characterization of surface roughness and floe geometry of...

Characterization of Surface Roughness and Floe

Geometry of Sea Ice over the Continental

Shelves of the Beaufort and Chukchi Seas

W. F. WEEKS, W. B. TUCKER III, M. FRANK,

and S. FUNGCHAROEN

ABSTRACT

Starting with the winter of 1975-76 and continuing to the present, remote sensing data have been collected in the coastal zones of the Beaufort and Chukchi seas to assess spatial and seasonal variations in the roughness of the upper surface of the sea ice. This paper reports on observations made primarily during the late winter and early spring of 1976 when the ice cover was at its maximum extent and very few leads were observed. The primary sensors used were a laser profilometer and an X-band side-looking airborne radar (SLAR) system.

The laser flights were made into the Beaufort Sea from land points at Barter Island, Cross Island (Prudhoe Bay), and Lonely and into the Chukchi Sea from Barrow, Wainwright, and Point Lay. Sample lines were 200 km long, and data were collected both out and back along each line. The heaviest ridging was found at Barter Island, and there was a general decrease in the number of ridges as one moved west into the Chukchi Sea. There was no strong variation in the mean ridge height along the coast. The general form of the distributions was a negative exponential. The most ridges are found in the first 50 km offshore at Barter Island and at Barrow and in the second 50 km on the other traverses, with the outermost 100 km of all the traverses consistently containing fewer ridges. The area near the AIDJEX triangle, located approximately 400 km off the coast, contained even fewer ridges (2.5 per kilometer, as compared with 9.4 at Cross Island). There was no correlation between mean ridge height and the number of ridges per kilometer, contrary to previous reports. An analysis was made of the probability of encountering very large ridges.

Examination of the SLAR images allows the determination of (a) the size distribution of multiyear floes within the nearshore ice pack (the distribution was a negative exponential, and the largest floe noted was 3.6 km in diameter) and (b) the variation in the areal percentage of deformed ice as a function of distance

300

Surface Roughness and Floe Geometry of Sea Ice 301

from the coast. This latter parameter showed a steady decrease with increasing distance north from the coast.

The laser and SLAR data support the idea of a more heavily deformed zone of sea ice existing over the continental shelf. The boundaries of this zone, however, appear to be diffuse.

INTRODUCTION

As part of its overall program to respond to the needs of petroleum development of the Alaskan continental shelf, the NOAA/BLM Alaskan Outer Continental Shelf Environmental Assessment Program collects data related to the identification and estimation of the potential hazards posed by the environment to offshore petroleum exploration and development. Along the margins of the Beaufort and Chukchi seas there is little debate about the identity of the principal hazard: it is the sea ice, a potential threat to offshore structures and operations during every month of the year. The forces exerted on offshore structures by drifting sheets of undeformed sea ice of uniform thickness are not well known, nor are the even larger forces produced by pressure ridges and ice pile-ups, which may have drafts of up to ten times that of the thickest undeformed ice.

If one hopes to design structures that will survive in regions of such highly deformed ice, one must have information on the sizes, frequencies, and spatial patterns of such ice accumulations. In the past, most such information, which could be described as limited at best, has been collected at sites away from the coast in an effort to characterize overall ridging patterns in the Arctic Ocean (Hibler et al., 1972; Tucker and Westhall, 1973). The exception to this is the recent study (September-October 1974 and April 1975) by Wadhams (1975) of the surface morphology of the ice in the Beaufort Sea just north of the MacKen-zie Delta. Inasmuch as his study area adjoins ours at the U.S.-Canadian border, interesting comparisons can be made between the two sets of data, although it should be kept in mind that they were obtained during different years.

