118
A. K. LIU AND C. Y. PENG
Day 32
Day 35
Day 38
Day 41
Day 44
300
300
300
300
300
250
250
250 ~
250
250
200
200
200
200 ~ 200
~
~
.
~ 150
"'150
150
150
150
~
100
100
100
100
100
50
50
50
50
50
00
50
100 00
50
100 00
50
100 00
50
100 00
50
100
Day 47
Day 50
Day 53
Day 56
Day 59
300
300
300
300
300
250
250
250
250
250
200
200
200
200
200
~ 150
"'-. ~ ~ ~"'-' -
~
150
150
150
150
"
~
100
100
100
100
~
100
50
50
50
50
50
00
50
100 00
50
100 00
50
100 00
50
100 00
50
100
km
km
km
km
km
Fig. 6. Summary of the ice edge locations near the St. Lawrence Island from February 1 to February
28,1992, with 3-day intervals
6.3.2
Bering Sea Polynya
In the Arctic, the coastal polynyas occur along coasts with strong steady off-shore
winds. In the open water in polynya, the cold winds cause oceanic heat loss and generate waves and turbulence. Figure 7 shows a low-resolution SAR image (100 m resolution) from February 27, 1992, where the opening of a polynya to the south side of the
St. Lawrence island is dearly visible. In this SAR image 1024 x 3072 pixels in size, the
wavelet transform using the Mexican-hat wavelet with a scale a = 128 units of pixel spacing [see Eq. (1)] was computed. Each pixel is 100 X 100 m2. With such a large wavelet
scale, zero-crossings can hardly exist. In general, using a 50% threshold for the results
of the wavelet transform with positive values yields an approximate ice edge. Note that
because of the large scale of the wavelet, the edge effect of the Fourier transforms
applied to carry out the wavelet transform may cover a significant portion of the
domain so that it is impossible to extract any meaningful approximate boundary. To