## 1 Introduction  
Active radar remote sensing plays a key role for high-resolution observations of the Arctic sea ice cover in the order of a hundred meters to a kilometer. Synthetic aperture radar (SAR) imagery carries information, for example, about sea ice concentration and extent, sea ice types and sea ice drift, e.g., [1]. Radar altimetry is the primary tool to estimate sea ice thickness with good accuracy on basin-scale [2, 3]. The altimeter measures the sea ice freeboard, i.e., the distance between the ocean and the sea ice surface, and ice thickness can then be calculated assuming hydrostatic equilibrium and auxiliary information about sea ice density, i.e., sea ice type, and snow load [4]. To reduce the dependence on auxiliary information, the direct retrieval of sea ice type from altimeter waveform data has been investigated and showed promising results, but also a large variability of waveform parameters [5, 6, 7, 8]. The introduction of the delay/Doppler or SAR altimeter has significantly increased the along-track resolution of spaceborne altimetric measurements [9, 10]. This results in better sensitivity to small features on the surface [11] and enables the ability to investigate the response of altimeter waveforms to small-scale variations of sea ice types such as large ridges, refrozen leads or multiyear sea ice floes embedded in first-year sea ice on spatial scales from a few hundred meters to a few tens of kilometers. Altimetric data alone is, however, not sufficient to unambiguously determine the origin of waveform changes over sea ice, because of temporal and spatial variability of waveforms and the impact of off-nadir specular reflectors [6, 7, 12]. Near-coincidental SAR imagery has proven a valuable source for verification of lead detection algorithms in altimeter data processing, e.g., [13; 14]. In the late 1980s, two studies have been undertaken with data from Seasat and Geosat to relate SAR backscatter and altimetric data over sea ice, but both were limited to one SAR image as a reference [15; 16]. Presently however, Copernicus' Sentinel-1 and Sentinel-3 two satellite constellations in addition to Radarsat-2/Radarsat-Constellation and CryoSat-2 provide an increasing and long term (Copernicus program) data availability especially at lower latitudes (below 81.5\({}^{\circ}\)). Furthermore, near real time availability of data [17] facilitates the use for short term observations of the sea ice cover. This allows us to perform, to the best of our knowledge, the first comprehensive study of the waveform evolution over different sea ice types with SAR imagery as high-resolution validation data. SAR imagery gives a high-resolution, two-dimensional picture of the current sea ice situation, but sea ice thickness can only be inferred from sea ice signatures with ambiguities [18]. Meanwhile, altimeter data provides a means to obtain sea ice thickness, but is sparse in nature due to the one-dimensionality of the measurements. To overcome these spatial limitations, averaging in space and time is commonly applied to obtain standard products of hemispherical sea ice freeboard and thickness, e.g., [19], at the expense of small-scale features. Combining these two information sources therefore has several potential benefits. On the one hand, sea ice type information, necessary for freeboard to thickness conversion [4], can be provided by SAR imagery in high-resolution [20] to reduce uncertainties in this critical step. On the other hand, the scarce thickness retrievals could be locally expanded to neighboring areas with similar sea ice signature in the SAR image regarding backscatter intensity and texture. In this paper, we use near-coincidental Sentinel-1 SAR imagery and CryoSat-2 altimeter data of three winter seasons in the Beaufort Sea to investigate the sensitivity of altimeter waveforms to small-scale (up to 10-20 km) variations of sea ice. We evaluate whether SAR imagery aids the interpretation of the sea ice situation and identification of sea ice types.