Dust Production Mechanisms - Mineral Dust: A Key Player in the Earth System - page 94

dust sources of the southern hemisphere (Australia, South America, South Africa).
These numbers show that the quantification of dust emissions is very uncertain, both
at the regional and at the global scale. These uncertainties are largely due to the
way dust emissions are represented in 3-D models, which in turn depends on our
understanding of dust production mechanisms and our capability to parameterise
them. In the following, the general understanding of the dust emission processes, the
way the different physical processes involved in dust emissions are modelled, and
the present limitations are discussed. The link between dust production mechanisms
and actual dust sources is also discussed briefly.

5.2 General Understanding

Dust emission is the final “product” of a succession of processes involved in
aeolian erosion of soils. Subsequently, describing dust emission means describing
the physical processes involved in soil erosion and the environmental factors that
influence them (e.g. Greeley and Iversen 1985 ). A detailed description of the physics
of sand movement and dust emission processes and their modelling can be found in
Shao ( 2001 ).
Aeolian erosion is a direct function of the wind velocity, but occurs only when
a threshold value has been reached. This erosion threshold results from the fact
that soil particles are subject to forces holding them at the surface: their weight
and interparticle cohesion forces I p . Interparticle forces can be reinforced by the
presence of water in the soil and as a result, the erosion threshold increases when
soil moisture increases. This threshold also depends on the presence of non-erodible
roughness elements, which cover and protect part of the surface and absorb a
fraction of the wind momentum, which otherwise would have been available to
initiate particle motion.
The wind shear stress acting on a horizontal surface can be related to the
vertical gradient of wind velocity U (Greeley and Iversen 1985 ), as schematically
illustrated in Fig. 5.1 and expressed in Eq. 5.1 :

D .@U=@ z / (5.1)

Where U is the wind velocity at the height z, and is the dynamical viscosity of the
air.

The wind shear stress can also be expressed as a function of the wind friction
velocity U * , which is the parameter most commonly used in aeolian erosion studies:

a U 2 D (5.2)

where a is the air density.
The wind velocity is defined at a given height z above the surface, while U * is
related to the vertical gradient of the wind velocity. In neutral conditions (no gradient