Geoscience Reference
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Fig. 3.6
Titan dunes observed
in a radar image from the T44
encounter. The dark dunes divert
around some bright hills, and in
some cases are blocked by them.
NASA/JPL/Cassini Radar team
Fig. 3.7
A hodograph of the
winds measured by the Viking
lander 1 on its first day on Mars.
At each hourly measurement the
wind velocity is described by a
vector from the origin to the
relevant point on the line. Note
that the downslope direction
(arrow) plays a significant role in
driving morning winds
boundary layer flows effectively just means the product of
width and speed) requires that flow accelerates through a
constriction or over a gentle hill, but may decelerate when
the flow can expand.
When a wind that may be strong enough to transport sand,
e.g., through a pass, is decelerated as the flow broadens into a
valley, the wind slows and the sand transport ceases. This
leads to a net deposition, and some dunefields are formed
this way (e.g., Coral Sands in Utah). A related effect is where
wind may be collimated by topography from several dif-
ferent directions, leading to a net local convergence which
sweeps sand into a local pile—the Stovepipe Dunes in Death
Valley (see Fig.
24.2
) form in this way.
The diurnal heating influence via albedo, thermal inertia,
or slope effects, often leads to a rather reproducible behavior
in wind direction (and to a lesser extent, speed) over the
course of a day. This is often plotted in a hodograph, a
vector diagram wherein the heads of vectors (representing
wind speed and direction) are joined from one hour to the
next (Fig.
3.7
). Usually these define a loop, with the wind
direction rotating through the course of the day.
Topography, even at the building or boulder level, will
influence the flow, in effect biasing the distribution of u' and
v' around an obstacle. Sand will accumulate around and in
the lee of bushes on playas or sabkhas to form small coppice
dunes or nebkhas. Streaks of sand deposition, related to
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