Winds and Atmospheres - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 32

Fig. 3.11 Plot of wind speed vs
time (upper panel) showing
fluctuating character. Such a
signal is often termed 'red noise'
or 'brown noise' by a color
analogy with the spectral
properties. When the wind speed
exceeds a threshold of about 5 m/s,
sand transport (measured by
SAFIRE detectors, see Chap. 17 )
suddenly picks up. The transport
signal is clearly very 'bursty' and
episodic. Figure courtesy Andreas
Baas

numerical models which are discussed in Chap. 18 . Ideally,
one can use a record of previous winds at a given location (or
at least nearby) to predict winds in the future, but of course
such records are all but nonexistent for other worlds. Low--
level winds can sometimes be estimated by tracking clouds
or dust devils in imaging from orbit, but generally must be
accomplished by in situ measurements (see also Chap. 17 ). A
brief summary of in situ measurements is given in Table 3.2 :
wind data from NASA planetary missions can be found at the
PDS Planetary Atmospheres node.

Windspeed distributions are highly asymmetric, not least
because speed cannot have a negative value. Windspeed
distributions are usually highly skewed; there is a long tail
of fast, but infrequent winds, and some statistical functions
that are often used to describe the probabilities include the
Rayleigh and Weibull functions (the latter, a flexible two-
parameter distribution, is widely used in the wind energy
industry). Note that because winds exceeding the saltation
threshold may be very infrequent (e.g., less than 1 % of the
time) it is sometimes most useful to plot the relative fre-
quencies on a logarithmic axis (Figs. 3.8 and 3.9 ).
Lorenz (1996) fit the in situ meteorological measure-
ments of the Viking landers at Mars with Weibull functions.
For the longest measurement sequence of 0-1040 sols
recorded by Viking 2, the hourly-averaged wind was calm
(below 1 m/s) for 14 % of the time, and the rest of the time
could be described overall by c = 3.85 m/s, k = 1.22. Note
that individual gusts could be much higher. Note also that
the winds over this nearly two-Martian-year period include
extended calm and windy epochs—fits to specific periods
would give different values. Fits to a couple of terrestrial
locations are given in figs, but note these data are at stan-
dard anemometer height (10 m) rather than the *1.5 m of
the Viking meteorology mast. Fenton and Michaels (2010)
study the Mars surface winds predicted by a Large Eddy
Simulation (see Chap. 18 ) and explore the effectiveness of
Weibull fits (as with the overall Viking fit above, Weibull
fits tend to underestimate the high-wind tail of the fre-
quency distribution).

3.4 Wind Speed Statistics

The net result of the planetary circulation is a time history
of winds at a given location. Two scalar parameters are of
particular concern at any place and time: namely, wind
speed, and direction. These two scalars can also be repre-
sented as a vector in 2-dimensional space (for the global and
regional scale at least, the third dimension and vertical
velocity can be ignored—at the dune scale, of course, this is
not the case).
A location's statistical wind properties over some length
of time are usually expressed in a couple of ways. First is a
histogram or probability distribution of wind speed. This is
of interest because winds fast enough to cause saltation
occur for only a small fraction of the time, so the simplest
statistical description—the average windspeed—does not
usefully inform how much aeolian transport occurs.