Dust Production Mechanisms - Mineral Dust: A Key Player in the Earth System - page 104

dust emission. Such processes can be natural processes, such as intense rainfall
or anthropogenic processes such as soil trampling by livestock or vehicle traffic.
The dynamics of crust formation and destruction can induce temporal and spatial
variability of dust emissions in semi-arid areas.
Some arid or semi-arid areas are located at sufficiently high latitudes to experi-
ence snow. In East Asia (35 ı N-50 ı N), for example, areas covered with snow can
persist in the spring, during the period when the highest frequency of dust storms is
recorded (Kurosaki and Mikami 2004 ). Snow protects the soil from wind erosion,
but the physical mechanisms involved have not been investigated extensively. Based
on a statistical analysis of synoptic reports, Kurosaki and Mikami ( 2004 ) proposed
an empirical parameterisation of the change of the erosion threshold as a function
of the degree of snow cover. Laurent et al. ( 2005 ) also found that for specific Asian
dust sources, snow cover could influence the modelled simulated dust emissions
significantly during the winter months.

5.4 Saltation

The amount of soil material set in a horizontal movement by the wind is generally
quantified through the so-called horizontal flux. The horizontal flux is the mass
of soil particles crossing a vertical surface of unit width and infinite height
perpendicular to the eroding surface and to the wind direction per unit time. The
saltation process has been investigated thoroughly both from a theoretical and
experimental point of view, in the field and in wind tunnels. In addition to the
prediction of saltation thresholds, numerous studies have been dedicated to the
quantification of the horizontal mass flux of saltating particles and its dependence
on U * and the influence of soil particle size and surface characteristics. Several
numerical expressions of the saltation flux have been proposed that generally
include a power dependence on U * and U t .
Assuming that above the erosion threshold the entire wind momentum is trans-
ferred to the surface by the saltating grains and based on dimensional arguments,
Bagnold ( 1941 ) suggested that the horizontal flux is proportional to U * to the
power of 3. Most of the expressions in the literature also exhibit a third power
law of U * to express the horizontal flux (e.g. White 1979 ; Owen 1964 ;Kawamura
1964 ). Such dependence has been experimentally observed as well in natural
situations as in wind tunnels, especially for large values of U * (Gillette 1974 , 1977 ,
1979 ; Gillette and Stockton 1989 ;Sörensen 1985 ; Leys and Raupach 1991 ;Shao
et al. 1993 ). These expressions are supported by theoretical considerations on the
mean path trajectories of the saltating particles as a function of U * andonthe
efficiency of momentum transfer to the surface. Most of these expressions assume
that the saltation layer has reached equilibrium and provide the horizontal flux as
a function of the wind velocity. In the field for a steady wind, the equilibrium
can be reached at a distance of about 600 m (Gillette et al. 1996 ). Horizontal
flux measurements performed with a high confidence level (5 % in mass) by