Geoscience Reference
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200
Fleetcher (1976)
Greeley et al. (1985)
IW82
SL00 3e-4
SL00 1.65e-4
SL00 5e-4
180
160
140
120
100
80
60
40
20
0
1
10
100
1000
Dp (µm)
Fig. 5.3
Threshold wind friction velocity (
U
t
) as a function of particle diameter based on Iversen
and White (
1982
) (IW82) and Shao and Lu (
2000
) (SL00) for different values of (3, 1.62 or
5
10
4
kg s
2
).
White dots
are experimental data from Fletcher (
1976a
,
b
)and
grey dots
from
Greeley and Iversen (
1985
)
Shao and Lu (
2000
) increases with the value of . It is slightly higher than that
predicted by Iversen and White (
1982
) for the value of recommended by the
authors, but can be adjusted to reach a similar minimum value. Differences in
the minimum threshold may have significant implications, especially on the dust
emissions frequency. Indeed, for a smooth and flat surface such as that encountered
in active dust source areas, a minimum
U
t
of 20 cm s
1
corresponds to a wind
velocity of 7 m s
1
at 10 m height above the ground (in neutral conditions, assuming
a
Z
0
of 10
3
cm, typical of a smooth surface), while a
U
t
of 24 cm s
1
corresponds
to a wind velocity of more than 8 m s
1
at 10 m height.
These theoretical curves have several implications for our understanding of dust
emission processes. Firstly, both theoretical and experimental results show very
high
U
t
(>100 cm s
1
) for particles in the single-micron range, that is, for typical
dust-sized particles. This corresponds to a wind velocity at 10 m height on the
order of 35 m s
1
10
3
cm) in neutral conditions.
Such a wind velocity is hardly ever reached during dust storms. This explains
why direct suspension or “uplifting” of dust-sized particles from the surface is not
commonly observed. Secondly, the existence of an optimum threshold for sizes
on the order of 60-100 m makes soils containing particles in this range (fine
sand in sedimentology) the most easily erodible. Thirdly,
U
t
increases with
D
p
for particles with diameters larger than 100 m and decreases for particles with
diameters smaller than 60 m. This implies that mobilisation of soil particles by
wind is a size-segregating process, in which the size distribution of the mobilised
for a smooth surface (
Z
0
D
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