Numerical Dust Models - Mineral Dust: A Key Player in the Earth System - page 215

9.9 Conclusion

Model simulations of dust emission, transport and deposition processes that are
performed to assess the spatio-temporal distribution of dust properties and study
interactions of dust and atmospheric processes can be carried out by either offline
models or computing dust processes online at regional or global scales. If dust is
embedded in the tracer routines of a global climate model or a regional weather
model, the dust processes are simultaneously computed with the atmospheric
dynamics. This approach allows considering atmospheric processes that operate
on short time scales. It also allows the simulation of feedback processes of dust
and dynamic processes. While models that simulate the atmospheric cycle of dust
particles as well as their influence on radiative fluxes at regional and global scales
are useful to quantify the direct radiative forcing of dust, the effect of changes in
meteorological parameters on dust emission is still not clear. As part of the global
aerosol model intercomparison study AeroCom, the results of 14 different global-
scale dust models were evaluated. It was found that the averages and seasonal
variability of vertically integrated mineral dust parameters like optical thicknesses
and Ångstrom exponents agree within a factor of two with observations. Less
agreement is found for surface concentration and deposition fields of mineral dust
particles. Also, the dust model results show large variations in dust concentrations
for particular regions.
A critical issue in dust models is the simulation of dust emission fluxes. While
the physical processes involved in dust emissions are well known and underpinned
by wind tunnel measurements, those processes depend on soil surface properties
like surface roughness and sediment availability that are not well documented for
most dust emission regions. As those regions lie in remote desert areas that are
difficult to access, methods will need to be developed to obtain the necessary
information on soil properties by remote sensing. Dust emission depends not only
on surface properties but also on surface meteorology that is usually provided by
the regional or global models used. A major problem in model-based assessments
of dust effects is that atmospheric models are often unable to reproduce the small-
scale wind events that are responsible for a large part of dust emission events. For
example, small-scale dry convective phenomena like dust devils and convective
plumes that are frequently observed in desert regions cannot be resolved even in
regional-scale models. While dust transport models usually perform well for long-
range transport, the simulated dust concentrations near dust sources for individual
events can vary by up to an order of magnitude for different models. This suggests
that the modelled dust distribution in the long-range transport may, at least in some
cases, be accurately reproduced for the wrong reasons. With only few anthropogenic
sources, dust effects are part of the natural climate system, but their role may
considerably change under changed climate conditions, even though the direction
is yet unclear. Subgrid-scale parameterizations of dust emission events must be
based on the actual wind processes that occur for the different regions at different
times of the year in order to be able to correctly predict changes in dust emission