Numerical Dust Models - Mineral Dust: A Key Player in the Earth System - page 202

1 1

u 2
tr
u 2

u tr
u

ˆ D C

The vertical dust flux is then derived from the horizontal saltation flux. As
example, Marticorena and Bergametti ( 1995 ) use an empirical expression for
relating the dust emission flux to saltation discharge by multiplying the saltation
discharge by a proportionality constant that depends on the clay content of the soil.
While this approach has been applied in various dust models, more physically
based descriptions of the emission processes during saltation and sandblasting have
been developed by, for example, Shao ( 2000 ) and Alfaro and Gomes ( 2001 ). In
those approaches the kinetic energy of saltating grains and the binding energy of
dust particles determine the emission factor ˛. In wind tunnel experiments, Alfaro
et al. ( 1998 ) were able to directly relate the kinetic energy of saltating particles
on a kaolinite clay bed to the size of released dust particles. According to these
wind tunnel experiments, the importance of disaggregation into smaller particles
due to saltating sand increased for increasing wind speeds (Alfaro et al. 1997 ).
Observational evidence by Washington et al. ( 2006 ) confirms the coincidence of
saltation and strong dust emission for 2-m winds exceeding 10 m s 1 (Washington
et al. 2006 ). In contrast, Kok ( 2011 ) found in a more recent analysis that the
size distribution of emitted dust particles would not depend on the surface wind
speed.
In addition to knowledge of the surface wind speeds in the dust source region,
information on both surface roughness and soil particle size distributions in soils
are required to compute dust emission fluxes. While surface roughness data in
unvegetated regions can be derived from satellite remote sensing data (Laurent et al.
2005 ; Marticorena et al. 2004 ; Prigent et al. 2005 ), the information on soil particle
size distributions relies on limited samples in dust regions.

9.3 Model Representation of Dust Source Properties

The particle size distribution in soils determines both u *tr and the size distribution
of emitted particles. Global estimates of soil size distributions that can be used in
global- or regional-scale dust models are, for example, derived from the standard
textural triangle, utilizing soil texture class data given in global soil property maps
such as the FAO/UNESCO Soil Map of the World (Zobler 1986 ). However, there
is a general lack of samples from dust source regions, and the wet sieving method
used to determine soil texture breaks up soil aggregates during the treatment, which
is therefore not appropriate for determining soil particle size distribution relevant
for dust emission processes. Thus for the purpose of dust emission modelling in
some regional studies, soil size distributions are taken from dry sieve techniques
that avoid the breaking of aggregates in the soils (Alfaro and Gomes 2001 ;Laurent
et al. 2006 ).