TECHNIQUES AND SAMPLING

The present study uses two types of remote sensing techniques, laser profilometry and side-looking airborne radar (SLAR). These techniques complement each other in that although they both "sense" the surface roughness of the ice they do so in quite different ways: the laser profilometer by measuring a continuous line profile of the topography of the upper ice surface and the SLAR by producing a two-dimensional map of the radar backscatter from the ice surface. Because in sea ice the strength of the radar return is almost totally controlled by the roughness of the upper ice surface, flat undeformed ice gives low returns (dark image) while pressure ridges and rubble fields give high returns (bright image). Therefore, on a SLAR image the two-dimensional pattern of high returns provides a map presentation of the areas of deformed ice. The different types of deformed ice features are commonly distinguished by their surface geometry; for

302 WEEKS, TUCKER, FRANK, and FUNGCHAROEN

example, pressure ridges are linear features that may be either straight or sinuous in map view, rafted ice chracteristically shows rectangular surface patterns, and rubble and hummock fields are areas in which essentially all the ice is deformed and in no particular pattern. Therefore, a laser profile by itself does not provide us with the information required to classify the ice features under study. Although we will for convenience refer to all the profiled ice features as ridges, we cannot in fact distinguish between rubble, rafts, ridges, and hummocks.

The profilometer used was a Spectra-Physics Geodolite 3A which contains a CW He-Ne laser and measures the distance between the aircraft and the upper ice surface to within a few centimeters. Because the aircraft (the NARL C-117) does not fly a perfectly level path, the resulting analog record contains both the high frequency distance changes produced by the pressure ridges and the low frequency changes produced by the porpoising of the aircraft. The records were analyzed manually by connecting smooth ice surfaces with a continuous hand-drawn curve. Ridge heights were then measured relative to this curve. Ridges were separated into 0.3 m wide class intervals and the smallest ridges measured were in the 0.6—0.9 m category. However, in reporting mean ridge heights, the slightly higher cutoffs (h()) of either 0.9 or 1.2 m are commonly used to facilitate comparisons with previous investigations. A ridge is said to be independent when its maximum elevation is not less than two times the shallowest troughs on either side of it. This criterion was also used by Wadhams (1975) and, in earlier laser and sonar studies, by Lowry (1974) and Williams et al. (1975).

The locations of the laser tracks discussed here are shown in Figure 1. They are oriented roughly normal to the coast and extend into the Beaufort Sea from land points at Barter Island (Kaktovik), Cross Island (Prudhoe Bay), and Lonely, and into the Chukchi Sea from Barrow, Wainwright, and Point Lay. On each profile, data were collected out 200 km and then, after a slight offset (~ 1 km), back to the point of origin. The coastal missions were flown between 23 and 25 February 1976. On 26 February the three legs of the AIDJEX triangle were also profiled.

The SLAR system used was a Motorola APS-94-XE1 (X-band) unit that operated in the real aperture mode from a Mohawk aircraft flown by the U.S. Geological Survey. The system imaged 25 km wide swaths on both sides of the aircraft that were separated by a 10 km blind spot beneath the aircraft. The horizontal resolution of the system was approximately 25 m in the near-range of the image. The images used were obtained on 1 May 1974 (north of Lonely) and on 23 September 1975 (off Barrow and Cape Simpson).

LASER RESULTS

Figure 2 shows the histograms expressed in terms of ridge frequency per 100 km of ice profiled versus ridge height for each of the six sample tracks shown in Figure 1. The general shapes of the profiles are exponential, agreeing with the earlier observations of Hibler (1975) and Hibler et al. (1972). If a sharp decrease


Figure 1. Location of laser sampling tracks on margins of Beaufort and Chukchi seas and sampling tracks between the AIDJEX stations (C, BF, and SB stand for the station names Caribou, Blue Fox and Snow Bird).

in ridge frequency occurs at low ridge heights, as suggested by Klimovich (1972), it must occur at heights of less than 0.6 m, where features other than those produced by ice deformation contribute significantly to the aggregate roughness of the ice surface.

During the three-day period that the laser profiles were obtained, the ice covers in both the Beaufort and the Chukchi seas were essentially continuous. Therefore, the number of ridges occuring during 200 km of laser track (shown in parentheses after the track names) can readily be converted to numbers of ridges per kilometer for each of the complete sample tracks, calculated for cutoff heights of both 0.9 and 1.2 m. In the text, unless specifically mentioned, we will use a 0.9 m cutoff and express the number of ridges per kilometer simply as the number of ridges.

