|
|
|
Kaufman, Y.J., N. Gobron, B. Pinty, J.-L. Widlowski and M. M. Verstraete (2002, accepted). Relationship between surface reflectance in the visible and mid-IR used in MODIS aerosol algorithm - theory. GRL, 29(23), doi:10.1029/. Data from the Moderate Resolution Imaging Spectroradiometer
(MODIS) instrument that flies in polar orbit on
the Terra and Aqua satellites, are used to derive the aerosol
optical thickness and properties over land and ocean. The
relationships between visible reflectance (at blue, rblue, and
red, rred), and mid-infrared (at 2.1 mm, r2.1) are used in the
MODIS aerosol retrieval algorithm to derive global
distribution of aerosols over land. These relations have
been established from a series of measurements indicating
that rblue 0.5 rred 0.25 r2.1. 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 rblue is consistently about 1/4 of r2.1 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 rred/r2.1 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 could be probably
mitigated by shadows due to uneven canopy and terrain on a
large scale. |
