Numerical Dust Models - Mineral Dust: A Key Player in the Earth System - page 201

operational dust forecasts (Chap. 10 ) . Such models are also better suited for
comparisons and interpretation of in situ observations made during intensive field
experiments.
This chapter provides first an overview of the parameterization of the dust source
regions, emission and deposition processes (that are described in detail in Chaps. 3 ,
4 , 5 , 6 , 7 and 8 of this topic) in regional- and global-scale dust models. Some aspects
of regional dust models are briefly addressed (for more details, see Chap. 10 ) . The
last part of this chapter describes relevant results of the intercomparison of global
dust models from the AeroCom project.

9.2 Dust Emission Modelling

The computation of a realistic distribution of dust emission fluxes is a central part
of the model-based description of the dust aerosol cycle, as the emissions depend
on both surface properties and surface wind speeds, and cannot be prescribed
externally as, for example, many anthropogenic emissions. Dust emission processes
are described in detail in Chap. 5 . Parameterization of dust emission in transport
models must take into account that dust emission processes are highly variable in
space and time and can occur at small local scales that may not be resolvable in the
regional- or global-scale dust models.
As described in Chap. 5 , dust emission fluxes from bare soils depend strongly on
the surface friction velocity u * , which in turn depends on the surface wind speed
at a reference height, aerodynamic surface roughness and atmospheric stability.
The emission of dust particles from the soil surface is initiated when the surface
wind friction velocity exceeds a threshold u *tr , which in turn depends on surface
properties such as the soil particle size distribution, surface roughness and soil
moisture. Dust emission is predominantly initiated by saltating soil particles,
which dislocate smaller clay-sized dust particles that can be transported over
large distances by a sandblasting process. In many models the computation of
dust emission fluxes is realized by first computing saltation discharge G (Hagen
et al. 1999 ), which in the literature is often referred to as horizontal dust flux.
Based on observations and theoretical considerations of the momentum balance of
the saltation layer, the magnitude of the saltation discharge depends on the cube
of friction velocity above u *tr (Bagnold 1941 ). Dust models use predictive flux
equations generally of the form

ˆ u
u tr

c a

g u 3

G D

with a being the air density, g the gravitational constant and c a dimensionless
adjustment factor, which depends on aerodynamic, surface and soil properties.
For the dimensionless function ˆ( u * /u *tr ), different expressions exist, for exam-
ple, the semi-empirically derived formulation by White ( 1979 ):