The largest mean ridge height (1.68 m) was obtained off Barrow, and there does not appear to be a systematic variation in this parameter along the coast. On the other hand, the number of ridges/km decreases systematically from a maximum value off Barter Island (7.80) to much lower values (3.16) in the Chukchi Sea. These results coincide with our visual impressions during the flights. The amount of deformed ice off of Barter Island was particularly impressive. The exception to the trend is Point Lay, which shows an increased number of ridges. The general ridge frequencies obtained were higher than values re-


A. Barter Is land ( 2 5 5 0 )

n B. Cross Is land ( 1 5 2 4 )

E. WoirMrigtit ( 8 7 0 )

2 0 0

100

-T 0

" -

H F. Poin Lay (1329)

1 '" 1 ' 1 i 2

(m)

Fi gure 2. Histograms showing ridge frequency per 100 km of ice profiled versus ridge height. The numbers in parentheses indicate the total number of ridges per 200 km of sampling track.

ported by Hibler et al. (1974) for an area roughly 70 km to the east of the Barter Island traverse (2.2 ridges/km in March 1971 and 1.3 ridges/km in March 1972, with/?,, = 1.2 m). They also were much higher than values obtained in April 1975 by Wadhams (1975) during the first 220 km of a traverse made north along longitude 135°W north of the Mackenzie Delta (0.22-1.34 ridges/km, h0 = 0 . 9 m). The mean ridge heights are, however, similar.

To examine the variations in the amount of deformed ice normal to the coast, we separated each traverse into four segments, each 50 km in length. The results are shown in Table 1. Variations in mean ridge heights are small, ranging between 1.48 and 1.74 m, and show no obvious pattern. Variations in the number of ridges, on the other hand, are quite pronounced and show a definite pattern. At Barrow and Barter Island, the largest number of ridges occur in the


TABLE 1 SUMMARY DATA FOR 50 km LASER TRAVERSES

(/;„ = 0.9 m)

Distance from Coast, km

150-200 100-150 50-100 0-50

150-200 100-150 50-100

0-50

Point Lay

4.04 4.16 5.98 3.44

1.49 1.54 1.50 1.57

Wain-wright

2.50 3.30 4.04 3.26

1.62 1.63 1.60 1.57

Barrow

Ridges per

2.92 3.38 2.82 3.52

Lonely

kilometer:

3.28 2.98 5.68 2.94

Mean ridge height, m: 1.57 1.69 1.70 1.74

1.48 1.58 1.62 1.57

Cross Island

3.14 3.60 7.56 6.00

1.58 1.56 1.59 1.57

Barter Island

3.79 4.27 6.63 9.40

1.52 1.56 1.49 1.63

first 50 km off the coast. In the other profiles the most deformed ice is found between 50 and 100 km from the coast. Beyond 100 km fewer ridges are encountered. Farther out along the Blue Fox-Snow Bird line in the AIDJEX array, approximately 400 km from the coast, the ridge frequency (2.56 ridges/km) is even smaller than at sites near the coast, with the one exception of the outermost 50 km of the Wainwright traverse. A histogram showing the frequency of ridges of different heights at AIDJEX is presented in Figure 3. As seen in Table 2, the mean ridge height at AIDJEX is also lower than values obtained from traverses of comparable length made nearer the coast.

Why the most ridges are found in the first 50 km at Barrow and Barter Island and farther off the coast at the other sites is not clear. Perhaps the ice just off the coast is continuously active throughout the winter at Barter and Barrow and therefore generates a large number of nearshore ridges, while at the other sites the fast ice edge gradually moves farther offshore, thereby limiting the amount of

1.22 2.44 3.66 4.6 Sail Height

Figure 3. Histogram showing ridge frequency per 100 km of ice profiled versus ridge height for the 259 km line between the AIDJEX stations Blue Fox and Snow Bird. The total number of ridges counted was 663 and the lower cutoff used was 0.9 m.


TABLE 2 SUMMARY DATA FOR COMPLETE 200 km LASER TRAVERSES

/?„ = 0.9 m h„ = 1,2 m

Point Lay Wainwright Barrow Lonely Cross Island Barter Island Blue Fox-Snow Bird

(259 km)

Mean Ridge Height, m

1.51 1.62 1.68 1.59 1.58 1.55

1.47

Ridges per km

4.46 3.28 3.16 3.90 5.11 7.80

2.56

Mean Ridge Height, m

1.76 1.88 1.93 1.84 1.86 1.83

1.80

Ridges per km

2.88 2.27 2.25 2.69 3.28 4.97

1.41

nearshore ice deformation. It is clear, however, that during February 1976 the ice near the coast was significantly more deformed than the ice farther seaward. This gives support, from the viewpoint of ice morphology and ice deformation, to considering the coastal marginal ice zone to be a separate ice province.

