SDS-WAS: DIFFERENT APPROACHES TO DUST FORECAST
EVALUATION
ENRIC TERRADELLAS1*, SARA BASART2, JOSÉ M. BALDASANO2, ANGELA
BENEDETTI3, GORAN PEJANOVIC4, ORIOL JORBA2, MALCOLM E. BROOKS5, ARLINDO
SA SILVA6, SARAH LU7, FRANCESCO BENINCASA2
1
AEMET, Barcelona, Spain
BSC-CNS, Barcelona, Spain
3
ECMWF, Reading, U. K.
4
SEEVCCC, Belgrade, Serbia
5
U. K. Met Office, Exeter, U. K.
6
NASA, Greenbelt, MD, U. S. A.
7
NCEP, College Park, MD, U. S. A.
eterradellasj@aemet.es
2
The World meteorological Organization’s Sand and Dust Storm Warning Advisory
and Assessment System (WMO SDS-WAS) has established a protocol to routinely
exchange products from dust forecast models as the basis for a common model
evaluation. A multi-model median, generated after interpolating the models fields to
a common grid mesh of 0.5 x 0.5 degrees, is also included in the evaluation. The aim
of this work is to present different approaches and to test the use of different
observational products in the evaluation system. It is also intended to find out which
approach and which observational data better reflect the model performance.
First, the forecasts of dust optical depth (DOD) at 500 µm are compared with the
aerosol optical depth (AOD) provided by the AERONET network for a number of
selected dust-prone stations. Since the AOD integrates the contributions of particles
of different origins, a first approach consists of restricting the comparison to
situations in which mineral dust is the dominant aerosol type. It is done by
discarding AERONET retrievals with an Ångström exponent (AE) 440-870 over
0.6. This comparison presents two obvious drawbacks: (i) it does not evaluate the
behavior of the models in those cases in which no dust is observed and (ii) it does
not corrects the departure associated with the presence of other types of particles. A
second approach consists of comparing the simulated DOD with the coarse-mode
AOD retrieved by AERONET. In this case, the comparison can be extended to all
situations. However, it is based on the assumption that all coarse-mode particles are
of mineral dust and that there is no dust in the fine mode. This hypothesis could
especially fail in certain coastal stations, where marine aerosol particles with a
similar size to that of dust particles are sometimes abundant.
Then, the DOD forecasts are compared with AOD retrievals from satellite-borne
instruments such as MODIS (including those obtained with the ‘Dark Target’ and
the ‘Deep Blue’ algorithms) and SEVIRI. Different approaches are also analyzed: (i)
evaluation restricted to geographical areas where mineral dust is the main source of
atmospheric aerosol, and (i) discarding of observations with AE over predefined
thresholds. The importance of taking into account the quality flags of the retrievals,
when available, is also discussed.