability Ispra, Italy)
Widlowski, Jean-Luc (Institute for Environment and Sustainability Ispra, Italy)
Verstraete, Michel M. (Institute for Environment and Sustainability Ispra, Italy)
Lau, William K. M.
Date Acquired
September 7, 2013
Publication Date
January 1, 2002
Subject Category
Earth Resources And Remote Sensing
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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