The present results are similar to the ridging patterns found by Hibler and Ackley (1973), who suggested that there is a band of deformed ice along the north coast of Alaska. These results are quite different from those of Wadhams (1975), who in April 1975, using similar techniques, observed a steady increase in the number of ridges as he flew north of the Mackenzie Delta to 76°N, as well as much lower ridge frequencies in general than those reported here. Wadhams's results strengthen our growing opinion that there are large year-to-year changes in both the degree of ridging and the ridging patterns along the edges of the Beaufort and Chukchi seas.

In the past, several workers have suggested simple linear correlations between either the number of ridges or the areal amount of deformed ice and the mean ridge height based on studies using aerial photography (Gonin, 1960), sonar (Hibler et al., 1972), and laser profilometry (Wadhams, 1975). Wadhams, whose results are most comparable to the present study, obtained a good linear correlation with most of the data he obtained during the summer of 1974 with his line running from an intercept of h = 1.20 m at 0 ridges to h = 1.44 m at 12 ridges. However, 27% of the 26 data points did not fall close to the final regression line and were labeled as anomalously high. All of these anomalous points had fewer than 4 ridges/km and ridge heights of 1.4-1.7 m and were located near the coast. One possible explanation for these anomalous points is the fact that they all appear to be from locations where there are significant areas with water depths of less than 20 m. Grounded ridges characteristically are higher than other ridges. They also tend to stabilize the ice around them reducing the total number of ridges that are formed. Wadhams's plot of his winter (April 1975) data is even more complex and does not show a simple linear correlation. He suggests that


when the data are separated into ice regions there is a positive linear correlation between frequency and mean height for each region.

Figure 4 is a similar plot of our results. It shows no obvious linear relations regardless of how the data are classified. We believe that in general a relation between mean ridge height and the number of ridges is reasonable. Why is it not apparent in our data? One possible consideration is that the coastal regions which we studied contained large areas where the ice is generally chaotically broken up into so-called rubble fields. The fields appear to be less common farther from the coast, where ice deformation appears as more normal ridging and rafting. For a given amount of ice deformation, a laser profile through a rubble field would be expected to indicate a large number of lower ridges, which would tend to obscure a linear relation between mean ridge height and the number of ridges.

Finally, it is of interest to attempt to work out the probability of encountering ridges of different heights. We will do this by following an approach outlined by Wadhams (1975), who observed that a semi-log plot of his laser data presented as log probability density vs. height was linear. Figure 5 shows a portion of our data plotted in a similar manner. All three data sets proved to be well described by straight lines with the largest deviations occurring at large ridge heights where very few ridges are present in a given class interval. If we then express these linear functions as negative exponentials and convert to meters, we obtain P(h)dh = 4.742 exp [-l.35Sh]dh for Barter Island (0-50 km) andP(h)dh = 7.870 exp [- l.720h]dh for AIDJEX (Blue Fox-Snow Bird), where P(h)dh is the probability that a ridge encountered at random will have a height in the range/? to (h + dh), given that its height is greater than 1 m. If these relations are assumed to hold for very high ridges, we can also calculate Pc(h), the probability that a ridge sampled

l .75r

1.70

1.65

S 1.60

1.55

.50

1.45 ~L 10 4 6 8

Ridges per Kilometer of Ice Cover

Figure 4. Mean ridge height versus the number of ridges per km of ice cover based on

values obtained from breaking each traverse into four 50 km sections.


Ridge Height 2 3 4 6 m

12 20 ft

>% ~~

i - 2

_ _

_

-

""

-

-

1 1 1

A

^ ^ n

\

i

^

i 1 i 1 i

\2 V̂ \ &\Barter Is.W

\ \ \0-50km A \ . \ V Cross Is.(o)

( A ) A I D J E X \ - « \

VV • * vV \ \ \ i i i i i

_ „ .

