A Physically Based Model of the
Form Drag Associated with Sastrugi
EDGAR L ANDREAS
INTRODUCTION
The neutral-stability, 10-m airice drag coeffi-
cients (CDN10) reported by Andreas and Claffey
(1995; henceforth, AC95) clearly depend on how
wind-driven snow alters the sea ice surface. Fig-
ure 1 shows the time series of Ice Station Weddell
(ISW) CDN10 values and hourly averages of the
wind speed and direction measured at 5 m.
These CDN10 values range between 1.27 103 and
2.54 103 but are not randomly scattered be-
tween these limits. Rather, they generally cluster
or depend coherently on time. For example, in
AC95 we focused on the events on Julian days 64
and 65 and 79 and 80. In Figure 1, we see that, on
these days, the 5-m wind speed was typically
8 m/s or higher, the wind direction was fairly
constant, and CDN10 decreased consistently with
time.
Our explanation for this sequence was that
winds of 68 m/s, and higher, redistribute the
mobile snow: snowdrifts perpendicular to the
wind erode; and the new drifts, built parallel
with the mean wind, streamline the surface. Con-
sequently, if these eroding winds persist and
their direction remains relatively constant for
just 12 hours, CDN10 will decrease by as much as
30%. If, after establishing a drift pattern, how-
ever, the wind shifts by as little as 20, it will face
a rougher surface, and measured CDN10 values
will increase.
There are several other events in Figure 1 that
demonstrate this same smoothing in the pres-
ence of a moderate, directionally constant wind
(for example, see days 86 and 87). Thus, the
sequence is repeatable and, in fact, seems to be
the key to understanding airice momentum ex-
change in the western Weddell Sea--and proba-
Figure 1. Time series of the Ice Station Weddell (ISW)
bly over most other sea ice regions as well.
CDN10 values reported in Andreas and Claffey (1995)
Form drag sustained by the snowdrifts must
and hourly averages of the wind speed and direction at
explain the behavior of the CDN10 values that we
5 m.
observed. Others have also recognized that form
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