Dust Aerosol Retrieval Over the Oceans with the MODIS/VIIRS Dark Target
algorithm. Part II: Non-Spherical Dust Model
Abstract
The Dark-target (DT) aerosol algorithm retrieves spectral Aerosol
Optical Depth (AOD) and other aerosol properties from
Moderate-resolution Imaging Spectrometer (MODIS) reflectance
observations. Over the ocean, the DT algorithm is known to contain
scattering-angle-dependent biases in its retrievals of AOD, Angstrom
Exponent (AE) and Fine Mode Fraction (FMF) for dust aerosols. Following
a two-step strategy to improve the DT retrieval of dust over ocean, for
which the first step is to identify dusty pixels (reported in ‘Part I’),
in this ‘Part II’, we report on construction of a new dust model lookup
table (LUT) and the strategy for applying it within the existing DT
algorithm. In particular, we evaluate different characterizations of
dust optical properties from a variety of frameworks and databases, and
compare them with the current DT retrieval assumptions. Substituting the
standard operational LUT with a spheroid dust model with identified
dusty pixels shows significant improvement when compared with collocated
AERONET-identified dusty pixels. The application of the new dust model
to dusty pixels reduces their AOD bias from 0.06 to 0.02 while improving
the fraction of retrievals within expected error (EE) from 64% to 82%.
At the same time, the overall bias in AE is reduced from 0.13 to 0.06,
and the scattering-angle-dependent AE bias is largely eliminated. In
testing with wo full months of data (April and July), the new retrieval
reduces the monthly mean AOD by up to 0.1 and 0.2 in the north Atlantic
and Arabian seas, respectively. The average AE and FMF are also reduced.