-

-

_

~

- -- 3 —

Figure 5. Log-linear plot of the probability density per 30 cm vs. ridge height for the first 50 km of the Barter Island traverse, the complete Cross Island traverse, and the traverse between AIDJEX stations Blue Fox and Snow Bird.

at random will have a height equal to or greater than h, fmmP(.{h) = J* P(h)dh. Values calculated from these relations are given in Table 3 and can be used to

estimate the likely maximum ridge height that can be expected if a given amount of ice moves past a sampling point such as an offshore platform. For instance, at Barter Island and at AIDJEX we observed 9.4 and 2.6 ridges/km. If we assume a commonly used mean annual drift rate of 2.5 km/day we would, over a year, encounter approximately 8600 ridges at Barter Island and 2400 ridges at the

TABLE 3 PREDICTION TABLE FOR OCCURRENCE OF HIGH RIDGES

Ridge Height

(h), m

4

6

8

10

12

14

Barter Island (0-50 km)

P(h) Pv(h)

2.1 x 10^2 i r r 1

1.4 x 10"3 t r r '

9.1 x 10~5 n r 1

6.0 x 10-6 i r r 1

4.0 x 10-7 r r r 1

2.6 x 10-* n r 1

1.5 x 10-2

1.0 x 10~3

6.7 x 10-3

4.4 x 10-6

2.9 x 1Q-7

1.9 x 1Q-8

AIDJEX (BF-SB)

/>(/?) Pc(h)

8.1 x 10-3 r r r 1 4.7 x 10~3

2.6 x 10-4 t r r 1 1.5 x 10^4

8.3 x 10-6 i r r 1 4.8 x 10^6

2.7 x 10-7 r r r 1 1.6 x 10~7

8.6 x 1Q~9 n r 1 5.0 x 1Q~9

2.7 x 10-10 n r 1 1.6 x 1Q-10


AIDJEX location. For these numbers of ridges, the likely maximum heights are 7.6 and 5.7 m respectively. Looking at this another way, we might ask how many ridges we must count before we would expect to find a ridge 10 m high in the sample. The answer is 2.27 X 105 and 6.25 x 106 for Barter Island and AIDJEX respectively which, assuming the same mean rate of ice drift, would require 26 and 1404 years. These examples emphasize the fact that to estimate the probability of encountering ridges of a given size in a specified amount of time, we must have accurate mean drift velocities for the nearshore ice. As our recent studies have shown (Tucker et al., in this volume), this information is poorly known for the coastal regions of the Beaufort Sea where offshore exploration most probably will start.

SLAR RESULTS

SLAR images are interesting in that they provide us with a different view of ridging, a map-like presentation in which areas of ice with a rough upper surface appear bright (strong radar backscatter) and flat ice surfaces appear dark (weak radar backscatter). On 1 April 1974, roughly the same time of year as the laser profiles were flown, a SLAR mission was completed starting at Lonely and flying due north for approximately 275 km. Good images were obtained over 25 km swaths on both sides of the aircraft. They were processed using a color densitometer with a planimeter attachment to obtain the area of bright return (deformed ice).

In making these determinations 5 x 5 km areas located at a constant lateral distance from the flight path (say at between 10 and 15 km) were examined along the complete sampling run. Then the image was repositioned to examine the next farther-out set of 5 x 5 km areas (say, at between 15 and 20 km). The densitometer was adjusted to compensate for differences in the return from similar ice conditions located at different distances (glancing angles) from the flight path. The results are shown in Figure 6. The symbols indicate data points measured at a constant lateral distance from the flight line. Although there is a large scatter in the results, changes in the lateral distance of the area being sampled from the flight line do not appear to affect the results systematically.

In interpreting these results it should not be assumed that if 60% of an area gives a bright return, exactly 60% of the ice surface is composed of deformed ice. However, large areas of bright returns should correlate with large areas of deformed ice, with the relationship between the two being approximately linear. Even considering the large scatter, Figure 6 shows a steady, approximately linear, decrease in the areal percentage of the ice cover that is deformed as one moves farther offshore. There is no sharp break in the data that would suggest a well defined boundary between the marginal ice zone and the polar pack ice zone located farther from the coast. Many times there is a sharp break between the deformed ice studied here and nearshore areas of undeformed fast ice. However, that break does not show on this particular traverse.


