Operational Dust Prediction - Mineral Dust: A Key Player in the Earth System - page 251

A10.2 Observations Used for the Dust Analyses

Main Products

The MODIS AOD product is used most widely due to its reliability and availability
in near real-time (Kaufman et al. 1997 ; Remer et al. 2005 ). Two separate retrievals
with different accuracies are applied over land and ocean. The former suffers
from higher uncertainties due to the impact of the surface reflectance. Several
other factors affect the accuracy of the retrievals both over land and ocean: cloud
contamination, assumptions about the aerosol types and size distribution, near-
surface wind speed, radiative transfer model biases and instrumental uncertainties
(Zhang and Reid 2006 ). The MODIS product provides the total AOD, such that
the partitioning between dust and other aerosol species is driven by the particular
analysis system and its underlying model. However, the standard MODIS Dark
Target method does not deliver data over bright surfaces where there is not enough
contrast between the surface and overlying aerosol layer. However, iron in desert
soils absorbs at blue wavelengths, and albedo in the blue part of the solar spectrum
is considerably darker than the mid-visible and red. This allowed the development of
the MODIS Deep Blue product (Hsu et al. 2004 , 2006 ). Deep Blue is not currently
assimilated at NRL, NASA or ECMWF, but it is expected to be incorporated into
their systems now that an error matrix has been established (Shi et al. 2012 ).
At the Met Office, the standard MODIS and MODIS Deep Blue (Hsu et al. 2004 ,
2006 ; Ginoux et al. 2010 ) products are being assimilated, and the AOD products at
550 nm wavelength from SEVIRI from Brindley and Ignatov ( 2006 ) and Brindley
and Russell ( 2009 ) are being monitored prior to being assimilated in the near
future. However, only a subset of observations can be used, as the forecast model
contains only dust rather than a full suite of aerosols. This restriction is achieved by
geographic filtering of the SEVIRI AOD and by using the MODIS standard product
aerosol-type flags over land and preferentially using the MODIS Deep Blue product
over bright desert surfaces. The presence of other aerosols in these regions of high
dust loading introduces uncertainty into the assimilation process.
CALIPSO (e.g. Winker et al. 2007 ) is the first satellite mission to have
made aerosol lidar observations routinely available from space. At MRI/JMA, the
CALIPSO Level 1B data have been successfully assimilated into the JMA dust
forecast model with a positive impact on the prediction of aeolian dust. A derived
CALIPSO product is also assimilated at NRL (Campbell et al. 2010 ; Zhang et al.
2011 ). In particular, Zhang et al. ( 2011 ) found that assimilation of lidar data had
a beneficial impact on the 48 h forecast. The same product is under study for
assimilation at ECMWF.