Winds and Atmospheres - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 33

convention, are those from which the wind blows. Although
wind rose diagrams can be constructed by hand, dedicated
software exists to generate them from wind data, and many
meteorological organizations make such wind rose dia-
grams and the summary data to generate them available on
the internet (Fig. 3.10 ).
Inspection of the wind rose for a site may allow one to
predict the dune morphology that might result. First, one
should only consider winds above the saltation threshold
since gentle winds will not generally cause sand transport. If
the fast winds are dominated by a single direction, then
transverse dunes or barchans are likely to result; if bimodal,
then linear dunes are to be expected, and if highly variable,
star dunes will form (see Chap. 6 ) .
While one can make a reasonable guess at the qualitative
dune type and orientation by eye, to quantitatively compute
the orientation and the sand fluxes (which will in turn allow
estimation of the formation time or migration rate of a
dune), the transport for each leg of the wind rose must be
computed to determine the resultant drift potential (see
Chap. 8 ).

Fig. 3.12 The turbulent intensity (standard deviation of wind speed
divided by the wind speed), as a function of local solar time, as
recorded by the telltale wind indicator on the Mars Phoenix lander.
Few measurements were made in the early morning, but it is clear that
the turbulence is highest in the early afternoon, when sunlight triggers
convective activity. The turbulence level is quite high, with fluctua-
tions in windspeed equal to about one third. (Data from Holsten-
Rathlou et al. 2008.)

As for Titan, Tokano (2010) reported near-surface
(*300 m) wind speed exceedance probabilities of wester-
lies and easterlies near the equator as calculated by a GCM.
He found that while easterlies were more probable for low
windspeeds (\0.6 m/s), strong winds were more likely to be
westerly, consistent with the general transport direction
indicated by the dune morphology. Winds of 0.8 m/s
occurred only 1 % of the time, and 1.3 % of the time only
0.01 % of the time, with westerlies about twice as frequent
as easterlies in each case. At a polar location for the late
summer season, Lorenz et al. (2012) reported a Weibull fit
for surface winds (10 m altitude) with c = 0.4 m/s and
k = 2.0. Clearly, Titan winds are much gentler than their
Martian and terrestrial counterparts. As yet, there are no
detailed surface wind predictions for Venus, although the
surface

3.6 Turbulence

The word 'turbulence' is often used in a rather dismissive
sense to describe the spatiotemporal complexity of the wind
field, as if invoking the term allows one not to think about
the problem further. (Further thinking might not get most of
us very far anyway—even the famous Richard Feynman
said: ''Turbulence is the most important unsolved problem
of classical physics.''). However, short-term fluctuations in
wind speed and direction are instrumental in sand transport,
and in contrast to the Bagnold-era consideration of mean
properties, many modern studies seek to explicitly quantify
the turbulence of the wind field and its effects, and mea-
surements Fig. 3.11 and modeling tools can now handle
this.
The usual way to treat turbulence is to consider the mean
flow (usually assumed horizontal in one axis, so denoted by
U(y)), with two orthogonal fluctuating components (u', v')
superposed on it (we'll consider only two dimensions here).
The first general quantity to consider is the 'turbulent
intensity'—how big those fluctuations are. For this to be
meaningful, the fluctuations have to be averaged over some
space or time. The root-mean-squared values r u , r v are
often used and divided by the mean speed U to express the
intensity I = r u /U. For a well-designed wind tunnel that
aims to minimize turbulence, I might be 1 % or lower. In
the open, or where flow obstacles are present, it can be
much higher: Fig. 3.12 shows measurements of I for the
Martian surface. At night when conditions are quiescent
(but windspeed is still a few m/s) I * 5 %, but builds

measurements of *0.6 m/s (see Chap. 14 ) are

rather similar to Titan.

3.5 Wind Direction: The Wind Rose

Wind speed alone is not enough to describe the likely
morphology of dunes—direction information is usually
desirable. The direction and speed frequencies over a period
(usually a year) are most succinctly displayed in a wind
rose. This is a polar diagram, where the wind direction is
binned into 8, 12 or 16 azimuths, and the length of the
radiating bar or wedge at that azimuth corresponds to the
relative frequency with which that direction occurs. There
are various conventions for encapsulating speed informa-
tion, e.g., showing the rose only for those winds above some
threshold, or (as in Fig. 3.10 ) dividing the bar into sections
corresponding to speed ranges. As always, the caveat must
be stressed that the directions shown, by meteorological