Statistical properties of sea ice surface topography in the Baltic Sea

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The results from helicopter-borne laser profiling of the ice surface in the Baltic Sea for March 1988 are presented. The laser was a PRAM III system with a measurement frequency of 1–4 kHz, a footprint of approximately 20 cm at a flight elevation of 100 m and a single-shot vertical resolution of 12.5 cm. The laser profiles were calibrated to remove aircraft motion and averaged to 1-m intervals which combined to produce an elevation accuracy of 1.8 cm. A total of 51 profiles with varying lengths from 6 to 24 km, in all 662 km, are analysed. The data were stratified into 5 groups representing different ice fields. The standard deviation of the surface elevations ranged from 7 to 14 cm. The spectrum of surface roughness displayed a red noise form (slope about − 1.5) through the wave-lengths 4 to 100 m. Ice ridges were identified using the Rayleigh criterion with a cutoff height of 40 cm. The mean ridge height ranged from 52 to 59 cm, the standard deviation 12 to 20 cm, and the maximum ridge height sampled was 197 cm. The number of ridges was 1.4 to 9.5 km −1 for 4 of the groups and up to 17.4 km −1 in one group representing an intense shear zone. Ridge heights were exponentially distributed whereas the log-normal distribution provided the best fit for the ridge spacings. Mean ridge height and ridge density were not strongly correlated. The amount of ridged ice was estimated to account for 1 to 18 cm of equivalent ice thickness while based on a crude extrapolation method rubble fields summed up to 2 to 10 cm. Deformed ice constituted 15 to 40% of the total ice volume. DOI: 10.1034/j.1600-0870.1993.t01-1-00004.x