γ = 0.15, represents a conservative but not insig-

creasing component perpendicular to the axis of

the sastrugi, *C*DN10 continues increasing. *C*DN10 has

nificantly small sastrugi coverage.

its maximum at Φ = 101.3, where it is 2.73 103

Figure 4 shows two sets of predictions for

for the γ = 0.15 and *h *= 10 cm case. Again, *m *and *n*

developed in the preceding section. Figure 4a

set the location for this maximum; this is where

sastrugi heights of 5, 10 and 20 cm, typical heights

fair agreement with the maximum value that we

observed on ISW, 2.54 103 (AC95).

represented in our snow-surface roughness sur-

As Φ increases toward 180, the model is entire-

veys (e.g., Fig. 2). The 10-cm case fits our observa-

tions quite well. Here, *C*DN10 is 1.431.45 103

ly speculative, since we have few data with which

when the flow is head-on or within 12 of head-

to compare such a reversal in the wind and be-

<

on (that is, when Φ ~ β). These values agree well

cause *C*R3 is the least certain of the form drag coef-

ficients. For now, the decrease as Φ approaches

with the nominal minimum value for *C*DN10 of

180 seems reasonable because the silhouette area

1.5 103 that we reported in AC95.

When Φ gets larger than 11.3 (see eq 3), the

is decreasing rapidly. But the absolute values for

wind can see the side ridge of the obstacle. In ef-

fect, it sees a larger silhouette area and a higher

In Figure 4a, the calculations with *h *values of 5

value of *C*R; *C*DN10 must thus increase. Figure 4

and 20 cm are less realistic than for *h *= 10 cm. For

shows that if the wind rotates just 20 from head-

on, *C*DN10 will increase by 1524%, in good agree-

8% less than our observed maximum; for *h *= 20

ment with our observations in AC95.

cm, the calculated maximum is 27% higher than

As Φ increases and the wind thus has an in-

what we observed. For all three heights, however,

5

γ = 0.15

4

h = 20 cm

3

10 cm

5 cm

103 CDN10

2

1

0

0

20

40

60

80

100

120

140

160

180

Wind Direction, Φ ()

5

h = 10 cm

4

3

103 CDN10

γ = 0.25

2

0.20

0.15

0.10

1

0

0

20

40

60

80

100

120

140

160

180

Wind Direction, Φ ()

CR2 = 0.30, CR3 = 0.30, c *= 0.25, *cw = 4 and cd = 0.6.

8