I ! I L _ 1 i i 0 100 2 0 0 3 0 0

Distance North of Coast (km)

Figure 6. Areal percentage of 5 x 5 km areas that show strong radar returns as a function of the distance north from Lonely, Alaska. The different symbols indicate different sets of data points measured at a constant lateral distance from the flight line. Based on images obtained on 1 May 1974.

Another use of SLAR images is to examine the geometry of multiyear floes in the marginal ice zone. This is best accomplished during the fall of the year when the thick "undeformed" multiyear floes stand out as clearly defined dark areas surrounded by either rings or broad areas of bright strong returns caused by highly broken-up newly formed first-year ice. Figure 7 shows histograms giving the frequency of old ice floes of different diameters located off Barrow (to the northwest) and Cape Simpson (to the north) as well as their length/width ratios. The ratios show that the floes are nearly circular. The largest length/width ratio observed was 5.2; the largest floe observed had a diameter of 3.6 km. The histograms are to a good approximation negative exponentials in form. Considering that these floes achieve their rounded shapes and sizes by rotation and abrasion during drift, the form of the size distribution may be explained by simply assuming that the size of a floe changes proportionally to its diameter D and the distance dL it has traveled since its formation. Therefore, dD = —aDdL, which when integrated givesD = /3 exp [—cd^]. Considering that the floes we are sampling have traveled a variety of distances, it is not surprising that their size distribution is approximately a negative exponential.

A C K N O W L E D G M E N T S

The study was supported by the Bureau of Land Management through interagency agreement with the National Oceanic and Atmospheric Administration,


under which a multiyear program responding to needs of petroleum development of the Alaskan continental shelf is managed by the Outer Continental Shelf Environmental Assessment Program (OCSEAP) Office. We would like to thank W.J. Campbell and H. Skibitzke for helping to arrange the SLAR missions and the Naval Arctic Research Laboratory for their support during the laser flights.

Cape Simpson—»H

1800 3 6 0 0 100 7 0 0 1500 2 5 0 0 M e a n F l o e D i a m e t e r (m)

Cape Simpson—*-N Borrow—»NW

5.2 1.0

L e n g t h / W i d t h

Figure 7. Histograms showing the frequencies of different floe diameters and length/width ratios based on SLAR images obtained on 23 September 1975. The areas studied were 1644 km2 (Barrow) and 2375 km2 (Cape Simpson).

REFERENCES

Gonin, G. B. 1960. Detection of the degree of ice hummocking by statistical analysis of aerial photography data. Problemy Arktiki i Antarktiki, 3, 93-100.

Hibler, W. D. 1975. Statistical variations in arctic sea ice ridging and deformation rates. Society of Naval Architects and Marine Engineers Ice Technology Symposium, pp. J1-J16.

Hibler, W. D., and S. F. Ackley. 1973. A sea ice terrain model and its application to surface vehicle trafficability, USACRREL Research Report 314, 20 pp., Hanover, N.H.

Hibler, W. D., W. F. Weeks, and S. J. Mock. 1972. Statistics of sea ice ridge distributions. Journal of Geophysical Research, 79(30), 5954-70.

Hibler, W. D., S. J. Mock, and W. B. Tucker III. 1974. Classification and variation of sea ice ridging in the western Arctic basin. Journal of Geophysical Research, 79(18), 2735-43.

312 WEEKS, TUCKER, FRANK,and FUNGCHAROEN

Klimovich, V. M. 1972, Characteristics of hummocks in shore ice. Meteorology and Hydrology, 5, translated by Dept. of Commerce, Wash., D.C., Joint Publ. Res. Serv. JPRS 59595.

Lowry, R. T. 1975. An experiment in ice profiling in Nares Strait and the Arctic Ocean. Journal of Glaciology, 75(73), 462-63.

Tucker, W. B., and V. H. Westhall. 1973. Arctic sea ice ridge frequency distributions derived from laser profiles. A1DJEX Bulletin, 21, 171-80.

Wadhams, P. 1975. Sea ice morphology in Beaufort Sea, Beaufort Sea Project Technical Report No. 36, 66 pp., Dept. of Environment, Victoria, B.C.

Williams, E., C. Swithinbank, and G. deQ. Robin. 1975. A submarine sonar study of Arctic pack ice. Journal of Glaciology, 15(13), 349-62.

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