<p class="first" id="P1">A high density of REMS wind measurements were collected in
three science investigations
during MSL’s Bagnold Dunes Campaign, which took place over ~80 sols around southern
winter solstice (Ls~90°) and constituted the first in situ analysis of the environmental
conditions, morphology, structure, and composition of an active dune field on Mars.
The Wind Characterization Investigation was designed to Available online 14 December
2016 fully characterize the near-surface wind field just outside the dunes and confirmed
the primarily upslope/downslope flow expected from theory and modeling of the circulation
on the slopes of Aeolis Mons in this season. The basic pattern of winds is ‘upslope’
(from the northwest, heading up Aeolis Mons) during the daytime (~09:00–17:00 or 18:00)
and ‘downslope’ (from the southeast, heading down Aeolis Mons) at night (~20:00 to
some time before 08:00). Between these times the wind rotates largely clockwise, giving
generally westerly winds mid-morning and easterly winds in the early evening. The
timings of these direction changes are relatively consistent from sol to sol; however,
the wind direction and speed at any given time shows considerable intersol variability.
This pattern and timing is similar to predictions from the MarsWRF numerical model,
run at a resolution of ~490 m in this region, although the model predicts the upslope
winds to have a stronger component from the E than the W, misses a wind speed peak
at ~09:00, and under-predicts the strength of daytime wind speeds by ~2–4 m/s. The
Namib Dune Lee Investigation reveals ‘blocking’ of northerly winds by the dune, leaving
primarily a westerly component to the daytime winds, and also shows a broadening of
the 1 Hz wind speed distribution likely associated with lee turbulence. The Namib
Dune Side Investigation measured primarily daytime winds at the side of the same dune,
in support of aeolian change detection experiments designed to put limits on the saltation
threshold, and also appears to show the influence of the dune body on the local flow,
though less clearly than in the lee. Using a vertical grid with lower resolution near
the surface reduces the relative strength of nighttime winds predicted by MarsWRF
and produces a peak in wind speed at ~09:00, improving the match to the observed diurnal
variation of wind speed, albeit with an offset in magnitude. The annual wind field
predicted using this grid also provides a far better match to observations of aeolian
dune morphology and motion in the Bagnold Dunes. However, the lower overall wind speeds
than observed and disagreement with the observed wind direction at ~09:00 suggest
that the problem has not been solved and that alternative boundary layer mixing schemes
should be explored which may result in more mixing of momentum down to the near-surface
from higher layers. These results demonstrate a strong need for in situ wind data
to constrain the setup and assumptions used in numerical models, so that they may
be used with more confidence to predict the circulation at other times and locations
on Mars.
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