Abstract
The statistics of ice ridging signatures was studied using a high (1.25 m) and a medium (20 m) resolution SAR image over the Baltic sea ice cover, acquired in 2016 and 2011, respectively. Ice surface profiles measured by a 2011 Baltic campaign was used as ground truth data for both. The images did not delineate well individual ridges as linear features. This was assigned to the random, intermittent occurrence of ridge rubble block arrangements with bright SAR return. Instead, the ridging signature was approached in terms of the density of bright pixels and relations with the corresponding surface profile quantity, ice ridge density, were studied. In order to apply discrete statistics, these densities were quantified by counting bright pixel numbers (BPN) in pixel blocks of side length L, and by counting ridge sail numbers (RSN) in profile segments of length L. The scale L is a variable parameter of the approach. The other variable parameter is the pixel intensity threshold defining bright pixels, equivalently bright pixel percentage (BPP), or the ridge sail height threshold used to select ridges from surface profiles, respectively. As a sliding image operation the BPN count resulted in enhanced ridging signature and better applicability of SAR in ice information production. A distribution model for BPN statistics was derived by considering how the BPN values change in BPP changes. The model was found to apply over wide range of values for BPP and L. The same distribution model was found to apply to RSN statistics. This reduces the problem of correspondence between the two density concepts to connections between the parameters of the respective distribution models. The correspondence was studied for the medium resolution image for which the 2011 surface data set has close temporal match. The comparison was done by estimating ridge rubble coverage in 1 km2 squares from surface profile data and, on the other hand, assuming that the bright pixel density can be used as a proxy for ridge rubble coverage. Apart from a scaling factor, both were found to follow the presented distribution model.
Highlights
The Baltic Sea is a semi-enclosed brackish sea water basin in northern Europe
Changing the percentage changes the connectivity of these pixel clouds but not their general delineation, and the same binary structures can be often identified in other Synthetic Aperture Radar (SAR) images with different resolutions and different acquisition parameters
The present approach proceeds from these observations by analysing SAR images in terms of local density of bright pixels 10 chosen by a certain percentage (BPP)
Summary
The Baltic Sea is a semi-enclosed brackish sea water basin in northern Europe. Baltic drift ice has dynamic nature due to forcing by winds and currents, which results in an uneven broken ice field with distinct floes, leads and cracks, brash ice 25 barriers, rafted ice and ice ridges. Better information about the distribution 10 of ridged ice would improve routing, icebreaker operation planning and the predictability of arrival times This would make the Baltic winter navigation system as a whole more efficient, safe, environmentally friendly and economical. In the Finnish-Swedish ice charts this information is coded by the degree of ice ridging (DIR), which is a numeral that seeks to quantify ridging 15 and to characterise navigational difficulty Icebreakers communicate their estimates of DIR values to the ice services where DIR values are assigned to ice chart polygons using the estimates, other data, and manually interpreted SAR images. Individual ridges are usually not resolved, the characteristics of the satellite instruments, images and acquisitions are varying, and there are sensitivities to ambient conditions and surface properties not related to ridges To this adds the fact that ground truth data and the SAR signatures cannot usually be matched if the ice has drifted or deformed in the meantime. The other, with 20 m resolution, is nearly concurrent with the surface data set which is used to validate the approach
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